Spatial variation in channel morphology and sediment dynamics: Gila River, Safford Valley, Arizona

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1 The Hydrology-Geomorptiology Interface: Rainfall. Floods, Sédimentation, Land Use (Proceedings of the Jerusalem Conference, May 1999). IAHS Publ. no. 261, Spatial variation in channel morphology and sediment dynamics: Gila River, Safford Valley, Arizona JANET HOOKE Department of Geography, University of Portsmouth, Buckingham Building, Lion Terrace, Portsmouth, Hampshire POI 3HE, UK c-mail: Abstract This paper aims to identify and explain the extent and pattern of spatial variability in channel and floodplain morphology and sediment dynamics of a semiarid river which is known to be highly dynamic. The study reach is a 100 km length of the Gila River in Safford Valley, southeast Arizona, and was mapped from large-scale aerial photographs of one date. Three major zones were identified in the valley floor: channel, active and phreatophyte zones. The width of the zones was measured at cross sections 1 km apart down the valley. The active zone is an important indicator of the extent of fluvial activity and is found initially to increase in width downstream from the canyon exit at the upstream end of the valley, then to vary in dimensions, but with a significant increase in width in the downstream quarter of the valley. Although the whole length of channel has been affected by the same major hydrological events, morphologically distinct reaches are identifiable. Various factors are hypothesized and examined as having a possible influence on this morphological variability through effects on sediment dynamics, including gradient and sediment supply; aspects of land and channel management are also assessed. The increase in active zone width at the lower end may reflect earlier clearances of phreatophytes, though it could also be due to more recent interaction of vegetation growth with groundwater and moisture levels. Key words channel management; floodplain; land management; phreatophytes; river channel; sediment supply; sediment transport; semiarid INTRODUCTION An important aspect of understanding how rivers behave is to elucidate the dynamics of change, the influences to which the river responds and the spatial variability of the response. A major aim of this work is to identify the overall controls on morphology and sediment dynamics, and the influence of various human and physical factors. This is of potential value both for understanding landscape change and for some aspects of management of rivers and valleys, particularly in arid and semiarid areas where streams are so variable in flow and mobile in their course, and where much of the human settlement is on the valley floors and therefore vulnerable. The purpose of this study is to analyse and explain the spatial variation of channel morphology and response using the Gila River in southeast Arizona. The study is complemented by work on the hydrological variations (Hooke, 1994) and on the temporal patterns of channel change in the valley (Hooke, 1996). The Gila River in Safford Valley, southeast Arizona has been selected for this study because a major project was carried out there by the US Geological Survey in the

2 252 Janet Hooke period This allows a comparison to be made between then and now, and utilizes the source materials on the channel characteristics and on flood impacts which are available. It is known that major changes in the character of the river have taken place during the last century, with a major phase of destabilization and channel widening occurring in the period , and a narrowing and stabilization during the 1940s and 1950s (Burkham, 1972). The Gila River The Gila River flows from the mountains of southwest New Mexico, westwards across southern Arizona, to its confluence with the Colorado just north of the Gulf of California. The study area is within the Basin and Range physiographic province, Safford Valley being a small basin, set between two ranges at either end through which the river flows in very narrow gorges or canyons (Fig. 1). The valley itself is a naturally arid area with an annual rainfall of 313 mm at Fort Thomas in the centre of the Valley, and a mean annual precipitation for the Gila basin to the head of Safford Valley of 424 mm. The river has perennial flow fed from precipitation and snowmelt in the mountains of New Mexico. The catchment area at the head of Safford Valley gauging station (US Geological Survey gauging station no. 4485) is km 2, the mean annual flow is îrrv 1, the mean annual maximum daily discharge is 269 m 3 s" 1, and the highest peak flow ever recorded is 3738 m 3 s" 1. The vegetation of the surrounding uplands ranges with altitude from desert shrub at the lower levels to coniferous forest on the mountain tops, with the edges of the valley supporting mainly mesquite. The floor of the valley adjacent to the channel now has dense stands of tamarisk (Tamarix pentandra) which came in as an invader plant to this area in the 1930s. Formerly, these wetter areas supported stands of cottonwood and seep willow (batamote). The valley is an important agricultural area, with cotton grown under irrigation, using water derived from the Gila River and groundwater. The area was part of the Apache Indian tribal lands prior to Anglo-American settlement, and the downstream part of the valley is within the San Carlos Indian Reservation. At the lower end of the study reach the river is impounded by the Coolidge Dam which was completed in The Gila River in Safford Valley is a perennial stream with a generally well defined channel zone but fringed by a broad zone which has been subject to fluvial processes within the last century. In very large floods such as the 1100 m 3 s'\ 10 year flood, inundation occurs over much of the valley floor. As the river emerges from the uplands the valley broadens, then is very wide around its confluence with the San Simon River which drains a major lowland area (Fig. 2). Downstream from Safford for about 30 km, the valley averages about 4.5 km in width, then it narrows between the adjacent Quaternary deposits to about 2.0 km from there downstream to the reservoir. The channel varies in characteristics both spatially and temporally but has a generally meandering habit with some straight reaches, though a greater tendency to braiding after large flood events. The characteristics of the channel were analysed by Burkham (1972) for several dates up to 1968 using map, aerial photograph and field evidence. He demonstrated that remarkable changes in channel width and morphology had taken place during the

3 Fig. 1 Location of Safford Valley in southeast Arizona.

4 Fig. 2 Map of Safford Valley in southeast Arizona.

5 Spatial variation in channel morphology and sediment dynamics 255 twentieth century, with a phase of very great widening and braiding in the period 1905 to 1917, then a gradual rebuilding of the floodplain and narrowing of the channel, with growth of phreatophytes, through to the 1960s. This included the period of invasion by tamarisk salt cedar with the dense phreatophyte zone reaching its maximum extent in the 1950s. The general sequence and timing of changes is coincidental with other fluvial changes in the American Southwest (e.g. Hereford, 1984; Graf, 1986a) and the causes of change have been much discussed (Cooke & Reeves, 1976; Graf, 1983b). The mobility and nature of channel changes has been analysed for a downstream reach of the Gila in central Arizona by Graf (1981). Hooke (1996) has analysed the temporal sequence of chamiel changes up to 1992 and has shown a complex relationship between channel form and characteristics of flood events, a conclusion reinforced by Huckleberry (1994) as a result of the 1993 flood. In order to understand the river behaviour and provide a perspective to the changes, it is necessary to analyse the spatial variability of characteristics. Analysis of the spatial patterns at one point in time is likely to help elucidate the overall controls on river behaviour. This is predicated on the assumption that it is major floods which influence the morphology of the channel and that these affect the whole length of the valley. The opportunity to assess the spatial variability was afforded by the availability of a set of colour aerial photographs dating from 1982 of the whole valley at a scale of 1:7200. These show the channel, deposits and vegetation in great detail. They also provide a useful baseline study in that they post-date a period of high flows in the 1970s but predate the largest flood on record in METHODS Figure 3 shows examples of some of the aerial photographs; channels, areas of fresh sedimentation, open bars and zones of various density of vegetation, with some old channel scars within them, can be distinguished. The cultivated areas and the dry uplands are also quite distinct. The area covered by sediment deposits and phreatophytes represents the zone of reworking and mobility of the channel. The spatial variation in channel, depositional area and total zone of mobility has, therefore, been measured by categorizing and mapping from the photographs, every parcel of land. A total of 11 categories were identified (Table 1) and maps such as that shown in Fig. 4 were produced for the whole valley. These categories were then grouped into three general zones: channel, active and phreatophyte zones, and the boundaries of each zone drawn in. Widths of each of the three zones were measured by sampling at cross sections spaced approximately 1 km apart down the valley. This produced 96 sampling points (cross sections 1-96) in the river length from Gila Box in the upper canyon to the Calva gauging station at the downstream end of the valley and upstream end of the reservoir. The methods worked well, with few zones difficult to distinguish except some scrubland and old wet channels. Field mapping was used to confirm the characteristics of each type of area. Sinuosity and meander/braiding characteristics were also measured from the aerial photographs. The active zone is of particular interest since it represents the zone which has been recently reworked by the river and is free of vegetation, except for sparse young plants.

6 256 Janet Hooke Fig. 3 Examples of 1982 aerial photographs of the River Gila: (a) sections 24-32; (b) sections 47-53; (c) sections 54-56; (d) sections 57-65; (e) sections 83-87; (f) sections

7 Spatial variation in channel morphology and sediment dynamics 257 Table 1 Categories used in mapping the River Gila valley floor. Zone 1 (Dark brown and water) Active, moist low-flow channel la (Dark green) Seepage zones, wet, with low plants 2 (Light brown/yellow) Fresh bars and unoccupied (dry), active channels 3 (Grey) Slightly older, active bars with bare sediment, scattered bushes 4 Scrubland, young phreatophytes, scattered bushes, old channels visible 5 Old abandoned channels 6 Dense phreatophytes 7 Structures, dykes, levées, etc. 8 Workings, gravel pits, anthropogenic features 9 Abandoned, rough fields 10 Cultivation 11 Upland Based on field observation of the river processes, it is suggested that the active zone is kept free by the erosional and flooding activity of the river rather than that the vegetation directly controls the active width, but the interaction is a complex one. The methods of analysis are based on an assumption that the patterns of variation may be complex and not necessarily amenable to linear statistical analysis. Possible factors influencing the spatial variability are each evaluated in turn, combining various types of field and documentary evidence and the data compiled in previous studies. All 1 2 mm 3 r^sh Fair Okte'Bar 4 Scrub Vcgdann EZZ3 5* g g 3 ES3 10 Cjl.w*ed rnkfc fuiits Zbnc rjsw Bau'ds'y Fig. 4 Example of mapping of the Gila flood plain from aerial photography.

8 258 Janet Hooke interrelations of quantifiable variables have been plotted and analysed statistically, and then the factors combined in a multivariate nonlinear analysis. The morphological variation is related to the sediment dynamics. A number of controls and causal factors of the spatial variation in morphology can be hypothesized including natural factors such as: valley gradient, valley width, geology and sediment supply. Human factors and structures can influence the river directly or indirectly, such as: dams and water extraction, dykes and embankments, channelization, bridges, water inflow, groundwater level, phreatophyte growth. Variations may also be regarded as inherent and may be revealed by autocorrelation analysis, (Thornes, 1980). In this paper, the possible influence of individual factors is identified, but the outcomes may be the result of these acting in complex and differing combinations down the valley and with different time rates of adjustment. Equilibrium theory has been shown to be inapplicable to such channels (Burkham, 1972; Graf, 1981; Wolman & Gerson, 1978). The aim of this paper is to assess the extent to which systematic patterns and controls are identifiable. ANALYSIS Zone widths The methods described above produced quantitative data on zone widths and these have been mapped against distance downstream in Fig. 5. Data on the spatial variation were supplemented by visual inspection of the photographs and subjective identification of different types of reach prior to the quantitative analysis. Very brief descriptions of these reaches are given in Table 2 to indicate their sequence downstream. The channel width is the actual wetted area and, where the channel is braided or anastomosing, this is the total of the multiple channels. This varies from about 20 m in the upper part to an average of about 50 m downstream, though with much variability. It corresponds with the low flow channels, which are usually contained between low banks and are quite well defined in the field. The active width is the variable of greatest interest since that is a measure of recent channel reworking and channel mobility. It is the zone kept free of vegetation by erosion and deposition and contains extensive bars. The width is highly variable, with values in the range m but an average in the main part of the valley of 313 m. The full vegetated width varies from 72 m to 1166 m. It can be seen from Fig. 5 that the active width increases rapidly as the stream emerges from the canyon section, with practically no margin of phreatophytes (cross section numbers CS 11-25). The active zone is then highly variable and bordered by a zone of phreatophytes to a major narrowing at CS 54 (Eden Bridge). Downstream from there to CS 73 the active zone is rather narrower but the phreatophyte zone is very wide. An abrupt change in the active width takes place from CS 73, though with the full width remaining comparable. Pattern Channel pattern is another measure of morphological characteristic. The channel pattern is obviously rather different in the confined section in the canyon where the sinuosity represents valley meanders. Downstream from there, sinuosity and pattern

9 2000 Channel widths Phreatophyte B Active Channel ^400 -, 300 -i ^200 fioch Valley widths Q o» i ' ) i i i-t r i ] -i , Valley Gradient Sinuosity , c 4 ra 2 S 0 m 6 *- S>. io 4 II , oj 300 H E200 "100 Braiding Index n Sediment supply llllll Il n i,,,,i, i l o, mill jii,, o, i n,, n,, o 1,11,1 n Maximum Sediment Size Dams Fig. 5 Spatial variation in morphological characteristics and other factors in the downstream direction, Gila River, Safford Valley, southeast Arizona.

10 Table 2 Characteristics of River Gila in Safford Valley. Section Widths Pattern Valley Flood plain vegetation Tributaries Sediment Structures CS1-5 Low Single, valley meanders, small Canyon Very little Small Little None point-bars CS6-11 Low Single channel, larger point Broader-canyon On upper parts of Some large Input of sediment None bars bars from tributaries CS Increasing Mostly single extensive bars Exit from canyon Little Few Large size Many CS24-32 Mostly Braided, complex Wide Increasing. Dense Mostly small Moderately large Many narrower along channels and washes size and abundant floodplain edge CS33-37 Wider Braided, large bars Wide Unvegetated bars. Major washes Abundant Several Thick vegetation on old channels CS38-46 Narrower Some multiple channels Wide Very dense Controlled Abundant Many CS47-53 Wide and Dominantly meandering Wide Extensive dense Major washes Abundant, Some very active extensive bars phreatophytes dominantly sand and gravel CS54-56 Very Straight, broad channel. Few Wide Dense Few Dominantly sand Few narrow bars CS57-65 Very wide Meandering, large loops Near valley wall Very dense and Many Sand and gravel Few extensive CS66-72 Wide Regular meanders Narrower Dense Several Sand and coarse Few CS73-82 Moderate Braided in parts Narrow Sparse, narrow Several Coarse Practically none CS83-87 Wide Multiple, ill-defined Narrow Bordering channels Several Coarse Few CS88-96 Wide Braids, extensive bars Narrow Bordering channels Many major Coarse Bridge inputs

11 Spatial variation in channel morphology and sediment dynamics 261 vary, with some distinct reaches being identifiable. Zones of multiple channels occur, particularly in sections 21-27, 33-37, and There is a large coincidence between these sections and high active widths, particularly in the downstream reaches. A distinctly meandering reach with a well defined single channel occurs in the section 58 to 74. The major boundary at 74 coincides with change from meandering to braided. A plot of sinuosity values against valley gradient reveals a weak, nonlinear inverse relationship. Relatively high sinuosity occurs in section and this corresponds with a low valley gradient section. The occurrence of multiple channels does not show any distinct statistical relationship to valley gradient but this is complicated by the problems of defining number of channels and by feedback adjustments. Tamarisk tends to increase sedimentation and channel plugging and therefore increase channel switching and mobility. Graf (1981) found greater mobility in the denser phreatophyte zones, but lack of vegetation would also mean that sediment is more easily moved and lateral erosion might be accomplished more easily. Field evidence indicates that tamarisk can be torn out in floods, though it is resistant, and that the stands are destroyed by lateral erosion. The depth to groundwater would influence phreatophyte growth so directions and pattern of seepage may be important. Structures in the channel such as dams, dykes and bridges would be expected to have fairly localized effects, though aggradation extending 11 km upstream in the 72 years since construction of a dam has been documented for the lower Gila by Graf (1988). Gradient This was measured from US Geological Survey 1: topographic maps which have contours at 10, 20 or 40 feet intervals depending on the scale of relief on the map. Valley slope is used since this is the overall control and the base for adjustment of the channel. Gradients within the valley range from to As expected, there is a general overall decline in gradient, as normal in long profiles, from the upper canyon downstream through Safford Valley. However, there are significant variations within the lowland part of the valley. Relatively steep reaches occur between CS 39 and 44, 50 and 53, 62 and 70, and there is a notable steepening of the valley at the downstream end. Shallow gradient reaches occur between CS 36 and 39 but especially 44-49, 58-61, and 79-89, with being a particularly low gradient reach. It would be anticipated that the water in floods would spill out onto the floodplain in shallower sections where the flood wave is transmitted more slowly and where channel banks are lower, with a tendency for aggradation in the reach. The spilling out would tend to cause braiding and an increased zone of moisture would enhance tamarisk colonization (Graf, 1982), though there would also be an increased zone of uprooting in large floods and prevention of colonization in periods of high floods. Comparison of the sequences show a tendency for flatter gradient reaches to be wider and have larger phreatophyte zones, as hypothesized. The steeper reaches appear to be narrower though this relationship is less clear towards the lower end. The steeper sections also tend to be straighter and, for example, in a particularly straight, narrow zone at Eden Bridge (CS 54), it is thought that coarse material transported in floods is

12 262 Janet Hooke flushed through the steep section. The low flow channel is relatively wide so aggradation of fines occurs in low flow. Width does appear to have some relationship to valley gradient but there are some anomalies. Gradient appears to exert an overall control but other factors are influencing. It does not account for the large increase in active width round CS 74. Valley width The valley is confined as far as CS 14 then broadens out as far as CS 44 (Pima), after which it narrows to a more restricted zone, CS It broadens again to CS 61, upstream of Fort Thomas then narrows significantly to about 2 km wide from CS 63. At its widest point the valley is over 6 km wide. It is 4 to 10 times the floodplain width and would not therefore appear to be a constriction on mobility. The river impinges on more consolidated rocks in the section downstream of Sanchez bridge (CS 25-30), then again upstream of Fort Thomas, in parts of section 61-82, where large meanders swing against the valley wall. The 1983 flood limits mapped by Garrett et al. (1986) do impinge on the edge of the upland in these sections, but in no location downstream of the canyon does the valley width appear to be limiting. There is no correspondence in the patterns of variation or full width, with those of valley width or distance from the valley wall. Geology The Gila River flows through an alluvial plain comprising an average thickness of 30 m of Quaternary silt, sand and gravel deposited by the river (Weist, 1971). Bordering the alluvial trench is higher land which grades gently up to the base of steep mountain slopes. Two principal terrace levels have been recognized and these pediment forms are capped by Pleistocene gravel and overlie Pliocene lake deposits. Knechtel (1938) cites a suggestion by Schwenssen (1919) that the boundary of the lake beds is at the Reservation boundary but Knechtel disagrees. The geological map of Graham and Greenlee counties shows a boundary between the Quaternary silt, sand and gravel and the lake strata, on the north side of the river near Bylas. It does not coincide exactly with the morphological changes and is not present on the south side. The alluvium is of the order of 30 m thick so there is no suggestion that the lake beds are close enough to control the river, though they do crop out in the terrace edges. The geological evidence is equivocal but a possible influence on groundwater and on the channel morphology cannot be dismissed. Sediment supply Sediment supply, both in calibre and amount, is well known to be a primary influence on channel morphology (e.g. Harvey, 1991). Sediment size and sources have been assessed in two ways. First, all tributary inflows and points of main channel impingement on the upland were mapped from the aerial photographs. The freshness

13 Spatial variation in channel morphology and sediment dynamics 263 of sediment, and erosion of many tributaries and evidence of whether they contributed to the channel, were also assessed in the field. Secondly, at 24 sample cross sections, maximum size of clasts was measured in the field. Although these measurements were taken at a different time from that of the photos, they still provide an indication of potential delivery of sediment from the upland and of comparative transport capabilities of sections. The nature of sediment supply in each reach is described in Table 2. There are four main potential sediment sources within the valley: - Ephemeral tributaries or washes draining the surrounding terraces and uplands Runoff in these is mostly generated by summer thunderstorms and each only flows very occasionally. From a programme of field measurements Burkham (1976a) calculated that the ephemeral tributaries each flowed on average only three days of the year. If the main channel of the Gila is near the upland margin, then these streams may contribute significant sediment, but many end in alluvial fans at the margin of the floodplain or within the phreatophyte zone. Those with a long distance to travel over the valley bottom tend to deposit their load before reaching the main channel. Many tributaries in the central portion of the valley have been channelized and many now have control structures on them to decrease sediment supply. Some were formerly very actively eroding, particularly in the period of arroyo formation, e.g. the San Simon stream (Cooke & Reeves, 1976). The most active contribution of this type at present occurs in the downstream portion of the valley approaching San Carlos reservoir. The ephemeral washes mostly drain areas of erodible Quaternary gravel, sand and silt. - Direct erosion of valley sides In a few locations, identified in valley width above, the main chamiel impinges directly on the older sediments of the valley. In places, particularly at the lower end, the sediments are coarse sands, gravels and cobbles. At the upstream end near Sanchez Bridge the laminated lake sediments are being eroded. In the upland section and at Black Point the river impinges on igneous rocks but rates of erosion are extremely low. Burkham (1972) considers that there is a low sediment load at the head of Safford Valley due to lack of sediment available in the mountains. Coarse sediment contributed from the mountainous parts of the catchment comprises mainly basalt, some andésite and some rhyolite. - Erosion from fields and irrigation ditches Some suspended sediment is present within the return flow of irrigation water draining from the fields and canals. All this sediment is fine silt and is thought not to be a significant contributor to total load nor to the basic morphology of the channel. - Erosion of channel banks This is a within-channel source of sediment and is a reworking of previously deposited material. The banks and bars being eroded vary in age but most are very recent. Composition of the banks varies from cobbles to clay and is spatially highly variable. Active bank or bar erosion is taking place at many locations along the channel but the complexity of the channels and valley floor morphology means that it is very difficult to assess whether there is net erosion or deposition in a section. The channel did historically carry a high sediment load as evidenced by the reservoir sedimentation, and descriptions of the turbidity of the water. Maddock (1940) considers that in its natural condition the Gila would have carried a relatively high sediment load, derived from the

14 264 Janet Hooke exposures of unconsolidated material. He was writing in 1940 at end of the period of accelerated soil erosion and arroyo cutting when there was a high contribution from tributaries. Calculations by the Technical Committee (Maddock, 1940) and by Lippincott (1900), in making calculations prior to the construction of the San Carlos reservoir, both estimate sediment load as -2% by volume of the inflow to the reservoir. Comparison with data from other reservoirs in the American West indicate this to be very high. Indeed in 1900, prior to the main period of channel widening, Lippincott said "No other stream is known in America which carries such large volumes of debris". However, given the nature of other streams such as the Rio Puerco and the Paria this is probably a statement of ignorance at the time. The San Simon formerly contributed much sediment but has been controlled since The overall sediment flow has probably decreased since the 1930s in line with that found for other parts of the Colorado basin (Graf, 1987). Discussion continues as to whether this should be attributed to climatic variations or to land use and management. The maximum size of clasts deposited in a channel reflects the flow competence but it is also dependent on supply. The field measurements (Fig. 5) show a gradual decrease in size of the largest clasts being transported in the present channel, from the upstream end of the study reach to about CS 54. Downstream from there there is a progressive increase in size so that 200 mm diameter cobbles are again found near Calva. The position of the main tributaries is plotted on Fig. 5 and it can be seen that the number of inflows decreases once the river leaves the upland, and is sparse through to CS 61, particularly in sections The number of inflows increases in the downstream reach but, in addition, these streams are closer to the upland and many more of them flow directly to the present active zone. This increase in size therefore appears to be due to the coarser inputs from the steep and active washes draining the adjacent uplands. Wider channels, with a greater tendency towards braiding would be expected where sediment supply of coarse material is higher. Figure 5 shows some correspondence between active widths, degree of braiding and sediment size, though not with number of tributaries entering. Because of the extent of the alluvial deposits in the valley and their ease of erosion, it is thought unlikely that sediment supply to the total load would be limiting. However, the pattern of sediment inputs and the decrease in size in the central part of the valley implies that coarse material is not abundant in that part and any which does occur is readily flushed through. Channel competence Alternatively, sediment size can reflect the competence of flow which in turn is influenced by the channel gradient and cross-sectional morphology. Burkham (1972) did speak of the "inability of the Gila River to move sediment through the valley". Calculations of the mean channel velocity, shear stress and dimensionless shear stress for the range of values of slope, width and depth in the study reach, using the various sediment competence curves available (e.g. in Graf (1971), Gardiner & Dackombe (1983)) give some indication of the range of competence. The range of values of shear stress is to 20.7 kg ra 2 which gives a range of competence of 58 to 333 mm using the Baker & Ritter (1975) equation, or 0 to 200 mm using the Shields criterion

15 Spatial variation in channel morphology and sediment dynamics 265 (Gardiner & Dackombe, 1983, Fig. 7.14). Values for sample sections are given in Table 3. Taking moderately high flows of recurrence interval four years (507 m 3 s" 1 ) the Baker & Ritter equation implies that most of the sections would be able to carry particles up to about 100 mm diameter but not every section would be competent to carry sediment greater than this size. Using dimensionless shear stress, the Shields criterion of 0.06 is found to be exceeded on almost all slopes, assuming depths of 1-2 m and when bed material is less than 60 mm average diameter. Although maximum diameters are higher than this at some sites, the amount of sand means that average sizes are unlikely to exceed this. The range of velocity, using a Maiming n value of 0.05, ranges from 0.94 to 6.25 m s" 1, which gives a lower limit of 5.0 mm diameter using the Hjulstrom (1935) curve. At moderate flood levels, three sample sections have competences of 12 mm (CS 48), 9 mm (CS 54) and 7 mm (CS 60). Calculations of stream power for the range of channel slopes in the study section range from 15.5 to 77.5 W m" 2 for a 60 m wide channel at 113 m 3 s" 1 flow, calculated as bankfull (Burkham, 1976b) at Calva, and from 43.9 W nf 2 on the lowest slopes to W m" 2 on the steepest section for the very highest discharge on record (3738 m 3 s" 1 ), assuming a 700 m wide flow. Specific (unit) stream power for sample sections are given in Table 3. For a moderately low flood, such as the maximum daily discharge in 1992 (374 m 3 s" 1 ) and using active width measured at sections, the specific stream power is lower in sections CS 63 and 86 than elsewhere. Using Grafs (1983a) equation, which is a combination of the shear stress (du Boys) and the Baker & Ritter equations, and a discharge of 507 m 3 s" 1, which is the twentieth highest peak flow of the last 79 years (and a Manning roughness coefficient of 0.05), the competence is calculated as 84 mm on the lowest channel slopes ( ) and 153 mm on the highest slopes (0.0042). Reversing the equation, a discharge equal to the highest peak discharge ever measured would be needed to move the very coarsest material through the Bylas section, where there is coarse material and tendency to braiding. A discharge of 880 m 3 s" 1 is needed to move 100 mm material on the low slopes, and discharge of only 133 m 3 s" 1 to move such material on the high slopes. These are equal to recurrence interval peak flows of about nine years and less than two years respectively. There is, therefore, some evidence that a complex combination of sediment supply and morphology influencing competence may have an influence and feedback effect on the morphology and mobility of the channel. A braided section is likely to leave large areas of wet sand which are readily colonized by tamarisk, and growth of those plants would then encourage sedimentation which could increase channel mobility. Fine sediment is basically not limited in any section since much is available from the channel banks themselves. If a channel is incised then colonization by vegetation could decrease mobility in low-moderate events but tamarisk is eroded in the very large events. Dams and water diversion Several weirs or small dams have been constructed on the main channel of the Gila (Fig. 5). These are water diversion structures and are the points where the irrigation water is taken off via the main canals. These structures could have two effects:

16 Table 3 Hydraulics of flow and competence for sediment transport. Site CS Moderate flood: Crit Q: no. S R W n D Power Unit power Stress Velocity Competence actual d rfloo (m) (m) (m J s' 1 ) (mm) (W m') (W nf 2 ) (kg nf 2 ) (ms'l (mm) (m 3 s" 1 ) (m 3 s"') Safford Thatcher Burkham Eden Burkham Bylas Calva

17 Spatial variation in channel morphology and sediment dynamics 267 (a) a sediment effect and through that an effect on gradient, and (b) an effect on the water budget, as well as being a direct alteration of the slope profile. Figures are available for the amount of diversion in each canal but some water is returned back to the main Gila channel and some water may be contributed by pumping into the canals. Calculations of the water budget within the Safford Valley have proved rather intractable (Baldys & Bayles, 1990). However, for the purposes of understanding the river morphology, it is thought unlikely that these diversions and the depletion of flow in the downstream direction have much effect since most of the changes in morphology take place during floods and the gauge records show that at these times water diversion for irrigation is comparatively insignificant (US Geological Survey Water Supply papers state "peak flows not materially affected by irrigation diversions"). Peak diversion is also at a different time of year from the major floods. (However, if it is moderate floods which determine morphology, then such diversions could become significant). It would be expected that the dams could have a significant effect on sediment supply and transport, and that differences up and downstream of dams would be detectable due to aggradation upstream and depletion of sediment downstream. Examination of the maps and the data plots in relation to the position of dams indicates that there is often a widening of the channel upstream but this is not consistent. The structures do vary in their nature and the extent to which they impound the flow of the channel. Field examination of several indicates that the effect is transmitted up to 1 km upstream so the effects tend to be localized and probably do not account for the major variations in morphology. Burkham (1972) reckoned that sediment accreted behind dams is flushed out in large floods. However, the dams would still act as control points in the longitudinal gradient profile of the channel. Overall, the active and phreatophyte widths show no obvious relation to location of dams and diversion structures. Water inflow Given the amount of pumping from groundwater and the amount of irrigation water diverted which is surplus to requirements, it is possible that flow increases downstream at certain times. At low flow in July 1992 there appeared to be more water in the chamiel downstream of Fort Thomas, than upstream. Again this is difficult to balance against transmission losses from the main channel. Once more it is thought that these variations in the water budget downstream become negligible during major floods. Overall, the discharge data from the two gauging stations at either end of the valley demonstrate that there is a net loss of water in the river downstream over an average year. Of more significance may be the main channel transmission losses and transformation of the flood wave downstream (Burkham, 1976b), rather than any water inflow. Dykes, embankments and straightening Because of the problems of erosion and flooding in the valley and the concern for maximum usage of water and minimum loss of water, much channel management has been earned out. On the other hand the river is so mobile and the major floods so

18 268 Janet Hooke powerful that many structures and works are temporary or ineffective. It is extremely difficult to obtain data on dates of construction of structures or on channel maintenance carried out. Mapping from the aerial photographs and field mapping indicate that two major types of structure are present, in addition to the diversion dams, and one major practice affecting the channel. Embankments have been built at the edges of the floodplain, bordering the cultivated areas and canals in many places. These vary in height and many are only mud banks. In some locations, higher, reinforced embankments have been built and these could have some effect on the channel mobility but most are far from the channel. The other structures are dykes of varying design stone, or wire mesh or iron which have been constructed at the edges of the floodplain or in some active zones, projecting into the channel. The idea of these is to reduce velocity, decrease erosion and encourage sedimentation. A major practice in the 1990s, and obviously important in 1982 from the photographs, is the practice of bulldozing or scraping and straightening the channel and pushing material up into a ridge on either side. This is done mainly as a water conservation measure, to create a more efficient low flow channel and thus reduce losses and increase water availability. No permanent structures are put in and the material is simply left as unconsolidated piles at the side of the channel, so the works are wiped out in major floods. However, in minor floods which can still achieve some erosion, they may guide the flow into a particular channel and away from vulnerable banks. Examination of the 1982 spatial variation in fluvial morphology shows little relation to these works at that time. Indeed, much of the straightening and dyking is in the widest zones, obviously the most variable and subject to diversion. The dykes have caused some sedimentation but overall have not materially affected the river behaviour. The long term effects of the straightening are difficult to determine because of the lack of records. The number of agencies involved in the management of the area, and the scope for landowners to undertake works, mean that an accurate picture would be very difficult to obtain. If the effective discharges for the morphology of the channel are the very large floods, then it seems unlikely that these works have a significant effect. If, however, it is the moderate flows or the general moisture levels which are important then the structures and works may be significant. Some embankments and straightening are associated with bridges. The bridging parts are mostly on narrow parts of the channel, whether induced or naturally occurring, but there is no evidence of channel migration into the bridges or of major modification caused by them (Gregory & Brookes, 1983). Phreatophyte clearance The major project by the US Geological Survey in the 1960s was set up to assess the effects and feasibility of phreatophyte clearance of the valley bottom to increase water availability. An experimental reach was cleared upstream of Calva but this only extended as far as Bylas bridge. It was found that the effect upon the hydrological budget would be negligible. However, in 1957 a scheme for vegetation clearance of the floodplain as a necessary precondition for the construction of a reservoir in the canyon just upstream of Safford Valley was approved. Negotiation between the agencies involved meant that

19 Spatial variation in channel morphology and sediment dynamics 269 funds were not released until 1970 (Kelley, 1970). Work started in the May but was suspended in June after a court injunction was imposed on behalf of conservation and wildlife groups. It is not known how much was cleared but it is thought to have been very little. The scheme never went ahead though it has been subject to thorough reevaluation (US Army Corps Engineers Report, 1987). According to the same report, "in 1971 the San Carlos Indians, using US Geological Survey research funds, cleared 24 miles of chamiel with an average width of 2500 feet" but it would appear this was the series of US Geological Survey clearances in the Calva reach over the period (Culler et al., 1982). They planted Bermuda, love and blue panic grasses. The other practice which has occurred and which affects the width of the full phreatophyte zone, as measured from the photographs, is that clearance on the outer edges has taken place to extend the cultivated land. One example of this in 1992 was noted. The nature and position of the phreatophyte boundaries implies that this is not a significant component of the variation. In 1982 a dense but young cover of tamarisk was present in the lower-most part of the reach, between Calva and the reservoir. In the section between Byles and Calva, affected by the clearance, most of the phreatophyte growth is confined to the edges of low flow channels and the moistest parts of the floodplain. The general evidence is that tamarisk can colonize areas quickly and grows very rapidly. It could be that the series of floods between clearance and 1982 prevented such growth, but this seems unlikely. Groundwater levels Phreatophytes such as tamarisk, willow and cottonwoods, grow where there is an abundant and continual water supply. It is notable that on the slightly elevated areas, e.g. between Bylas and Calva, the tamarisk quickly disappears and mesquite becomes dominant. The width of the total zone may therefore be related to the availability of moisture. However, the relationship to the width of the active zone is less easily determined. It could be that, if phreatophytes less easily take hold in that reach, the river is able to keep a wider active zone. Throughout the lower reach where the active zone becomes wider it is fringed by phreatophytes, so it is difficult to explain the active width through water limitation and generally the groundwater is closer to the surface nearer to the channel (Culler et al, 1982). However, examination of the aerial photographs in detail indicates that the tamarisk in this downstream reach is mostly confined to old channels. Tamarisk has a deep tap root but groundwater must be within reach of this (~5 m). The absence of tamarisk may therefore indicate lower groundwater levels in this reach. This could possibly be accounted for by lack of replenishment from irrigation water. Although upstream there is pumping from groundwater, there is also possibly supply to the channel through the flood irrigation practised. Culler et al. (1982) suggest that water can move to or from the alluvial aquifer, to the channel. Further investigation is therefore needed to establish whether the groundwater levels are mainly influenced by the irrigation or by the replenishment from the river. It is also possible that there are changes in the stratigraphy of the valley floor, affecting groundwater levels.

20 270 Janet Hooke DISCUSSION The previous analysis has illustrated the complexity of combinations of factors and of their interaction. It is suggested that such detailed analysis of evidence provides much greater insight into processes than a statistical approach. However, the combined effect of all these factors has been analysed in a multiple regression analysis. The variables show remarkably little multi-colinearity. The combined model is significant and gives an r 2 value of 51% but this is still a low level of explanation. Analysis of effects of omitting an observation on the coefficients (DBETA plots) show very "well behaved" data. The partial residual plots show little pattern for any variable. Obviously some variation does remain unexplained and several additional arguments may be put forward. The remaining variation may be random and a function of the sampling down the valley. This is also related to the possible insensitivity of some variables, e.g. the structure presence or absence does not include the distance of the sampled cross section from the structure. In addition, particular patterns of morphological variation have been identified as characteristic of semiarid channels, most notably the beaded appearance of width and their autocorrelated functions (e.g. Thornes, 1980). Some tendency for this is present, but the downvalley trends and the effects of structure are superimposed on this. What is very notable is that the marked increase in the width of the active zone at CS 74 coincides with the boundary of the Indian Reservation. This implies that some land management practice may be having an effect since neither slope nor sediments would appear to provide an explanation of that spatial change. The variation of characteristics in the middle part of the valley may, however, be at least partially explained by sediment supply relative to competence, and the influence on channel pattern, which in turn affects the width of the active zone. CONCLUSIONS This analysis has shown that considerable variation occurs in the morphology of such a channel and floodplain, even within a section which is primarily affected by the same hydrological events. Certain overall patterns appear to be related to the valley gradient, particularly the degree of braiding and the width of the full fluvial zone. The characteristics of the sediment load appear to be closely related to the supply of coarse material from the tributaries but also the competence of the sections. In spite of several large floods prior to the date of this mapping, channel and land management practices, notably a programme of clearance of phreatophytes, would appear to have an important effect. Elsewhere the active zone, from the nature of its boundaries and processes operating within it as seen in the field and on air photographs, would appear mainly to reflect the lateral mobility of the channel rather than it being restricted by the growth of tamarisk. Considerable difficulties are associated with this type of causal analysis because of feedbacks, interactions and the non-equilibrium behaviour of these channels. The actual behaviour and morphology of the channel depends on the balance between the various components identified, the timescales of their effects and their spatial propagation. This requires complex spatial and temporal modelling. However, to

21 Spatial variation in channel morphology and sediment dynamics 271 construct this the operation and effects of individual components need to be understood. This paper has analysed in detail the field, documentary and photographic evidence to assess these effects, and shows how both an understanding of the processes and hydraulics, and a knowledge of the history of events and management, are necessary to the explanation of form. Acknowledgemeuts I should like to thank the Department of Geography, Arizona State University, for the provision of facilities during my study visit and particularly Professor Will Graf for all his help and the provision of data. I am grateful to the Department of Geography, University of Portsmouth, for study leave, to Rosemary Shearer and Bill Johnson for diagrams, to Carol Derrick and Margaret Fairhead for wordprocessing and to Robert Perry for computing assistance. REFERENCES Baker, V. R. & Rilter, D. F. (1975) Competence of rivers to transport coarse bedload material. Geol. Soc. Am. Bull. 86, Baldys, S. & Bayles, J. A. (1990) Flow characteristics of streams that drain the Fort Apache and San Carlos Indian Reservations, east-central Arizona, US Geological Survey Water Resources Investigations Report Burkham, D. E. (1972) Channel changes of the Gila River in Safford Valley, Arizona, US Geological Survey Professional Paper 655-G. Burkham, D. E. (1976a) Flow from small watersheds adjacent to the study reach of the Gila River Phreatophyte Project, Arizona. US Geological Survey Professional Paper Burkham, D. E. (1976b) Hydraulic effects of changes in bottom-land vegetation in three major floods, Gila River in southeastern Arizona. US Geological Survey Professional Paper 655-J. Cooke, R. U. & Reeves (1976) Arroyos and Environmental Change in the American South-West. Oxford University Press, Oxford, UK. Culler, R. C, Hanson, R. L., Myrick, R. ML, Turner, R. M. & Kipple, F. P. (1982) Evapotranspiration before and after clearing phreatophytes, Gila River flood plain, Graham County, Arizona. US Geological Survey Professional Paper 655-P. Gardiner, V. & Dackombe, R. (1983) Geomorphological Field Manual. Allen and Unwin, London, UK. Garrett, J. M., Roeske, R. H. & Bryce, B. N. (1986) Flood of October 1983 in southwestern Arizona-areas of inundation in selected reaches along the Gila River. US Geological Survey Water Resources Investigations Report A. Graf, W. H. (1971) Hydraulics of Sediment Transport. McGraw-Hill, New York, USA. Graf, W. L. (1981) Channel instability in a sand-bed river. Wat. Resour. Res. 17, Graf, W. L. (1982) Fluvial response to the spread of tamarisk in the Colorado River Basin. Geol. Soc. Am. Bull. 89, Graf, W. L. (1983a) Flood-related change in an arid region river. Earth Surf. Processes Landf. 8, Graf, W. L. (1983b) The arroyo problem - paleohydrology and paleohydraulics in the short term. In: Background to Paleohydrology (ed. by K. J. Gregory), John Wiley, New York, USA. Graf, W. L. (1986) Fluvial erosion and federal public policy in the Navajo nation. Phys. Geogr. 7, Graf, W. L. (1987) Holocene sediment storage in canyons of the Colorado Plateau. Geol. Soc. Am. Bull. 99, Graf, W. L. (ed.) (1988) The Salt and Gila Rivers in Central Arizona. Department of Geography Publ. no. 3, Arizona State University, Tempe, Arizona, USA. Gregory, K. J. & Brookes, A. (1983) Hydrogeomorphology downstream from bridges. Appl. Geogr. 34, Harvey, A. M. (1991) The influences of sediment supply on the channel morphology of upland streams: Howgill Fells, northwest England. Earth Surf. Processes Landf. i6, Hereford, R. (1984) Climate and ephemeral-stream processes: twentieth-century geomorphology and alluvial stratigraphy of the Little Colorado River, Arizona. Geol. Soc. Am. Bull. 95, Hjulstrom, F. (1935) Studies of the morphological activity of rivers as illustrated by the River Fyris. Bulletin of the Geological Institute of the University of Uppsala 25, Hooke, J. M. (1994) Hydrological analysis of flow variation of the Gila River in Safford Valley, south east Arizona. Phys. Geogr. 15, Hooke, J. M. (1996) River responses to decadal-scale changes in discharge regime: the Gila River, SE Arizona. In: Global Paleohydrology (ed. by J. Branson, A. Brown & K. J. Gregory), Geol. Soc. Special Publ. 115, London, UK.