Suspended sediment balance for the mainstem of Changjiang (Yangtze River) in the period

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1 HYDROLOGICAL PROCESSES Hydrol. Process. 25, (211) Published online 14 February 211 in Wiley Online Library (wileyonlinelibrary.com) DOI: 1.12/hyp.7996 Suspended sediment balance for the mainstem of Changjiang (Yangtze River) in the period Marwan A. Hassan, 1 * Michael Church, 1 Yunxia Yan, 2 Olav Slaymaker 1 and Jiongxin Xu 2 1 Department of Geography, The University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z2 2 Institute of Geographical Sciences and Natural Resources Research, Key Laboratory for Water Cycle and Related Land Surface Processes, Chinese Academy of Sciences, Beijing 111, China Abstract: Suspended sediment dynamics during the period are examined along the mainstem of Changjiang (Yangtze River). The period represents a basin condition prior to major changes in land management policy and dam building on the river s mainstem. The downstream sediment dynamics reflect basin geology and topography and channel morphology. Sediment exchange within the mainstem was calculated by the development of reach sediment balances that reveal complex temporal and spatial patterns. There is relatively little sediment exchange in the upper, bedrock-controlled reaches, with systematic increases in the downstream alluvial reaches. Degrading, transfer, and aggrading reaches were identified. Relations between input and output in all reaches were significant but no relation was found between sediment exchange and input/output. Comparison between short-term (22 years) and long-term (52 years) records demonstrates the importance of the record length in studying the suspended sediment dynamics in a large fluvial system. The longer record yielded better correlation and different trends than the shorter record. Sediment transfer (output/input ratio) changes downstream: the dominance of the upstream contributing area in sustaining the appearance of net degradation through most of the river system highlights the importance of reach length on characterisation of suspended sediment dynamics in large fluvial systems. Copyright 211 John Wiley & Sons, Ltd. KEY WORDS Suspended sediment; fluvial sediment balance; Yangtze River; sediment exchange; channel morphology; reach sediment balance Received 1 September 21; Accepted 7 January 211 INTRODUCTION Suspended sediment plays a major role in the hydrogeomorphological and ecological functioning of a river basin. The amount of suspended sediment transport is related to the rate of soil erosion in the landscape, and erosion from the channel banks and bed. Sediment from the landscape is transported through the channel network toward the mainstem and the river basin outlet. In lowlands, transported sediment settles and is temporarily stored in the channel bed, backwater channels and, during overbank flow events, on the floodplain. Poor land management practices may result in high suspended sediment concentrations as the result of excessive erosion. Excessive sediment may impact river ecology, create navigation hazards and flooding, and necessitate flood control structures and sediment dredging. Since the preponderance of all sediment that is evacuated from the landscape through large river systems moves in suspension, it is also, more fundamentally, an important index of landscape change. Owing to expense and logistical problems, most sediment balance studies have been conducted in relatively small basins. The study of sediment dynamics in a large basin poses many challenges, including quantification of * Correspondence to: Marwan A. Hassan, Department of Geography, The University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z2. mhassan@geog.ubc.ca sediment sources and major sediment reservoirs, estimation of sediment load from tributaries, and within-reach (channel and floodplain) sediment deposition and entrainment. Estimation of sediment storage within-reach is an essential step toward understanding channel processes and in the construction of a reach sediment balance (e.g. Walling et al., 1998; Owens et al., 1999; Smith et al., 23a, b). In spite of that, only a handful of studies have attempted to quantify reach sediment balances for medium and large basins (e.g. Phillips, 1991; Walling et al., 1998; Singer and Dunne, 21; Smith et al., 23b; Ali, 29). This could be due to lack of data or to the need to deal with processes on multiple spatial and temporal scales. Furthermore, the study of sediment dynamics for such basins is complicated because of complex geology, climate, land use, and landscape history. Because of sparse and poor quality data, De Boer and Ali (22) questioned the reliability of datasets for large basins over an extended period of time. Within the last two decades, a large body of literature has been published on Changjiang focussing on a range of topics including temporal trends and impacts of dams on sediment flux to the ocean (e.g. Lu and Higgitt, 1998; Higgitt and Lu, 21; Yang et al., 25, 27; Liu et al., 27; Wang et al., 27; Zhu et al., 27; Chen et al., 28; Dai et al., 28; Chu et al., 29; Xu and Milliman, 29), mostly based on the records of one or Copyright 211 John Wiley & Sons, Ltd.

2 234 M. A. HASSAN ET AL. Figure 1. Changjiang Basin showing the distribution of gauging stations with sediment records used in the analyses, longitudinal profile of the mainstem, study reaches, and cross-section through the basin geomorphological units. Reaches: 1. Zhimenda-Shigu; 2. Shigu-Pingshan; 3. Pingshan-Zhutuo; 4. Zhutuo-Cuntan; 5. Cuntan-Yichang; 6. Yichang-Luoshan; 7. Luoshan-Hankou; and 8. Hankou-Datong only a few monitoring stations. In developing a sediment budget for Changjiang, Wang et al. (27) suggested quantifying relations among sediment transport, storage, yield and sediment mining along the tributaries and the mainstem of the river. In the process of sediment budget construction they identified storage areas in gullies and tributaries, attempted to estimate the amount of material deposited along the middle and lower reaches, and to estimate sediment mining demand. Of the total estimated soil erosion from the upper Changjiang, Wang et al. (27) estimated that 76% is stored in gullies and tributaries and only 24% reaches Yichang station on the mainstem. They stress that the closure of the Three Gorges Dam and increasing demands for sediment will likely have a negative impact on the sediment budget of the reaches below Yichang. However, they did not consider spatial and temporal patterns of sediment storage within the main river channel. Nor did they develop a sediment budget for the whole basin, but focused on certain sub-basins and reaches. To explore potential effects of climate shifts, land use changes, and sediment mining on long-term trends of sediment fluxes, Dai et al. (28) examined temporal trends for a small number of sediment monitoring stations along the mainstem (Yichang and Datong) of Changjiang. They concluded that dam construction was the dominant factor contributing to the decline in sediment delivery to the ocean (Syvitski et al., 25). Recently, Hassan et al. (21) examined temporal patterns of within reach storage in four reaches of Changjiang between Pingshan and Datong utilising longterm (52 years) sediment monitoring data. They identified aggrading, degrading, and transfer reaches; then related the temporal trends to dam constructions, land management, and flood magnitude. However, because of data limitations, they did not include in their analysis most of the upper Changjiang mainstem. Furthermore, their reaches were long, making it difficult to assess the spatial scale of within-reach sediment dynamics. Changjiang offers an excellent opportunity to estimate the suspended sediment dynamics based on changes in sediment input and output because of the extensive sediment monitoring program. In this study, we characterize the suspended sediment dynamics along eight reaches of the Changjiang mainstem, extending from Zhimenda in the Tibetan Plateau to Datong at the head of the river s estuary (Figure 1) using a sediment balance approach. The study reaches cover a wide range of channel morphology, flow and sediment transport regimes, climate, and land use, providing an opportunity to examine both spatial and temporal patterns of the suspended sediment dynamics in a large river system. Two out of the eight reaches in this study are the same as those analysed by Hassan et al. (21) and will be used to compare shorterterm (this study) with longer-term sediment budgeting. In spatial terms, the present study extends over almost twice the length of river that Hassan et al. (21) studied. Furthermore, the shorter reaches allow us to develop a more detailed spatial sediment budget which is likely to improve our understanding of the reach dynamics. Our analysis encompasses the period , when the largest number of monitoring stations was active in the basin. Whilst land use change is an ongoing phenomenon in the basin, the most substantial soil conservation programs and water utilisation projects were implemented in Copyright 211 John Wiley & Sons, Ltd. Hydrol. Process. 25, (211)

3 FINE SEDIMENT BALANCE 2341 the early 198s and therefore, our study provides information on sediment dynamics prior to more recent major changes in land management in the basin and dam construction on the mainstem. Changjiang Basin and study reaches Changjiang, which takes a number of names (e.g. Tongtian, Jinsha Jiang) along its length, is the largest river in China, with a drainage area of 1Ð94 ð 1 6 km 2 and a length of 64 km. The river flows from its source in the Dangla Mountain Range in Qinghai Province eastwards to the East China Sea (Figure 1). Traditionally, the basin is divided into three parts: the upper, generally mountainous and hilly part extends all the way to Yichang (draining about 59% of the contributing area); the middle part refers to the section from Yichang to Hankou (draining about 87% of the contributing area), where Lake Poyang drainage meets the river; the downstream part is from Hankou to Shanghai (Chen and Gupta, 21; Chen et al., 21; Chen et al., 28). The estuarine reach of the river extends from the tidal limit at Datong to the river mouth (Figure 1). Changjiang flows through four physiographic regions: Qinghai-Tibet Plateau, Sichuan Basin, Changjiang Gorges, and Jianhan Basin (Li et al., 21). The upper Changjiang drains part of the Plateau of Tibet, which is still being uplifted as the Indian and Eurasian plates collide. The bedrock comprises igneous, marine sedimentary, and metamorphic rocks with thick deposits of overlying sediments. From the Plateau of Tibet, Changjiang flows through gorges deeply incised into mountainous plateaus consisting of Palaeozoic and Mesozoic rocks. In its lower reaches, the river flows across basin fills of Cenozoic fluvial sediments deposited as the river has migrated across its lower basin. The mean temperature and precipitation are 4Ð2 C and 27 mm near the basin s headwater, 2 C and 869 mm in Sichuan Basin, and 17 C and 146 mm in the middle and the lower parts of the basin, respectively (Wang et al., 27). The annual discharge increases downstream from Ð46 ð 1 4 m 3 /s at Pingshan to 1Ð4 ð 1 4 m 3 /s at Yichang, 2Ð3 ð 1 4 m 3 /s at Hankou, and 2Ð8 ð 1 4 m 3 /s at Datong (Figure 1) (Chen et al., 21; Wang et al., 27). During the study period, there were a number of high flow events along the mainstem of Changjiang. Table I presents a summary of annual maximum peak discharge at three stations along the river mainstem. The 1998 event, the most recent flood that caused extensive Table I. Summary hydrological characteristics of three stations along the mainstem Pingshan Yichang Datong Annual Discharge (m 3 /s) Mean annual flood (m 3 /s) years event years event years event damage to property and loss of human life, has an estimated return period of 18 years at both Yichang and Datong (Hassan et al., 21). Over the last five decades large changes in land use and soil conservation measures aimed at reducing soil erosion have brought under management large areas of the basin, resulting in a major decline in sediment delivery to the East China Sea (Lu and Higgitt, 1998, 1999; Walling, 1999, 26; Chen et al., 21; Walling and Fang, 23; Syvitski et al., 25; Wang et al., 27; Chu et al., 29). Our period of study precedes the major changes that have occurred since 198. To gain further insight into the possible effect of changes in land management and water control we have conducted trend analysis on the flow and suspended sediment records of three of the principal mainstem stations, Pingshan, Yichang, and Hankou. Pingshan gauges drainage from the uppermost, least developed part of the basin; Yichang is at the exit from the Three Gorges and the downstream limit of upland drainage, while Hankou is midway between Yichang and the estuary. The analysis extends from the beginning of the records in the early 195s to 2. We purposely deleted the post-2 period because the effect of the closure of the Three Gorges Dam severely skews the analysis. Data and cumulative departures are displayed in Figure 2. The analysis shows that, within the study period , flows and sediment yield at Pingshan were consistently below the long-term mean. The individual rises in 1966 and 1974 indicate the largest two floods in the period of record, but do not break the overall trend. At Yichang, similar conditions were observed until 198 (the 1974 flood here being the flood of record for the period), when a shift occurred to aboveaverage runoff and sediment yield. At Hankou, flows were generally below average until 198, but somewhat more variable. However, sediment yield at this station was average or above average throughout the period. The end of the study period (1985) marks the major turning point in the sediment record, as it does at Yichang. At all three stations, short excursions in the flow and sediment records occur in phase. We conclude that the records are sufficiently trend-free to represent a regime situation for the period of study. Fourteen sediment monitoring stations are located along the mainstem of the river between Zhimenda and Datong (Figure 1). On the basis of basin physiography, contributing area, data availability, and location of sediment monitoring stations along the main channel, the mainstem of the river was divided into 8 reaches (Figure 1) for the development of reach suspended sediment balances for the period The contributing area to each reach ranged between 138 ð 1 3 km 2 and 289 ð 1 3 km 2 with an average of 213 ð 1 3 km 2. The uppermost two reaches, Zhimenda-Shigu and Shigu- Pingshan, are located within the Eastern mountains where the river channel is confined and deeply incised into bedrock, forming deep canyons (Chen and Gupta, 21). The confined river flows in rapids through high canyons with large surface gradients (Figure 1). In these reaches, Copyright 211 John Wiley & Sons, Ltd. Hydrol. Process. 25, (211)

4 2342 M. A. HASSAN ET AL. (a) Pingshan 1.8 Sediment Water (d) Pingshan 1 8 Sediment Water Scaled cumulative departure (b) Yichang (c) Hankou 1 Sediment load (Mt) (e) Yichang (f) Hankou Water yield (1 9 m 3 ) Figure 2. Data of annual mean flow and annual suspended sediment load at (a) Pingshan; (b) Yichang; and (c) Hankou. Scaled cumulative departures (for details, see Outcalt et al., 1997) for trend detection are shown in panels (a) (c) for the same stations. In the departures plot, rectilinear segments indicate periods of record that are, at the least, first-order stationary. A descending plot indicates flows below the overall mean of the record; an ascending plot indicates flows above the overall mean channel width, depth and slope range between 5 and 15 m, 5 2 m, and Ð1 Ð4 ð 1 3, respectively (Chen et al., 21). Channel gradient, river morphology, and flow regime change significantly downstream from Pingshan station (Figure 1; Chen et al., 21; Li et al., 21). The Pingshan-Zhutuo and Zhutuo-Cuntan reaches are within Sichuan basin where the channel gradient declines significantly. The Cuntan-Yichang reach is confined and deeply incised into bedrock within the Three Gorges region. Downstream from Pingshan, the river meanders within a relatively narrow floodplain, with point and mid channel bars dominating the channel morphology. The Gezhouba and Three Gorges dams are situated upstream from Yichang station, with a large reservoir extending upstream from each. The middle part of the river was divided into two reaches, Yichang-Luoshan and Luoshan-Hankou. Between Yichang and Hankou stations, the meandering reach is characterized by point and mid channel bars, islands, cutoff meanders, and extensive floodplain. Channel width and depth for the reach ranges between 1 and 2 km and 6 and 15 m, respectively. Finally, the reach between Hankou and Datong meanders with a wide floodplain (Chen et al., 21). The river has a relatively wide (2 4 km) deep channel (1 2 m) with an almost flat slope (Ð5 1Ð ð 1 5 ) before entering the estuary (Figure 1). Large lakes such as Dongting and Poyang reduce the amount of sediment delivered to the river s mainstem. Most of the middle and lower reaches of Changjiang are confined by dykes, bridges, and walls. DATA AND METHODS Data Annual sediment load, annual flow and precipitation data were obtained from the Changjiang Commission. The Commission regularly collects sediment samples, determines sediment concentrations, and calculates annual load (for more details see Hassan et al., 21). The number of monitoring stations in the basin Copyright 211 John Wiley & Sons, Ltd. Hydrol. Process. 25, (211)

5 FINE SEDIMENT BALANCE 2343 Table II. Summary contributing area for the eight study reaches Reach Contributing area (Km 2 ) Upstream gauged area (%) Main tributaries gauged area (%) Small tributaries gauged area (%) Un-gauged area (%) Zhimenda-Shigu Ð64 Ð Ð 17Ð36 Shigu-Pingshan Ð94 Ð 3Ð74 3Ð31 Pingshan-Zhutuo Ð82 22Ð84 5Ð43 1Ð91 Zhutuo-Cuntan Ð17 18Ð2 Ð64 1Ð17 Cuntan-Yichang Ð95 Ð Ð41 2Ð63 Yichang-Luoshan Ð71 2Ð26 ¾Ð ¾Ð Luoshan-Hankou Ð2 9Ð55 Ð94 2Ð49 Hankou-Datong Ð26 9Ð51 1Ð5 2Ð18 ranged between 1 in the 195s and 334 in the 198s. During the period , except for 3 years in late 196s and , the number of monitoring stations exceeded 3. The spatial distribution and number of monitoring stations are critical for the development of regional sediment yield relations and the estimation of sediment delivery from ungauged basins. Therefore, we focused our analysis on this period because it has the highest and the most widely distributed number of monitoring stations in the basin. Reach sediment balance In the construction of reach sediment balances we followed the formulation for Changjiang adopted by Hassan et al. (21). Here, we will provide a brief review. The reach sediment balance consists of three components: input, output, and storage. In terms of input, sediment is delivered to a reach from upstream, tributaries, and bank erosion. For the Changjiang reaches, input from upstream is the flux of sediment passing the monitoring station located at the upstream limit of the reach. Similarly, input from tributaries with monitoring stations is determined by the sediment flux at the river mouth. In order to estimate sediment input from non-gauged tributaries, we developed regional sediment yield relations for the basin following the methodology described in Church et al. (1999), and adopted by Hassan et al. (28, 21). This method involves two steps: (1) kriging, in ArcGIS to establish the regional pattern of sediment yield; and (2) mapping, to interpolate sediment yield for basins with no monitoring stations. To avoid possible biasing effects of land use changes, sediment control measures and climate shifts, we developed yearly regional relations rather than an average for the whole period. Table II provides a summary of the contributing areas for the study reaches. For all reaches except the uppermost one, the ungauged area ranged between near-zero and 3Ð3%. The uppermost reach the least monitored area (Figure 1) has an ungauged contributing area of 17Ð4%. Input terms Bank erosion may be an important part of reach sediment input, especially in the lower parts of Changjiang. However, such information is not regularly measured and therefore difficult to estimate. For confined reaches and in the absence of floodplains, bank erosion is limited and probably introduces relatively small errors into the sediment balance. In the middle and lower reaches of Changjiang, bank erosion, cutoff meanders and channel shifts have been documented (e.g. Chen et al., 21; Wang et al., 27). Since the 197s, however, inputs from bank erosion have been reduced significantly by dyke, wall, and bridge building. In the absence of routine monitoring, the net effects of bank erosion and deposition, along with channel bed changes, are effectively factored into our estimate of net change in sediment storage along the channel. That is, sediment storage in the channel banks is considered to be part of channel storage. The uppermost two reaches are incised into bedrock and, therefore, fine sediment inputs from bank erosion and floodplain are insignificant for the time period under consideration. However, these reaches are likely to experience inputs from mass movement. Detailed information on sediment delivery from mass movement to the mainstem of the river is not available. Field evidence indicates that most of the mass movement activity occurs in the Shigu-Pingshan reach (Cai, 1998; Wang et al., 27; Xu, 29). Owing to steep terrains, a large number of gullies and landslides that deliver sediment to the mainstem are concentrated around the river reach between Panzhihua (near the confluence of the Yalongjiang (Figure 1) and Pingshan station (Wang et al., 27). For this river reach, Wang et al. (27) reported that the sediment yield is more than 4 times higher than the basin s average. Furthermore, they stated that a large amount of sediment is stored in downstream reaches of the tributaries and gullies adjacent to the mainstem. Along the Panzhihua-Pingshan reach, Cai (1998) and Xu (29) documented 258 gullies (one gully for every 3 km channel length) that deliver sediment to the mainstem. In addition, 687 slides and slumps were observed near the mainstem with an estimated volume of sediment of 3Ð1 billion m 3 (Cai, 1998). Therefore, mass wasting activity contributes large amounts of sediment to the Shigu-Pingshan reach, part of which may be recorded as material originating in the reach and evacuated past Pingshan. However, that portion of the material that remains deposited within the reach is not recorded in our sediment balance and represents a negative bias in our estimate of sediment exchange within the reach. Copyright 211 John Wiley & Sons, Ltd. Hydrol. Process. 25, (211)

6 2344 M. A. HASSAN ET AL. Output terms Outputs from a study reach include sediment exported downstream, floodplain deposition associated with flooding, sediment exported in water diversions, and sediment mining. Output along the main channel is the sediment flux passing through the monitoring station at the downstream end of the reach. Annual sediment diversion output was determined from data monitored by the Changjiang Commission. However, sediment output into floodplains is not measured and is difficult to estimate. Output to floodplains is limited because of dykes and retaining walls and deposition within the dykes can be considered to be, like bank storage, effectively a part of the sediment exchange within the channel. However, more distant floodplain deposition, where it occurs, remains a source of error in our sediment balance. Error term Sediment mining is a major human impact on the middle and lower Changjiang reaches. A wide range of values has been reported in the literature (e.g. Chen et al., 25, 28; Dai et al., 28; Chu et al., 29) but the total is difficult to substantiate because of illegal mining and lack of records. Dai et al. (28) asserted that, since most of the mined sediment is sand, this output will have little impact on the wash load (silt and clay) sediment balance. Furthermore, mining activity has intensified in recent years, so that our period of interest precedes the most significant impact of this activity. Sediment mining, however, remains the main source of error in our reach analysis. The sediment budget Given the limitations reported above, the Changjiang reach sediment budget has been reduced to Storage change D Input upstream C tributary output mainstem C diversion C error Storage change in this model is in fact the change in sediment stored in the channel and on the banks inside the dykes because input from and output to the floodplain are limited and sediment mining volumes remain unknown. The error term depends largely on sediment mining and floodplain deposition, and on resident mass wasting deposits in the Shigu-Pingshan reach, and represents a bias term. Suspended sediment texture To further assess the character of our suspended sediment reach balance, we examined the texture of the suspended load along the mainstem from Shigu to Datong (Figure 1). At Shigu the farthest upstream station with available data the median of the median sizes in the period is Ð44 mm, with a maximum observed value of Ð78 mm (Figure 3). For the river reaches between Shigu and Pingshan, a slight Median grain size (mm) Shigu Longjie Dukou Pingshan Yichang Hankou Median 25 75% 1 99% Datong Figure 3. Variation in median grain size of suspended sediment load along the mainstem of the river during the period downstream decline in median and maximum values is evident. Further downstream, a measured drop in the range and median particle size is evident. This likely reflects the major decrease in channel gradient below Pingshan. Hassan et al. (21) examined the particle size distribution of the suspended sediment for the four lowest stations for the period They reported values lower than the data reported in Figure 3. For example, at Pingshan station, the median was Ð44 mm between 1964 and 1985, and declined to Ð21 mm in the period This decline is probably due to changes in landuse and the construction of dams (Xu, 27). To compare the bed material below Yichang with its suspended material, we examined the data of Wang et al. (29). They found that the bed material in the reaches between Yichang and Datong (Figure 1) consists mainly of sand in the Ð1 Ð4 mm range. In comparison, about 25% of the suspended load at Yichang is fine sand (95% <Ð18 mm). In reaches upstream of Yichang, however, we expect a coarser bed which might include coarse sand, gravels, and larger material. Data on bed material for reaches upstream of Yichang are not available. However, bedload measurements conducted upstream of Cuntan station show that material larger than 5 mm moves during floods (Zhou and Zhian, 1994), indicating a relatively coarse bed which includes gravels. Since we are considering suspended sediment, mainly in the silt clay range, we assume that apparent losses from the suspended load within a reach (that is, positive contributions to storage change) to be sediment that has settled from the water column. However, it seems quite probable that some portion of the settled sediment has joined the bedload and moved out of the reach. Accordingly, our analysis directly quantifies only the suspended sediment dynamics in the river. However, the bed in the middle and lower river consists mainly of sand, so that there appears to be a significant disjunction between suspended silt/clay dynamics possibly including sand Copyright 211 John Wiley & Sons, Ltd. Hydrol. Process. 25, (211)

7 FINE SEDIMENT BALANCE 2345 in the Ð1 mm fraction and the dynamics of coarser sand. This circumstance suggests that little of the finer material does join the bedload it is, rather, deposited in backwaters and opens the possibility to obtain some insight into the suspended sediment balance of the reach. We will, then, assign within reach losses from the suspended sediment load as contributions to suspended sediment storage, and vice versa. Other sources of error More basic sources of error in the Changjiang reach balance including flow and sediment concentration measurements and load estimates. The Changjiang Commission collects sediment samples at hydrological gauging stations located on the mainstem and main tributaries. The concentration and load are estimated from daily grab samples (bottle) at a single section, increasing to 5 6 per day during floods. To calculate the channel average, suspended sediment concentrations are occasionally determined along 25 or more verticals, usually 3 6 points for each vertical. The average for each vertical is used to calculate the cross-section suspended sediment concentration. Then, the established relation between crosssection averaged suspended sediment concentration and the single vertical is used to convert the daily single vertical to a daily cross-section averaged value (for more details, see Hassan et al., 21). Given that the Commission does not publish error estimates, it is difficult to estimate them. By comparison with a program with similar sampling intensity conducted on Fraser River, British Columbia (McLean et al., 1999), we estimate the error for the annual load to be in the order of š1%. This figure was adopted in this study as a figure of merit for error in the annual suspended sediment load estimates from all river stations. Regional estimation is another source of error, especially for the upper reach with an estimated non-monitored area of 17Ð4%. A fixed value of 3Ð11 Mt (from the regional analysis) was assigned for this term. This source of error is reduced significantly for the rest of the reaches. Storage change is the residual term in the sediment balance, hence the error assigned to it is the pooled error of all the other terms. Calculations were carried out in absolute figures. For the 6 lower reaches, the results yield outside error estimates ranging between 13 and 19% of reach sediment input, adopted as a figure of merit for comparison. For the Zhimenda-Shigu and Shigu-Pingshan reaches, the calculated error was 39 and 124%, respectively, which is likely due to very small values of sediment input into the reach. Estimated errors for individual years, on the same comparative basis, ranged between 13 and 24%, except for the upper two reaches (up to 46% for Shigu-Pingshan reach). Two examples To illustrate the manner in which we calculated a reach sediment balance we present examples from Pingshan- Zhutuo and Yichang-Loushan reaches in the upper and middle Changjiang for 1964 (Figure 4). For the Pingshan- Zhutuo reach there are five inputs, upstream, two main (a) Pingshan-Zhutuo (1964) InSmall 22. ± 2.2 (6.4%) InUpstream 219. ± 28.9 (63.5%) Minjiang 78.8 ± 7.88 (22.9%) (b) Yichang-Loushan (1964) InSmall 8.8 ±.88 (1.3%) InUpstream ± 62.1 (97.9%) InRegional 74.3 ± 3.11 (2.2%) Storage 33.5 ± 39.8 (9.72%) Diversion ± 24.4 (35.%) Tuojiang 17.3 ± 1.73 (5.%) InRegional 4.3 ± 3.11 (.6%) Storage 11.4 ± 8.3 (1.64%) Chenglingjie basin 62.9 ± 6.29 (9.%) OutDownstream 311. ± 31.1 (9.3%) OutDownstream ± 44.2 (63.3%) Figure 4. Two examples of components of reach sediment budgets: a) Pingshan-Zhutuo, and b) Yichang-Loushan reaches for Values in parenthesis are the values in percentage. For explanation of errors see text tributaries (each with drainage area >2 ð 1 3 km 2 ), small tributaries (<2 ð 1 3 km 2 ), and non-monitored tributaries estimated using regional relations, and one output through the main channel (Figure 4). The 2 ð 1 3 km 2 discrimination between large tributaries and small tributaries has no scientific significance but was motivated by the graphic limitation of the balance presentations. For the reach, most of the sediment is contributed from upstream and large tributaries (Figure 4). The nonmonitored landscape contributed only an estimated 2Ð2%. Overall, the sediment balance was positive, resulted in 9Ð7% of the input being stored in the reach. However, this result is comparable with the pooled error estimate, so the change in storage might be minimal. Mainstem input and output contribute, by themselves, an outside error margin of š38ð Mt to the total error estimate of š39ð1 Mt, indicating their total dominance of the error structure. Inputs to the Yichang- Loushan reach come from upstream, non-monitored tributaries and small monitored tributaries. For this reach there are two outputs, through the mainstem and diversions (Figure 4). In terms of balance, most of the sediment was contributed from upstream and relatively small amounts from small tributaries that join the river in this reach. For the year, a small positive sediment balance of 1Ð6% of the input was recorded (Figure 4). Again, the mainstem stations contribute 95% of the outside total error assigned to the storage term. RESULTS Sediment balance Temporal patterns of input, output, and storage for the study reaches are presented in Figures 4 and 5. In the Copyright 211 John Wiley & Sons, Ltd. Hydrol. Process. 25, (211)

8 2346 M. A. HASSAN ET AL. Input, output (Mt) (a) Zhimenda-Shigu Output 4 Input (b) Shigu-Pingshan (c) Pingshan-Zhutuo (d) Zhutuo-Cuntan (e) Cuntan-Yichang (f) Yichang-Luoshan (g) Luoshan-Hankou 2 (h) Hankou-Datong Year Erosion Deposition Figure 5. Temporal patterns of sediment input and output for the period for the eight study reaches upper two reaches, Zhimenda-Shigu and Shigu-Pingshan, the output curve follows closely that of the input but is higher, indicating net erosion over the study period. In the Zhimenda-Shigu reach the difference between input and output is relatively small, indicating relatively modest loss of sediment (Figures 5a and 6a). Positive sediment storage was recorded only in1971; a relatively large amount of sediment accumulated in the reach in 1971 and then about the same amount was evacuated a year later (Figure 5a)). In the Shigu-Pingshan reach, the sediment output was consistently well above the input resulting in the appearance of significant net erosion throughout the period (Figures 5b and 6b). Significant changes in sediment Storage change (Mt) (a) Zhimenda-Shigu 5 (b) Shigu-Pingshan (c) Pingshan-Zhutuo (d) Zhutuo-Cuntan (e) Cuntan-Yichang (f) Yichang-Luoshan (g) Luoshan-Hankou (h) Hankou-Datong Year Figure 6. Temporal patterns of reach sediment exchange for the period for the eight study reaches including the estimated error range for the sediment exchange storage were recorded in the years 1965, 1966, 1968, and 1974 (Figure 6b)). Episodic large inputs from mass movement initiated on the steep slopes adjacent to the mainstem (Cai, 1998; Wang et al., 27; Xu, 29), rather than net channel erosion, probably explain the persistently larger outputs from the reach. In this view, erosional processes are very active in the Shigu-Pingshan reach. Flood data are not available for the stations upstream of Pingshan, but the Pingshan record casts some light Copyright 211 John Wiley & Sons, Ltd. Hydrol. Process. 25, (211)

9 FINE SEDIMENT BALANCE 2347 on the sediment history. On the basis of flood analysis for the period at Pingshan station (Hassan et al., 21), the 1966 and 1974 floods were the largest and second largest on record, respectively, while the 1968 flood has a return period >3 years and was the sixth largest event on record. These relatively large events may be associated with the observed major sediment evacuation from the reach observed in these years (Figure 6b)). In comparison, these same years were the years of major net sediment deposition in the next downstream reach. Downstream of Pingshan station, the river channel gradient declines dramatically (Figure 1) and major changes in the channel morphology are evident. The Pingshan-Zhutuo reach shows a clear depositional trend (Figure 5c)), with modest storage of sediment in most years. Significant sediment storage occurred in 1966, 1968, and 1974 (Figure 6c)), in mirror image of the major sediment evacuation years in the immediately upstream reach. For the two remaining reaches of the upper Changjiang (Zhutuo-Cuntan and Cuntan-Yichang), the input and output were about the same, resulting in modest sediment exchange (Figures 5d, 5e, and 6d and e)), with slightly more negative values in the downstream reach. Furthermore, the two reaches have about the same range of sediment input and output for the period, implying continuous sediment throughput in both reaches. However, there are some differences in terms of sediment storage between the reaches (Figure 6d) and e)). The temporal trends of input and output reveal major sediment throughput in 1966, 1968, and 1974 the same years in which major sediment exchanges were occurring in the upstream reaches. However, 1981 and 1984 also become relatively prominent in the records of these reaches. The middle Changjiang is a meandering river extending from Yichang to Hankou with three main tributaries joining the river in this reach (Figure 1). The river between Yichang and Hankou was divided into two reaches: Yichang-Luoshan, and Luoshan- Hankou (Figure 1). They show similar temporal trends of input and output (Figure 5f) and g)) but they differ in sediment exchange (Figure 6f) and g)). The Yichang- Luoshan reach shows a negative sediment balance in most years, although the error margin includes the zero datum, whereas consistently and significantly positive sediment storage occurred in the Luoshan-Hankou reach. The lower Changjiang is represented by the Hankou- Datong reach which is located upstream of the estuary (Figure 1). The sediment output curve closely follows that of the input showing minor changes in sediment storage within the reach (Figures 4h) and 5h)). In most years, a negative balance in the sediment storage is evident (Figure 6h)). Overall, a net sediment evacuation from the reach was observed, but the figures consistently fall within the margin of error. Cummulative input (percentage) (a) Zhimenda-Shigu (c) Pingshan-Zhutuo (e) Cuntan-Yichang (b) Shigu-Pingshan Regional Large tributaries Small tributaries Upstream (d) Zhutuo-Cuntan (f) Yichang-Luoshan 8 (g) Luoshan-Hankou (h) Hankou-Datong Year Figure 7. Relative suspended sediment contribution from upstream, main tributaries, small tributaries, and non-monitored basins for the eight study reaches. Note the truncated scale in the lower panels. All scales are equivalent Sediment delivery from the landscape To further explore spatial patterns of sediment delivery to the examined reaches, we plot the relative contribution of each source during the study period (Figure 7). Sediment inputs to the reaches were grouped into four categories: (1) from upstream, (2) from small tributaries (<2 ð 1 3 km 2 ), (3) from large tributaries (>2 ð 1 3 km 2 ), and (4) from non-monitored tributaries. For the Zhimenda-Shigu reach, most of the incoming sediment arrives from upstream (¾6%) or from nonmonitored basins (¾4%) (Figure 7a)). The prominence of the latter source is an artifact of the lack of gauging Copyright 211 John Wiley & Sons, Ltd. Hydrol. Process. 25, (211)

10 2348 M. A. HASSAN ET AL. Fine sediment transfer ratio Pingshan Yichang Shigu Luoshan Zhutuo Cuntan Hankou Datong Pingshan Zhutuo Yichang Luoshan Shigu Cuntan Hankou Datong 2, 4, 6, 8, 1,, 1,2, 1,4, 1,6, 1,8, Drainage basin area (km 2 ) Figure 8. a) Downstream changes in sediment transfer ratio (reach output/reach input) for each study reach. b) Sediment transfer ratio calculated as an aggregate ratio starting from the reach most upstream. In each step, a downstream reach was added to create a cumulative ratio of the sediment transfer as reach output/drainage basin aggregated input. The 1%, 25%, median, 75% and 9% quantile bars are shown in this remote region. A different outcome was obtained for the Shigu-Pingshan reach; a relatively small proportion of sediment was delivered from upstream (¾25%). In contrast, large proportions of sediment were delivered from either small or large tributaries (¾3 and 25%), while about 2% derived from non-monitored basins (Figure 7b)). These two reaches drain high and mountainous terrain, respectively, and the possibility must be considered that a significant amount of sediment is delivered to the rivers by landslides and debris flow. As discussed above (Wang et al., 27), the Shigu-Pingshan reach indeed experiences large inputs from debris flows and landslides, consistent with the substantial sediment contributions from tributaries of all classes in this reach. In contrast, the small magnitude of tributary contribution in the proximal Zhimenda-Shigu reach argues against the significance of direct mass wasting to channels in the high plateau, but it must be recognized that we have least gauging information from this region. Episodic inputs are, further, a plausible reason for the large fluctuations in sediment exchange in the Shigu-Pingshan reach (Figure 6b)). The relative amount of sediment delivered from small tributaries and non-monitored basins declines significantly downstream. For Pingshan-Zhutuo and Zhutuo- Cuntan most of the sediment derives either from upstream or main tributaries (Figure 7c and d). For the four remaining distal reaches, more than 8% of the sediment was delivered from upstream sources (Figure 7e to h), implying high sediment throughput from the high and mountainous proximal reaches. Sediment transfer along the mainstem To better understand downstream trends in sediment transfer through the river system and the sediment balance, we calculated a sediment transfer ratio, reach output/reach input, for each reach. For this analysis, we consider all 14 stations located on the river s mainstem. The results are plotted in Figure 8a) as medians and percentiles over all years. Based on the median values and the 25 75% range we classify the reaches as follows: (1) degrading (i.e. output exceeds input: ratio >1) reaches upstream of Pingshan, (2) transfer (output equals input: ratio ¾1) Pingshan- Luoshan reaches, and (3) aggrading (input exceeds output: ratio <1) downstream of Luoshan (Figure 8a)). Generally, variability in the sediment transfer ratio declines downstream. This outcome is consistent with our earlier observation that sediment throughput along the mainstem becomes an increasingly dominant component of the sediment source as one proceeds downstream. To explore the influence of the reach length on the reach sediment balance and transfer ratio, we calculated an aggregate ratio starting from the reach that was most upstream. In each step, a downstream reach was added to create a cumulative ratio of the sediment transfer as reach output/drainage basin aggregated input. At the lowest reach, for example, the ratio is a summation for the whole river from Zhimenda to Datong, compared with sediment output at Datong. Again, we used all 14 stations located along the mainstem. The analysis yields a different downstream trend in the sediment transfer (Figure 8b)) than that obtained for the single reaches (presented in Figure 8a)). For most of the reaches, except the lowest Copyright 211 John Wiley & Sons, Ltd. Hydrol. Process. 25, (211)

11 FINE SEDIMENT BALANCE 2349 two, the transfer ratio was >1, implying net degradation for most of the river length. However, as in the single reach analysis, the lowest two reaches indicate net aggradation (Figure 8b)). In fact, all reaches indicating sediment balance in the single reach analysis (Figure 8a)) became part of a net degradational drainage basin in the cumulative analysis, emphasising the dominance of sediment sources in the reaches above Pingshan (where the aggregate ratio reaches a maximum) in contributing suspended sediment to the river. Suspended sediment relations: input, output and reach sediment storage Relations between annual sediment input and output for the eight reaches were explored in order to better understand the suspended sediment dynamics along the mainstem of the river. All relations are presented in Figure 9 and the summary statistics in Table III. All functional relations are statistically significant (at p<ð1 for statistical significance Table III). In terms of degradation/aggradation, the eight reaches can be classified into four categories which are generally consistent with earlier findings. In the first group the relation falls entirely above the 1 : 1 line within the range of the data, indicating degradation over the study period. The slope of these relations is also >1, implying more active sediment evacuation in larger events. This group comprises the two uppermost reaches (Zhimenda-Shigu and Shigu- Pingshan). The second group, in which the relation falls entirely below the 1 : 1 line within the range of data, indicating persistent aggradation, include two reaches Pingshan-Zhutuo and Luoshan-Hankou (Figure 9). The slope of these relations is <1, implying greater sediment gain in larger events. A third group, comprising the three reaches between Zhutuo and Luoshan exhibit data scattered about the 1 : 1 line, implying approximate sediment balance. Most of the actual data fall above the line in the Cuntan- Yichang reach, implying mild degradation, consistent with earlier analyses. In each of these cases, the slope of the relation is slightly less than 1, suggesting that in large events, processes shift from net sediment evacuation toward sediment accumulation. The distal reach, Hankou- Datong, is singular in that, whilst the data scatter about Table III. Summary statistics of functional relations between sediment output and input for period Reach R 2 N F a ša b š b Zhimenda-Shigu Ð Ð47 32Ð86 22Ð6 1Ð17 Ð12 Shigu-Pingshan Ð Ð43 121Ð21 36Ð51 3Ð2 Ð3 Pingshan-Zhutuo Ð Ð73 74Ð61 13Ð39 Ð72 Ð4 Zhutuo-Cuntan Ð Ð35 35Ð55 24Ð7 Ð92 Ð5 Cuntan-Yichang Ð Ð89 67Ð83 26Ð76 Ð9 Ð5 Yichang-Luoshan Ð Ð2 134Ð43 33Ð29 Ð82 Ð5 Luoshan-Hankou Ð Ð91 78Ð9 37Ð88 Ð75 Ð7 Hankou-Datong Ð Ð33 115Ð24 57Ð7 1Ð49 Ð13 N is number of observations, F is F-test, a is regression intercept, and b is regression slope. All results at significant at p<.1. the line of balance, there is a sharp gradient in the relation that they define such that sediment accumulation occurs during modest events and evacuation of sediment occurs during large ones. In addition to input-output relations we explored relations between input, output and storage. The amount of stored sediment within a reach is likely to depend, among other things, on the magnitude of the sediment input. On the other hand, the output will likely depend, among other things, on the amount of sediment stored with the Output (Mt) (b) Shigu-Pingshan 3 2 Best fit 1: (a) Zhimenda-Shigu (c) Pingshan- Zhutuo (d) Zhutuo-Cuntan (e) Cuntan-Yichang (f) Yichang-Luoshan (g) Luoshan-Hankou (h) Hankou-Datong Input (Mt) Figure 9. Relations between annual sediment output and input for the eight reaches. Table III provides statistical details Copyright 211 John Wiley & Sons, Ltd. Hydrol. Process. 25, (211)

12 235 M. A. HASSAN ET AL. reach. Therefore, we performed relations between input and storage and storage and output (26 trials in total). The analysis revealed either a weak or no relation for most of the cases. 6 4 (a) Pingshan- Zhutuo 1:1 Length of record and trends in sediment balance The record selected for analysis spans 22 years, which is rather short for examining temporal trends. Hassan et al. (21) analysed 4 reaches, which are aggregates of the lower 6 reaches that have been considered in this study for the period (Table IV). Therefore, we have an opportunity to compare our short-term view (this study) with longer-term trends (reported by Hassan et al., 21) derived from a study of about twice the temporal duration. To do so, we aggregated the Pingshan-Datong reaches into the four reaches of the earlier study: Pingshan-Zhutuo; Zhutuo-Yichang; Yichang- Hanjianngkou and Hankou-Datong. Figure 1 shows a direct comparison of the data, distinguishing the early and late periods of the longer dataset. Above Yichang, the data generally plot in the same field that is, the data overlap and exhibit similar variability though in the Zhutuo-Yichang reach the post-1985 data extend the data field to lower sediment loads and there is a conspicuous drop in sediment output after 22 (Yichang station is below the Three Gorges Dam). Below Yichang, the data to 1985 plot consistently, but the post-1985 data, in general, define a regime of lower sediment flux in which the post-22 data plot low but are consistent with the overall relation. Output (Mt) (b) Zhutuo-Yichang (c) Yichang-Hankou (d) Hankou-Datong Short term Long term Post Three Gorges Dam Post Three Gorges Dam 6 Table IV. Summary statistics of functional relations between sediment output and input for the periods and Reach R 2 N F a ša b šb period Pingshan- Zhutuo Zhutuo- Yichang Yichang- Hankou Hankou- Datong period Ł Pingshan- Zhutuo Zhutuo- Yichang Yichang- Hankou Hankou- Datong Ð Ð73 74Ð61 13Ð39 Ð72 Ð4 Ð Ð11 136Ð74 35Ð7 Ð85 Ð7 Ð Ð73 188Ð42 29Ð56 Ð4 Ð5 Ð Ð34 115Ð24 57Ð7 1Ð46 Ð12 Ð Ð5 65Ð2 11Ð9 Ð79 Ð4 Ð Ð16 67Ð7 26Ð7 1Ð1 Ð56 Ð Ð22 116Ð4 13Ð3 Ð48 Ð23 Ð Ð95 29Ð26 14Ð6 1Ð1 Ð4 Notes: Ł After Hassan et al. (21). N is number of observations, F is F-test, a is regression intercept, and b is regression slope. All results at significant at p<.1. 2 Post Three Gorges Dam Input (Mt) Figure 1. Relation between sediment output and input in four reaches for two periods, and For the reaches above Hankou, the mean relation for the longer-term data is similar to that of the shorterterm. In the distal reach (Hankou-Datong), however, there is a conspicuous regression toward overall sediment balance in the longer-term data (Table IV). Indeed, in all cases, the longer-term data regress toward the condition of overall sediment balance, even though they come nowhere near that condition in the Yichang-Hankou reach. In general, the longer-term relations are the stronger ones, as well, a result that derives mainly from the extended range of the data. The comparison between the time-span budgets demonstrates the importance of Copyright 211 John Wiley & Sons, Ltd. Hydrol. Process. 25, (211)