Each basin is surrounded & defined by a drainage divide (high point from which water flows away) Channel initiation

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Hope Dorsey
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1 DRAINAGE BASINS A drainage basin or watershed is defined from a downstream point, working upstream, to include all of the hillslope & channel areas which drain to that point Each basin is surrounded & defined by a drainage divide (high point from which water flows away) Channel initiation Type of downslope movement of precipitation depends on ) infiltration capacity of surface, which is controlled by soil texture & structure antecedent moisture vegetation other surface conditions 2) intensity and duration of precipitation

2 water moving downslope as runoff moves rapidly & has erosive ability: channels begin to develop when the erosive force, F (function of slope angle, water depth) of the overland flow exceeds the resistance, R (function of vegetation, nature of surface) of the surface being eroded soil profile precipitation zone of percolation groundwater flow throughflow saturation overland flow Hortonian overland flow runoff water table ground water

3 Stages of overland flow rainbeat impact cratering thread flow integration of raindrops, flow around surface grains sheet flow integration of thread flow, water level rises above roughness elements rill flow sheet flow concentrates into small, parallel streamlets of water, which grow by micropiracy as master rills concentrate water & entrench to small channels small channels tributaries, grow by headward erosion or sapping Sapping involves throughflow; subsurface water moves along percolines zones of greater soil depth & moisture content, or in pipes horizons or surfaces of limited permeability with enlarged pores

4 rill erosion, Peru sheetflow erosion around plant roots

5 Sapping occurs in the saturated zone, where percolines meet the surface as seeps or springs, undermining overlying material & causing the collapse of valley head walls Sapping networks are controlled by the direction of groundwater rather than surface flow (eg. joints), & produce ampitheaterheaded valleys like those on the Colorado Plateau, Hawaii, and Mars (??) Piping also affects network development occurs in the unsaturated zone, where differences in permeability create hydraulic head & differential erosion Network evolution direct observation on recently exposed sites flume studies

6 Cienega Creek, Arizona

8 Pawnee Buttes, Colorado Mississippi

10 Alton, Utah Owl Canyon, Colorado

11 Box canyon, Dinosaur Ntnl Monument, CO Dead Horse Point State Park, Utah

12 The basic steps in network growth are initiation of channels elongation through headward growth elaboration/branching as tributaries are added the order & relative importance of these steps depends on such external factors as slope Ergodic hypothesis: space substitutes for time Random-walk model: recognizes element of randomness (each choice has equal probability of occurrence) in the development of networks

13 drainage density drainage density field observation age of till surface (0 3 yrs) flume experiment time (hours)

14 Network Analysis the study of network patterns is necessary to understand the controls on their formation the early approaches to network analysis were largely qualitative, focusing on the importance of time & geo structure at the largest scale, the most commonly used classification at present divides drainage patterns on the basis of their planview shapes other classifications focus on other aspects of the drainage net, determining quantitative, but dimensionless, numbers from ratios facilitates comparison between basins such ratios represent linear, areal & relief morphometric components of the basin linear stream number areal drainage density relief relief ratio hypsometric integral

15 Rapidly expanding drainage network, nw Australia

16 Stream numbering: quantify magnitude of basin, or relation/number of tributaries to trunk (5 th order basin, for eg.) Horton (945) Strahler (952) Shreve (967)

17 drainage density Drainage density: average length of streams per unit area controlled by interactions among geology climate age of basin drainage density increases as resistance or surface permeability decreases drainage density links the form attributes of the basin to the underlying processes variation of drainage density with climate & lithology shale sandstone arid temperate tropical

18 Relief ratio: relief morphometry deals with the vertical dimensions of a drainage basin relief ratio is the maximum basin relief divided by the longest horizontal distance of the basin parallel to the main stream Hypsometric integral: distribution of mass above a datum (see text) The morphology of a drainage basin can be viewed as the connection between the way water moves into the basin, & the way it moves out

19 Water input is one of the controls on morphology (types & distribution of channels), and morphology is one of the controls on hydrology (flow characteristics) climatic regime water basin morphometry water hydrology

20 Stream discharge: volume of water passing a given channel cross section during a specified time interval Q = w d v = A v (in ft 3 /s or m 3 /s) Velocity is not evenly distributed varies at cross section varies downstream (generally increases) Discharge measured at gaging stations by US Geological Survey & other agencies; measured hourly, daily, at peak flows, etc Discharge is usually estimated from a rating curve

21 discharge gage or stage height Rating curve discharge big floods average flows Flow duration curve droughts % of time flow equalled or exceeded specified discharge

22 Gaging weir, Rocky Mountain NP, CO First gage in the US, Embudo, Rio Grande, New Mexico

23 Colorado River at Lees Ferry, AZ Weir gage, Fool Creek, Fraser Experimental Forest, CO Boat-mounted current meter

24 discharge Geomorphologists study the magnitude & frequency of flows in order to determine the effect of various flows on channel & basin morphology use flow duration curves & flood-frequency analysis R = (n + )/ m R = recurrence interval (yrs) n = number of discharge values in sample m = rank of flow year flood... recurrence interval

25 discharge Flood hydrograph peak runoff increasing basin size time base flow

26 Sediment Yield Basin morphology also controls the movement of sediment into the channels & out of the basin Sediment yield: the amount of sediment leaving the basin Sediment yield does not necessarily equal erosion rate because of sediment storage on slopes (colluvial) & in channels (alluvial) Controls on sediment yield are climate mainly precipitation (Langbein-Schumm curve) vegetation screens & binds regolith basin size small = high sediment yield due to steep slopes & channels elevation & relief rock type/erosional resistance eg. clastic sedimentary rocks have higher sediment yield than crystalline rocks land use

27 sediment yield sediment yield Langbein-Schumm curve seasonal tropics effective precipitation (cm) Land use construction aggradation forest cropping woods & urban stable aggradation grazing scour-stable scour

28 May 990 June 990 September 990

WATER ON AND UNDER GROUND. Objectives. The Hydrologic Cycle

WATER ON AND UNDER GROUND. Objectives. The Hydrologic Cycle WATER ON AND UNDER GROUND Objectives Define and describe the hydrologic cycle. Identify the basic characteristics of streams. Define drainage basin. Describe how floods occur and what factors may make