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
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
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
rill erosion, Peru sheetflow erosion around plant roots
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
Cienega Creek, Arizona
Pawnee Buttes, Colorado Mississippi
Alton, Utah Owl Canyon, Colorado
Box canyon, Dinosaur Ntnl Monument, CO Dead Horse Point State Park, Utah
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
drainage density drainage density field observation age of till surface (0 3 yrs) flume experiment time (hours)
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
Rapidly expanding drainage network, nw Australia
Stream numbering: quantify magnitude of basin, or relation/number of tributaries to trunk (5 th order basin, for eg.) 3 2 2 2 2 2 2 2 2 3 3 3 5 7 8 Horton (945) Strahler (952) Shreve (967)
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
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
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
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
discharge gage or stage height Rating curve discharge big floods average flows Flow duration curve droughts % of time flow equalled or exceeded specified discharge
Gaging weir, Rocky Mountain NP, CO First gage in the US, Embudo, Rio Grande, New Mexico
Colorado River at Lees Ferry, AZ Weir gage, Fool Creek, Fraser Experimental Forest, CO Boat-mounted current meter
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....... 00-year flood... recurrence interval
discharge Flood hydrograph peak runoff increasing basin size time base flow
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
sediment yield sediment yield 0 000 2000 Langbein-Schumm curve seasonal tropics effective precipitation (cm) 800 860 900 960 Land use construction aggradation forest cropping woods & urban stable aggradation grazing scour-stable scour
May 990 June 990 September 990