Dams, sediment, and channel changes and why you should care Gordon E. Grant USDA Forest Service PNW Research Station Corvallis, Oregon
Dam effects on river regimes FLOW (Q) SEDIMENT (Qs) TEMP CHEMISTRY DAM Channel morphology Aquatic Biology
Dam effects on river regimes FLOW (Q) Channel morphology SEDIMENT (Qs) DAM Dams may or may not affect the flow regime Virtually all dams affect the sediment transport regime by trapping sediment Aquatic Biology
pre-dam post-dam Discharge record 1920-2000, Colorado River below Glen Canyon Dam
25 Peak Discharge, thousands of cubic feet per second 20 15 10 5 0 70 60 50 40 30 20 Deschutes River near Madras, Drainage area 7,820 mi 2 Deschutes River at Moody Drainage area 10,500 mi 2 Project Construction Dec. 1964 Feb. 1996 10 0 1900 1920 1940 1960 1980 2000 Water Year Pre- and post- dam discharge record for Deschutes River, Oregon
Dams trap sediment Cougar Reservoir drawdown 2002, SF McKenzie River, Oregon
If we understand how dams change flow and sediment regimes, can we predict downstream channel response?
Principle of River Equilibrium A graded stream responds to a change in conditions in accordance with Le Chatelier's general law: "if a stress is brought to bear on a system in equilibrium, a reaction occurs, displacing the equilibrium in a direction that tends to absorb the effect of the stress." J. Hoover Mackin, 1948. The concept of the graded river. GSA Bulletin; v. 59; no. 5; p. 463-512; DOI: 10.1130/0016-7606(1948)59[463:COTGR]2.0.CO;2
Using Lane s Balance to predict downstream changes & channel narrowing Lane s Balance Applied River Morphology, Dave Rosgen, 1996
less sediment A. Dam traps sediment, no change to flow
coarsening lower slope less sediment incision, narrowing A. Dam traps sediment, no change to flow
less sediment?? less water B. Dam traps sediment and reduces flows
less water C. Dam reduces flows, sediment influx below dam
fining higher slope less water aggradation C. Dam reduces flows, sediment influx below dam
Textural coarsening Clackamas River, Oregon
Incision / bed degradation Arno River at Empoli
Large decrease in eddy sand bars Grand Canyon
Post-dam bed degradation Williams & Wolman, 1984
ARBITRARY ELEVATION, IN METERS Channel narrowing 30 25 20 30 28 26 COTTONWOODS SALTCEDAR & RUSSIAN OLIVES 24 WILLOWS 22 20 0 1912 1928 1997 20 40 60 80 100 120 140 DISTANCE, IN METERS Green River, Utah; Grams and Schmidt, unpublished data
Framework for downstream channel response Equilibrium Point
How do we bring hydrologic and sediment transport effects together? Δ Sediment Supply Aggradation Textural shifts at confluences Island and bar construction Vegetation enchroachment Channel aggradation Degradation Bed scour Armored channel Bar and island erosion Channel degradation, narrowing Δ Transport Capacity
Δ Sediment Supply Rio Grande River? Colorado River (135 km) Yangtze River (673 km) Δ Transport Capacity
A summary of sorts Although dams change water and sediment regimes, the nature and direction of those changes depends on the dam itself and how it s operated Trajectories of downstream change are semi-predictable using simple analytical models Changing flow and sediment regimes translate to changes in channel form and aquatic habitat
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Sandy River Other examples of channel response
Loss of secondary channels (simplification) 1938 1993 Colorado River
1871 Lodore Canyon, Green River early 1990s Grams and Schmidt, 2002
How do we bring hydrologic and sediment transport effects together? Geology Rock properties (erodibility) Deformation and structure Processes Sediment Supply Topography Relief Drainage Density Sediment Transport Regime Climate Precipitation type, intensity and duration Hydrologic Regime Basin Scale Geologically-controlled landforms and events ( history ) Dams Channel and valley floor morphology Channel and reach scale
Transport capacity T cr t ( Q Q ) t Q Fraction of time threshold for sediment transport is exceeded (grain size dependent)
Change in transport capacity due to dam T * T T post pre Ratio of post-dam to pre-dam fractional transport time
Change in sediment supply due to dam S * S S B A Ratio of below dam to above dam sediment supply
dam S*<<1 S*<1 but increasing Pre-dam Post-dam
Flaming Gorge Dam Downstream Changes in Flow and Sediment Flux, Green River Utah
Aggradation Reach 3: initial narrowing (10%) now aggrading Reach 2 narrowing (13%) degrading? Green River (464 km) Reach 1 narrowing (13%) degrading (0.7m) Degradation
Pre-dam loads were 35-40% sand (Topping et al., 2000) Glen Canyon Dam Colorado River
Elevation (m) Aggradation 950 1956 2000 935 0 Distance Downstream (km) 25 Colorado River (135 km) Degradation
Elevation above sea level (m) 10/1/57 10/1/61 10/1/65 10/1/69 10/1/73 10/1/77 10/1/81 10/1/85 10/1/89 10/1/93 10/1/97 Deschutes River, Oregon Aggradation Deschutes River pre-pelton (160 km) 422.7 422.5 1957-1998 0.1 m incision in 40 years Deschutes River pre-dam (160 km) 422.3 422.1 421.9 2/3/65 2/26/5812/31/64 2/24/82 2/10/96 Degradation 421.7 Time
Aggradation Rio Grande River Yangze River (673 km) Degradation
Evaluating altered sediment regimes Pre-dam as reference point Reservoir sedimentation rates Magnitude/frequency of sediment transport Ratio of above dam to below dam sediment supply Volume Timing/mechanisms of supply Caliber
Q Linking hydrograph changes to geomorphic response Peak discharge Armor breakup (Phase II) Initiation of motion (Phase I) Active bars inundated Secondary channels inundated Time