1. Geomorphic Effects of Dams on Rivers Gordon Grant

2. Hydrologic Regime  The hydrologic “regime” Geology Geology Climate Climate Topography Topography  Changes in flow (Q) due to manipulation Direct = dams, diversions Direct = dams, diversions Indirect = climate shift, forest practices Indirect = climate shift, forest practices  Flows are influenced in magnitude, duration, frequency and time There are consequences for manipulation There are consequences for manipulation

3. Hydrograph – McKenzie River, OR Flood control dams

5. Run of the River Dam-No Flood Control Capacity

7. Sediment Regime  Much harder to get sediment data than hydrologic data  Use reservoir sedimentation rates – Japan surveys once a year Influenced by other dams Influenced by other dams Landuse history Landuse history  Ratio of sediment supply above and below dam  Sediment budgets

12. Channel Response  Incision occurs immediately downstream of dam  Bed incises to equilibrium then stops  Channel narrowing in which peak flows are affected  Loss of secondary channel  Colonization of bars by vegetation (traps sediment)  Alluvial fans stabilize

15. How to bring hydrologic and sediment transport effects together?  How is the dam affecting the river? Channel response Channel response Flow and sediment Flow and sediment

24. Geologic Influence  Geology plays a major role in hydrology High Cascades vs. Western Cascades streams High Cascades vs. Western Cascades streams

26. Dam Effects  High cascades Very little sediment derived Very little sediment derived Little sediment accumulated behind dams Little sediment accumulated behind dams  W. Cascades Deeply dissected Deeply dissected More sediment accumulation More sediment accumulation

31. Summary  Dams alter… Sediment regime Sediment regime Hydrologic regime Hydrologic regime River morphology River morphology