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Geomorphic Effects of Dams on Rivers

Geomorphic Effects of Dams on Rivers. Gordon Grant. Hydrologic Regime. The hydrologic “regime” Geology Climate Topography Changes in flow (Q) due to manipulation Direct = dams, diversions Indirect = climate shift, forest practices

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Geomorphic Effects of Dams on Rivers

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  1. Geomorphic Effects of Dams on Rivers Gordon Grant

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

  3. Hydrograph – McKenzie River, OR Flood control dams

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

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

  6. 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

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

  8. Geologic Influence • Geology plays a major role in hydrology • High Cascades vs. Western Cascades streams

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

  10. Summary • Dams alter… • Sediment regime • Hydrologic regime • River morphology

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