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S. Fork Nooksack River , WA

S. Fork Nooksack River , WA. Reasons for Land Clearing. Agriculture Lumber Mining Urban Development. Impacts of Land Clearance. - Cross section of a graded stream in equilibrium. Land clearance for agriculture and lumber removes forest vegetation and exposes soil What happens?.

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S. Fork Nooksack River , WA

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  1. S. Fork Nooksack River , WA

  2. Reasons for Land Clearing • Agriculture • Lumber • Mining • Urban Development

  3. Impacts of Land Clearance - Cross section of a graded stream in equilibrium • Land clearance for agriculture and lumber removes forest vegetation and exposes soil • What happens?

  4. Sediment Load vs. Discharge From Leopold et al. (1964) – Rio Grande River near Bernalillo, N.M. G = pQj where G is suspended load and j is between 2 and 3

  5. Assessing Impacts on Channel Morphology ParameterUpstreamAt siteDownstream Velocity Discharge Roughness Slope Area Load

  6. - Sediment load increases at an exponentially greater rate than discharge causing deposition - Decrease in sediment supply with reforestation will result in incision and return to prior river level but with higher river banks • Deposition resulting from deforestation causes increase in slope capable of transporting the higher sediment load, thus bringing the system back into equilibrium – slow process (102-103 years) • Reforestation decreases load and so incision relaxes the higher slope created by earlier aggradation – faster (101-102 years)

  7. Lower Poultney River, VT

  8. Merrimack River, NH

  9. Impacts of Urbanization - Cross section of a graded stream in equilibrium • Housing development clears vegetation and paves surface • What happens?

  10. Courtesy of VT DEC – River Management

  11. Assessing Impacts on Channel Morphology ParameterUpstreamAt siteDownstream Velocity Discharge Roughness Slope Area Load

  12. Impacts of Urbanization - Increased discharge causes erosion - Tendency is for incision in clay - And channel widening in sand - Both decreasing slope due to incision and increasing channel area due to widening decrease the capacity, bringing the system back into equilibrium with the increased discharge

  13. Effect of Urbanization on Flood Hydrographs

  14. Channel Incision Near Santa Fe, New Mexico Control Sections Urbanized Sections From Dunne and Leopold (1978)

  15. Increased Discharge Results From: • Greater percentage of impervious surface • Filling of wetlands • Reconfiguration of drainage network • Smoothing of land surface • Loss of floodplain storage

  16. Hydrologic Changes Resulting From Urbanization From MacBroom (1998)

  17. Effect of Wetlands on Peak Runoff Percentage of Watershed Covered by Wetlands 0.0 0.2 1.0 3.0 5.0 Peak Flow Reduction Factor 1.00 0.97 0.87 0.75 0.72 From MacBroom (1998) - A watershed with 3% wetlands can have peak flows 25% smaller than a similar watershed without wetlands

  18. Loss of Floodplain Storage From MacBroom (1998)

  19. From Gould (1975)

  20. From Gould (1975)

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