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S. Fork Nooksack River, WA
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Reasons for Land Clearing Agriculture Lumber Mining Urban Development
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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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Sediment Load vs. Discharge G = pQ j where G is suspended load and j is between 2 and 3 From Leopold et al. (1964) – Rio Grande River near Bernalillo, N.M.
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Assessing Impacts on Channel Morphology Parameter Upstream At site Downstream Velocity Discharge Roughness Slope Area Load
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- 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 (10 2 -10 3 years) - Reforestation decreases load and so incision relaxes the higher slope created by earlier aggradation – faster (10 1 -10 2 years)
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Lower Poultney River, VT
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Merrimack River, NH
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Impacts of Urbanization - Cross section of a graded stream in equilibrium - Housing development clears vegetation and paves surface - What happens?
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Courtesy of VT DEC – River Management
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Assessing Impacts on Channel Morphology Parameter Upstream At site Downstream Velocity Discharge Roughness Slope Area Load
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- 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 - Increased discharge causes erosion Impacts of Urbanization
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Effect of Urbanization on Flood Hydrographs
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Channel Incision Near Santa Fe, New Mexico From Dunne and Leopold (1978) Control SectionsUrbanized Sections
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Increased Discharge Results From: Greater percentage of impervious surface Filling of wetlands Reconfiguration of drainage network Smoothing of land surface Loss of floodplain storage
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Hydrologic Changes Resulting From Urbanization From MacBroom (1998)
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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 - A watershed with 3% wetlands can have peak flows 25% smaller than a similar watershed without wetlands From MacBroom (1998)
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Loss of Floodplain Storage
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From Gould (1975)
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