S. Fork Nooksack River, WA. Reasons for Land Clearing Agriculture Lumber Mining Urban Development.

S. Fork Nooksack River , WA

Reasons for Land ClearingAgricultureLumberMiningUrban 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?

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

Assessing Impacts on Channel MorphologyParameter Upstream At site DownstreamVelocityDischargeRoughnessSlopeAreaLoad

- 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)

Lower Poultney River, VT

Merrimack River, NH

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

Courtesy of VT DEC River Management

Assessing Impacts on Channel MorphologyParameter Upstream At site DownstreamVelocityDischargeRoughnessSlopeAreaLoad

- 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

Effect of Urbanization on Flood Hydrographs

Channel Incision Near Santa Fe, New MexicoFrom Dunne and Leopold (1978)Control SectionsUrbanized Sections

Increased Discharge Results From:Greater percentage of impervious surfaceFilling of wetlandsReconfiguration of drainage networkSmoothing of land surface Loss of floodplain storage

Hydrologic Changes Resulting From UrbanizationFrom MacBroom (1998)

Effect of Wetlands on Peak RunoffPercentage of Watershed Covered by Wetlands0.00.21.03.05.0Peak Flow Reduction Factor1.000.970.870.750.72- A watershed with 3% wetlands can have peak flows 25% smaller than a similar watershed without wetlandsFrom MacBroom (1998)

From MacBroom (1998)Loss of Floodplain Storage

From Gould (1975)

From Gould (1975)