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 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.

5. Assessing Impacts on Channel Morphology Parameter Upstream At site Downstream 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 (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)

9. Lower Poultney River, VT

13. Merrimack River, NH

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

15. Courtesy of VT DEC – River Management

16. Assessing Impacts on Channel Morphology Parameter Upstream At site Downstream Velocity Discharge Roughness Slope Area Load

17. - 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

18. Effect of Urbanization on Flood Hydrographs

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

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

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

22. 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)

23. Loss of Floodplain Storage

25. From Gould (1975)