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SECTION 4 Water Quality Evaluation Figure 4 -3 Estimated Total Suspended Solids Loads Per Acre - Per Year in the River Road Santa Clara Basin (within the UGB) Estimated TSS Pounds Pounds River Road Santa Clara Basin Relative to the Range of TSS Pounds Per Acre Per Year in the per Acre Per Acre Per Year in Other Eugene Basins River Road Santa Clara per Year Basin Existing Development 231 Potential Increase from 126 Development of Vacant Land Potential Increse from 71 Drywell Decommissioning Total Buildout 428 100 200 300 400 500 600 700 Note: The pollutant load estimates are based on the following assumptions: 1) new development would occur without the inclusion of water quality best management practices: 2) during an average year, all flows from drywells (and, hence pollutant loads) would be infiltrated and would not discharge to surface waters; 3) all drywells were assumed to be located in residential areas; and 4) decommissioning of drywells would result in those discharges being transferred, untreated, to surface waters. Stream Stabili In addition to pollutant loads discharged to surface waters, an additional water quality issue is streambank erosion. As urbanization occurs, changes to the natural hydrology of an area are inevitable. Hydrologic changes associated with development include both an increase in the volume of runoff and an increase in the peak rate of runoff, as illustrated by the storm hydrograph comparison shown in Figure 4 -4. These changes occur in response to site clearing, grading, and the addition of impervious surfaces and maintained landscapes. In addition to hydrologic changes associated with urbanization, activities within and adjacent to waterways such as vegetation removal, construction of retaining walls, weirs, fences, bridges and other features, can affect stream stability and, ultimately, water quality. Collectively, these activities can produce the following impacts to stream corridors: • An increase in streambank and streambed erosion; • Increased deposition of newly eroded debris and sediment, which reduces flood conveyance capacity; • Damage to riparian habitat; • Reduced streamflows during the dry season as a result of reduced infiltration and hence groundwater recharge; • Increased water temperatures in the summer due to reduced and hence more shallow streamflows; and • Increased maintenance needs and liabilities. 0:A25695978 Eugene RR -SC Final Basin PlanVMaster Plan \FINAL2- 2010AMaster Plan 3 -11 -10 FINAL Word Version AW Sec 2 Corr.doc A —L