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<br />11-2 <br /> <br />I <br />I <br />,I <br />I <br />I <br />I <br />I <br />, <br />I <br />I <br />a <br />I <br />I <br />I <br />I <br />.1 <br />I- <br />II <br />I <br /> <br />of the watershed varies significantly. Bear Peak and steep mountain slopes aligned <br />with the Flatirons are on the west. David's Draw, which is also on the west side, <br />has some dense development. There are moderately steep to mildly sloping <br />agricultural areas, open space, riparian cottonwood groves, and the gravel pit area <br />in and along the valley bottom. There is a large foothills tributary on the east which <br />includes the Marshall Reservoir and a downstream basin which has steeply sloped <br />valley sides with a mixture of brush, grass, and trees. On the east side of the <br />watershed there are moderately sloping agricultural areas which naturally drain to <br />South Boulder Creek, but whose drainage is interrupted by several irrigation <br />ditches. <br /> <br />The wide floodplains and multiple flow paths of South Boulder Creek provide an <br />important balancing effect by storing, routing, and slowing the flashy peak runoff <br />rates from the steep slopes and tributary streams. From Highway 93 to Highway <br />36 the main channel conveys day to day flows, and whatever flood flows do not <br />spill upstream. The majority of the 1 OO-year flood will be carried in the West Valley <br />Overflow route. However, the West Valley Overflow is forced to carry much of the <br />water back to South Boulder Creek by the Upper and Middle Flatirons Quarry Pit <br />(CU) levees just upstream of Highway 36. Some minor amount of the overflow will <br />not return to South Boulder Creek and will be conveyed by either Dry Creek Ditch <br />No.2 and or limited overbank conveyance to join the South Boulder Creek spill just <br />downstream at Highway 36. <br /> <br />Downstream of Highway 36 and the main channel of South Boulder Creek there are <br />another 6.2 square miles of potential tributary area (42.5 square miles below Gross <br />Reservoir, 135.7 square miles total basin). This basin ranges in elevation from <br />5170 feet M.S.L. at the confluence with Boulder Creek to 6050 feet M.S.L. at the <br />top of the Viele watershed. The watershed is largely urbanized with notable <br />exceptions being the riparian corridor directly along the main channel of South <br />Boulder Creek, the highest portion of the Viele watershed, the golf course, and <br />some of the hills on the east side of the watershed. The valley bottom is a mile <br />wide and is an alluvial fan, with varying cross slope direction and some moderate <br />down valley slope. <br /> <br />The Viele watershed is on the southwest side of the basin, and begins on a ridge <br />just south of the confluence of Bear Canyon Creek and Fern Canyon. It has four <br />significant storage facilities and an outfall channel on the south side of the valley <br />that crosses Highway 36 at the Foothills Interchange and then joins South Boulder <br />Creek just north of South Boulder Road. <br /> <br />IRRIGATION DITCH DIVERSIONS <br /> <br />There are numerous water supply and irrigation diversions and ditches in Eldorado <br />Canyon and the downstream watershed. Normally, the diversion effect of flood <br />