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1 <br /> ' Boulder Creek. Ground elevations in the valley are often lower west of Dry Creek <br /> Ditch No. 2, therefore, flood flows can be carried west of this Ditch. <br /> From Highway 93 to Highway 36, another nine square miles are potentially <br /> tributary to South Boulder Creek at Highway 36 (36.0 square miles below Gross), <br /> ranging in elevation from 5350 MSL to 8461 MSL at Bear Peak. The character of <br /> the watershed varies significantly and includes Bear Peak and steep mountain <br /> slopes aligned with the Flatirons on the west; David's Draw which is also on the <br /> west side and has some dense development; moderately steep to mildly sloping <br /> agricultural, open space, riparian cottonwood groves, and gravel pit areas along the <br /> ' valley; a large foothills tributary on the east which includes the Marshall Reservoir <br /> and a downstream basin which has steeply sloped valley sides with a mixture of <br /> brush, grass, and trees; and moderately sloping agricultural plains areas on the east <br /> ' side of the valley which naturally drains to South Boulder Creek, but whose <br /> drainage is interrupted by several irrigation ditches. <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. As documented in the Phase I report, the majority of the 100 year <br /> flood will be carried in the West Valley Overflow route. However, the West Valley <br /> Overflow is forced to carry much of the water back to South Boulder Creek by the <br /> Upper and Middle Flatirons Quarry Pit levees just upstream of Highway 36. Some <br /> minor amount of the overflow will not return to South Boulder Creek, probably no <br /> more than 300 cfs, which will be conveyed by Dry Creek Ditch No. 2 and limited <br /> overbank conveyance to join the South Boulder Creek spill just downstream at <br /> Highway 36. <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 (42.2 square miles below Gross <br /> Reservoir, 135.9 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, the highest <br /> portion of the Viele watershed, the golf course, and some of the hills on the east <br /> side. The valley bottom is a mile wide and is an alluvial fan, with varying cross <br /> slope direction and, fortunately for drainage function, some moderate down valley <br /> 8 slope. The Viele watershed is on the southwest side of the basin, and begins on a <br /> ridge just south of the confluence of Bear Canyon Creek and Fern Canyon. It has <br /> four significant storage facilities and an outfall channel on the south side of the <br /> valley that crosses Highway 36 at the Foothills Interchange and then joins South <br /> Boulder Creek just north of South Boulder Road. <br /> ' ll _ 2 <br />