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<br />Because of the steep mountain slopes and narrow valleys in Clear <br />Creek County, development has, by necessity, occurred near the <br />streams and often within the flood plain. City streets and state <br />and interstate highways cross the flood plains and often constrict <br />the free-flowing streams with bridges and embankments. Continuing <br />development within the study area is expected, and pressures leading <br />to intensified flood plain use will probably accompany such development. <br /> <br />2.3 Principal Flood Problems <br /> <br />Clear Creek, Soda Creek, Virginia Canyon, and Chicago Creek flow <br />through Idaho Springs in narrow channels with relatively steep <br />slopes. Large flows through the city are characterized by very <br />high velocities and unstable water-surface elevations. Bridges <br />appear to be the greatest constraints on passage of floodflows. <br /> <br />Flooding in Idaho Springs is primarily a function of the spring <br />snowmelt, or possibly snowmelt in conjunction with rain-an-snow. <br />Summer rainstorms do not appear to be the major cause of flooding, <br />although heavy hailstorms occasionally cause drifts of hail that <br />block drainage paths and create some flooding. <br /> <br />Deposition of large amounts of sediment, including sand, gravel, <br />and small boulders, has occurred at the base of steep mountain <br />gulches in Idaho Springs following heavy runoff periods. These <br />deposits occasionally cover roads and affect flooding in the manner <br />that a permanent alluvial fan would affect flooding. Gilson Gulch <br />and the Gulch above 8th Avenue are affected in this manner. <br /> <br />Clear Creek has been gaged at two locations within Idaho Springs <br />for periods of 2 and 4 years. Neither station had a record of <br />sufficient length to analyze for flood frequencies. <br /> <br />A search through past newspapers and interviews with local residents <br />revealed that no significant flooding has occurred in Idaho Springs. <br />The largest flood that anyone can recall occurred in 1956 due to <br />the failure of the Georgetown Lake Dam. Flows in excess of 5000 cubic <br />feet per second (cfs) were recorded on Clear Creek above Golden <br />from that dam failure. It is estimated that floodflows through <br />Idaho Springs were approximately 5500 cfs. No evidence was found <br />of any structural flooding. A flow of 6130 cfs was recorded by the <br />streamflow gage at Lawson, Colorado. No frequency has been computed <br />for the 1956 flood. <br /> <br />4 <br />