FLOOD10363 ‎(2)‎

• County: Gunnison
• Community: Gunnison County
• Stream Name: East River, Cement Creek, Alkali Creek, Ohio Creek
• Title: Gunnison County Flood Hydrology, Contract # W99-D-0018 Final Report
• Date: 11/1/2004
• Prepared For: US Army Corps of Engineers, CWCB and Gunnison County
• Prepared By: Tetra Tech, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The 1 percent chance exceedence flow for the Gunnison River from Ohio Creek to Tomichi Creek, with a
drainage area of 1,012 square miles, is 10,200 cfs; whereas the 1 percent chance exceedence flow for
Tomichi Creek, with a drainage area of 1,061 square miles is significantly less at 2,540 cfs. The large
difference in flow values is due to the differences in topography between the two contributing watersheds.
The contributing watershed to the Gunnison River reach is dominated by rugged, mountainous terrain
subject to large amounts of snowfall. The watershed of T omichi Creek is primarily a high mountain valley
area, and while it still receives snowfall, the volume is significantly less than that of the Gunnison River
reach watershed. The lower elevation areas have higher temperatures and longer snowmelt runoff period
than the upper watersheds, which will hold snow longer and will have more rapid snowmelt runoff periods.
Both factors contribute to the large difference in the 1 percent chance exceedence flows for the two
reaches.

The results from the flood frequency analyses for the study reaches indicate logical trends in values that
increase in the downstream direction. The resulting flood flows for each reach are presented in Table 7.2.
This study provides the necessary flood flows to complete the hydraulic analyses for the FEMA FIS study
along each of the eight reaches for the associated flood events.

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