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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5.3 REACH 4 - EAST RIVER FROM CEMENT CREEK TO ALKALI CREEK

5.3.1 Description of Reach 4

The East River between Cement Creek and Alkali Creek is approximately 6 miles long with a drainage
area of 238 square miles at its downstream end. The USGS gage East River below Cement Creek (ID#
09112200) is located approximately 4 miles above the downstream end of the reach.

5.3.2 Flood Records for Reach 4

The record for the East River below Cement Creek gage includes recorded peak flows for 1964 through
1972, 1980, 1981, and 1994 through 2000; however, a longer record is needed to complete an
appropriate evaluation of flood flows in this reach. I

The REGFREO program was used to determine the correlation of the available flow data to nearby gages
on the Gunnison River system. A high degree of correlation (>0.9) between the East River below Cement
Creek gage and the following gages was found.

· Taylor River at A1mont (09110000)
· Taylor River below Taylor Park Reservoir (09109000)
· East River at A1mont (09112500)
· Gunnison River near Gunnison (09114500)
· Slate River near Crested Butter (09111500)

The East River at A1mont gage was chosen to expand the data set for the Slate River near Crested Butte
daily stream gage flows. The East River at A1mont gage has a continuous record from 1935 through
2000. The East River gage was chosen since it had the highest degree of correlation, the longest
available period of record, and unprocessed results. Three of the gages had been processed to remove
the effects of the Taylor Park Reservoir. The resulting family of flow records is presented in Appendix A,
Table A.3.

5.3.3 Results for Reach 4

Before any analysis was completed, the record for both the East River at A1mont and East River below
Crested Butte gages were filtered for rain on snow events, using the 0.5 inch criteria discussed in Section
4.1.1. One rain on snow event was found in the record of the East River below Cement Creek gage. The
East River at Almont results are described in Section 5.4.

The REGFREO and HEC-FFA programs were used to perform the analysis on the flow record in
Appendix A - Table A.3. Prior to using the REGFREO program, HEC-FFA was used on both the East
River at A1mont and East River below Cement Creek data to test for low and high outliers based on a
methodology described in Bulletin 17B. Any outliers were manually removed from the REGFREO data
set. The 1977 flow for the East River gage was determined to be a low outlier. The results from the
analysis are presented in Figure 5.3.3.1. Additionally, the adopted statistics and the 10, 2, 1, and 0.2
percent chance exceedence flows are presented in Table 5.3.3.1.

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