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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5.6 REACH 7 - OHIO CREEK UPSTREAM OF THE CONFLUENCE WITH THE GUNNISON RIVER

5.6.1 Description of Reach 7

The reach length of Ohio Creek from the confluence with the Gunnison River up to the 51 N-15S township
line is approximately 12 miles with a contributing drainage area of 167 square miles. The USGS Gage
Ohio Creek near Gunnison (ID# 09114000) recorded data for six years from 1945 to 1951, and a nearby
gage, Ohio Creek above Mouth near Gunnison (ID#091139S0), recorded data from 1995 through 2000.
Further upstream, the USGS Gage Ohio Creek near Baldwin with a drainage area of 121 square miles
(ID# 09113500) recorded data from 1940-1950, 1959 through 1971, and 1980 through 1981.

Significant tributaries including Mill Creek, Carbon Creek, and Castle Creek are all located above the
gage near Baldwin. No significant side inflows are apparent below this gage; however, there are several
ungaged diversions on Ohio Creek. A detailed evaluation of the diversions is not included in the analysis
of the stream gage records for this study.

5.6.2 Flood Records for Reach 7

The existing gage records do not provide an adequate data set for flood frequency analysis. The Ohio
Creek near Baldwin gage recorded daily and peak flow data for twenty-seven years, representing the
upper reach of the creek. The Ohio Creek near Gunnison and Ohio Creek above Mouth near Gunnison
represent the lower half of the reach. Together, the two gages represent nine years of record.

The REGFREQ 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) exists between the Ohio Creek near
Gunnison and the Ohio Creek at Baldwin. Since no other gage presented an acceptable correlation with
the Ohio Creek near Gunnison, the Ohio Creek at Baldwin was chosen to expand the data set for the
Ohio Creek near Gunnison daily stream gage flows.

Attempts were made to use data from Reach 9 (Gunnison River from Ohio Creek to Tomichi Creek) and
Reach 6 (Gunnison River from A1mont to Ohio Creek) to extend the peak flow record for both the upper
and lower portion of Reach 7. Daily values for Reach 6 were subtracted from the daily values for Reach 9
to create a set of daily values that in theory represent the flow in Ohio Creek. (It is the only major tributary
on the two reaches of the Gunnison). Unfortunately, this resulted in a large number of negative results
and generally unsatisfactory data. Possible reasons for the unsatisfactory results include the construction
of Reach 6 data using both natural and deregulated flow versus the entirely deregulated Reach 9 flows,
and subtle interactions caused by the deregulation procedure.

5.6.3 Results for Reach 7

Before any analysis was completed, the data record for both the Ohio Creek gages were filtered for rain
on snow events, using the 0.5 inch criteria discussed in Section 4.1.1. No rain on snow events were found
in either Ohio Creek gage record.

The REGFREQ and HEC-FFA programs were used to perform the analysis on the flow record in
Appendix A - Table A.6. Prior to using the REGFREQ program, HEC-FFA was used on both the Ohio
Creek gage data sets to test for low and high outliers based on a methodology described in Bulletin 17B.
No outliers were found. The results from the analysis are presented in Figure 5.6.3.1. Additionally, the
adopted statistics and the 10, 2, 1, and 0.2 percent chance exceedence flows are presented in Table
5.6.3.1.

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