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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This was done to account for the fact that a large portion of the flow at the Gunnison near Gunnison gage
is unregulated flow contributed by the East River and Ohio Creek. However, the contribution from the
change in storage is better represented by the ratio determined for the gage closer to the reservoir.
Multiplying both the change in storage flow and the regulated flow by 1.0687 yields results significantly
lower than the results for the upstream reach, Gunnison River from Almont to Ohio Creek; therefore, the
alternative procedure is implemented for this study. For both gages, the estimated travel time from the
reservoir to the gage is less than 12 hours. Therefore, a lag is not applied.

5.0 INDIVIDUAL REACHES

The eight reaches, for which the peak flow analysis is desired, are analyzed. In addition, two more
reaches are included on the Taylor River since their results are needed to support the analysis of several
of the eight study reaches. Two of the eight reaches had sufficient stream gage data to warrant an
analysis based on the stream gage data alone. The other six study reaches and the two additional
reaches are altered or supplemented with nearby streamgage data. The characteristics, streamgage
information, correlation information, and results of analysis are presented below for each reach.

5.1 REACH 2 - SLATE RIVER TO CONFLUENCE WITH EAST RIVER

5.1.1 Description of Reach 2

The Slate River reach extends from the confluence of the Slate River and the East River approximately
10 miles upstream to the confluence with Oh-Be-Joyful Creek with a drainage area of approximately 70
square miles. Flows into the river are almost entirely from snowmelt runoff.

5.1.2 Flood Records for Reach 2

There are reported peak flows from the USGS gage Slate River near Crested Butte for 1940 through
1951 and 1994 through 2000; however, a longer record would be more appropriate for completing an
analysis of flood flows in this reach.

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) between the Slate River gage and the
following gages were found.

· Taylor River at A1mont (09110000)
· East River at A1mont (09112500)
· East River below Cement Creek (09112200)
· Gunnison River near Gunnison (09114500)

The East River at Almont gage was chosen to expand the data set for the Slate River near Crested Butte
daily stream gage flows. The East River at Almont gage has a continuous record from 1935 through
2000. Although the other three gages noted above had slightly higher correlation, the East River gage
was chosen due to the longer available period of record and unprocessed results. The data from the
Taylor and Gunnison gages had been processed to remove the effects of the Taylor Park Reservoir.

No data are available for the upstream portions of the Slate River above the confluence with Coal Creek
and the town of Crested Butte. Data are available from Coal Creek for five years from 1941 through 1946.
These data indicate that from March through June, the contribution from Coal Creek is generally 10 to 20
percent of the USGS recorded streamgage flow of the Slate River near Crested Butte. If flood flows were
needed for the portion of Slate Creek above Crested Butte, the results for the lower reach could be
reduced by 10 percent; or the results for the downstream reach could be used as a conservative value for

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