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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7.0 SUMMARY AND CONCLUSIONS

Analyses of flood flows along eight reaches in Gunnison County, summarized in Table 7.1, were
completed. The Slate River, East River, Taylor River (tributary to the Gunnison), and Gunnison River are
dominated by rugged, mountainous watersheds subject to large amounts of snowfall accumulation.
These rivers have peak runoff events due primarily to snowmelt. On occasions, rain has occurred during
the time of the peak snowmelt runoff.

The upper portion of the Ohio Creek watershed is located in a mountainous region; however, the lower
portions are located at lower elevations with a wider floodplain as compared to the Slate, East, and
Gunnison Rivers. The majority of the Tomichi Creek watershed is located in a high mountain valley
region. While ninety percent of the peak flows are due to snowmelt runoff, there are eight years of record
out of sixty-three, where the peak flow is due to cloudburst events of greater then 0.5 inches of
precipitation occurring during the snowmelt runoff season. Replacement of the precipitation-influenced
events with the next highest snowmelt only event resulted in a better fitting frequency curve. The
replacement of rain on snoW events when found was then applied for the entire study.

Table 7.1

Study Reaches

Approximate Approximate Elevation at
Lower end of
Study Reach Drainage Reach Reach Description
Reach Length (mi) Area (mi2) (ft)
2 10 70.1 8,827 Slate River to confluence with East River
3 1 160.4 8,800 East River from the Slate River to Cement Creek
4 6 238 8,440 East River from Cement Creek to Alkali Creek
5 7 289 8,006 East River from Alkali Creek to the Taylor River at
Almont
6 6 766 7,700 Gunnison River from Almont to Ohio Creek
7 5 167 7,700 Ohio Creek from Township Line to the Gunnison River
8 2 1,061 7,629 Tomichi Creek from the Gunnison River upstream to
the Gunnison Airport
9 5 1,012 7,655 Gunnison River from Ohio Creek to Tomichi Creek

A specific review of the contributions from Alkali Creek was completed. Alkali Creek flows into the East
River along the reach from Cement Creek to Almont. The Alkali Creek drainage basin is lower in
elevation, and snowmelt runoff from this basin is not significant. There is essentially a one-to-one
relationship between the East River flows below Cement Creek and the East River flows at Almont; this
further supports the conclusion that there is no indication of significant inflows from Alkali Creek. No
further rainfall runoff modeling of this basin or other basins is suggested.

Due to lack of sufficient data for some reaches, correlation between streamgages was used to expand
and complete the record of annual peak flows. To extend the limited data sets, correlation of this data to
extensive data from other similar reaches are created and used in order to perform flood frequency
analyses for these reaches (see Section 5).

The only development in the basin that has a significant effect on flood flows is Taylor Park Dam along
the Taylor River. While this dam is an irrigation storage facility, it does impact flood flows along the Taylor
River and the Gunnison River (Reaches 6 and 9). The regulated gage data at the Taylor River below
Reservoir, Taylor River at Almont, and Gunnison River near Gunnison are all used to create unregulated
flow data sets using reservoir storage information. These data are used for the flood frequency analysis to
yield results of natural unregulated conditions.

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