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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5.2

REACH 3 - EAST RIVER FROM SLATE RIVER TO CEMENT CREEK

5.2.1 Description of Reach 3

The East River between the Slate River and Cement Creek has a reach length of approximately 1 mile
with a total drainage area of 208 square miles at the downstream end of the reach. This reach is
designated separately to identify flows in the East River above the confluence with Cement Creek. This
reach is located near the Crested Butte South community. No streamgage data is available for this reach.

5.2.2 Flood Records for Reach 3

There are no streamgage records for this relatively short reach of the East River that extends from the
confluence with the Slate River to the confluence with Cement Creek.

Flows in this reach are equivalent to the flows in the downstream reach of the East River (Reach 4 - East
River from Cement Creek to Alkali Creek) less the contribution from Cement Creek (approximate
drainage area of 30 square miles at the confluence with the East River). Unfortunately, there is no
common period of record between the gages. Therefore, it was not possible to create a data set by
subtracting the Cement Creek contribution to the East River below Cement Creek.

Above Cement Creek, two USGS streamgages, the East River near Crested Butte and Slate River near
Crested Butte, are located a short distance upstream of the confluence of these two rivers. The combined
drainage area of the two gages is 160 square miles, approximately 75 percent of the drainage area of the
reach. The ungaged 48 square miles of drainage area is located in the lower portion of the watershed and
will contribute a proportionally smaller flow than the upper drainage area.

To better estimate the contribution from the ungaged drainage area, the data from the Cement Creek
gage were used to create a relationship between the combined Slate River near Crested Butte and East
River near Crested Butte gage data. A ratio of 5 percent was used to account for the ungaged drainage
area. This was applied to the daily data set of the combined Slate and East Rivers near Crested Butte to
produce an estimated daily average flow data set for Reach 3.

Adding the daily flows from the gages yields the flow for Reach 3. Before creating the flood record for
East River above Cement Creek, the relationship between the peak and daily average flow was required
to determine the estimated peak flow record for Reach 3. The relationships indicate that the peak flows in
the East River near Crested Butte is 1.21 and Slate River near Crested Butte is 1.06 of the reported
average daily flows on the corresponding day (refer to Figure 5.2.2.1). The R2 value for both relationships
is 0.95.

Yearly peak values are determined for this reach by applying a weighted average of the East River near
Crested Butte and Slate River near Crested Butte gages based on drainage area. The resulting ratio of
14 percent was used to estimate the peak discharges from the maximum annual daily averages of the
Reach 3 data set.

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