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

I
I
I
I
I
I,
I
I
I
I
I
I
I
I
I
I
I
I
I

5.4.4 Review of Drainage Area versus Flow and Contributions from Alkali Creek

Calculations indicate that flows from Alkali Creek do not have a significant impact on flows in Reach 5.
Although the East River drainage area at Almont is larger (289 square miles) than the drainage area
below Cement Creek (238 square miles), results from the previously discussed regression analyses
indicate that there are little, if any, side inflows to the East River between Cement Creek and A1mont.
There is essentially a one-to-one relationship between the East River flows below Cement Creek and the
East River flows at Almont. This applies to peak flows as well as average daily flows.

Based on field visit observations and review of USGS topographic maps, the Alkali Creek basin was
determined to be semi-arid, relatively small, and at a lower elevation. Based on this review, further study
of the flows from Alkali Creek does not appear to be warranted. The East River at Almont USGS
streamgage appears to be sufficient for providing flow data for the reach of the East River below Cement
Creek. .

5.5 REACH 6 - GUNNISON RIVER FROM ALMONT TO OHIO CREEK

5.5.1 Description of Reach 6

The upstream section of the Gunnison River from Almont to Ohio Creek represents Reach 6. This reach
has an approximate reach length of six miles and a total drainage area of 766 square miles. No
stream gage data are available for this reach. Flows in this reach are affected by operations of Taylor Park
Reservoir.

5.5.2 Flood Records for Reach 6

Although no streamgage records are available for this reach, the USGS stream gages on the East River at
Almont and Taylor River at A1mont are both located immediately above the confluence of these two rivers.
Adding the daily flows from these gages yields the flow into Reach 6. Before creating the flood record for
Gunnison River at A1mont, the gage flow data from the Taylor River at A1mont are transformed from
regulated flow data to unregulated flow data (see Sections 4.3 and 5.9). The relationships indicate that
the peak flows in the East River at A1mont is a factor of 1.11 larger than the average daily flow on the
corresponding day (refer to Figure 5.5.2.1) The R2 value for the relationship is 0.9888.

The daily average flow data from each stream gage are added to compute the flow into Reach 6. The
annual maximum daily value is identified for each year, and the peak to daily relationship was used to
estimate the instantaneous peak value for each year from the combined daily values. Table 5.5.2.1
presents the estimated peak flow record for Reach 6.

The REGFREQ program was then used to determine the correlation of the newly created flows to nearby
gages on the Gunnison River system. A predictably high degree of correlation (>0.9) between the Slate
River gage and the following gages were found.

· Taylor River at Almont (09110000)
· East River at A1mont (09112500)

The East River at A1mont gage was chosen to expand the data set for the Slate River near Crested Butte
daily stream gage flows. Since the Gunnison at A1mont gage was created by summing the East River at
Almont and Taylor River at Almont gage, there would be no advantage to including the Taylor River at
Almont gage in expanding the data set for the Slate River near Crested Butte.

26