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<br />5.4 REACH 5 - EAST RIVER FROM ALKALI CREEK TO ALMONT
<br />
<br />5.4.1 Description of Reach 5
<br />
<br />The East River between Alkali Creek and the Taylor River (at Almont) is approximately 7 miles in length
<br />and has a total drainage area of 289 square miles at the downstream end. The contributions from Alkali
<br />Creek are relatively low since the Alkali Creek basin is more arid and lower in elevation than the upper
<br />drainage basins (see Section 5.4.4 for more detailed discussion). Most of the Alkali Creek sub-basin is
<br />below 10,000 feet. There is an extensive record from the USGS gage East River at Almont (ID#
<br />09112500) that is located at the downstream end of this reach.
<br />
<br />5.4.2 Flood Records for Reach 5
<br />
<br />The USGS record for the East River at Almont is representative of the flows along this entire reach. The
<br />streamgage record includes reported peaks for 1913, 1916, 1918 through 1921, and 1935 through 2000.
<br />The reported family of flows for the USGS gage East River at Almont is presented in Appendix A, Table
<br />A.4. A comparison of the East River flows below Cement Creek and at A1mont indicates that the
<br />contributions from Alkali Creek are not significant.
<br />
<br />5.4.3 Results for Reach 5
<br />
<br />Before any analysis was completed, the data record for the East River at A1mont gage was filtered for rain
<br />on snow events, using the 0.5 inch criteria discussed in Section 4.1.1. Five rain on snow events were
<br />found in the record of the East River at Almont gage.
<br />
<br />There are a small number of scattered water year data available before 1935 that were not used in the
<br />analysis for the following reasons.
<br />
<br />· Over 65 years of continuous data available without pre 1935 data
<br />· Little known about the missing intervening years, i.e. whether drought or wet
<br />
<br />The REGFREQ and HEC-FFA programs were used to perform the analysis on the flow record in
<br />Appendix A - Table A.4. Prior to using the REGFREQ program HEC-FFA was used on both the East
<br />River at A1mont data to test for low and high outliers based on a methodology described in Bulletin 17B.
<br />Any outliers were manually removed from the REGFREQ data set. The 1977 flow for the East River gage
<br />was determined to be a low outlier. The results from the analysis are presented in Figure 5.4.3.1.
<br />Additionally, the adopted statistics and the 10, 2, 1, and 0.2 percent chance exceedence flows are
<br />presented in Table 5.4.3.1.
<br />
<br />Table 5.4.3.1 Flood Frequency Analysis Results for Reach 5 - East River from Alkali Creek to
<br />Almont
<br />
<br /> Std. Flow (cfs)
<br /> Mean Dev. Skew Chance Exceedence
<br /> 10% 2% 1% 0.2%
<br />Peak 3,337 0.140 0.000 3,284 4,212 4,599 5,495
<br />1-Day 3,307 0.140 0.000 3,061 3,927 4,287 5,122
<br />3-Day 3.281 0,140 0.000 2,887 3,703 4,043 4,830
<br />5-Day 3.267 0.140 0.000 2,795 3,585 3,915 4,677
<br />7-Day 3,254 0.140 -0.100 2,703 3,419 3,710 4,365
<br />15-Day 3.205 0.140 -0.100 2,415 3,055 3,314 3,899
<br />30-Day 3.155 0.140 -0.100 2,152 2,722 2,954 3,475
<br />60-Day 3,069 0.150 -0.200 1,810 2,295 2,487 2,914
<br />
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