PROJ01918 ‎(2)‎

• Contract/PO #: FS0077FX
• Contract Type: Grant
• Water District: 0
• Loan Projects - Doc Type: Feasibility Study

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Stream Flow Data

Historic stream flow data for Leroux Creek were obtained from gauge 1345. The gauge readings
are available for the period of 1937 through 1955 and the period of 1961 through 1968. Earlier
studies were conducted by USBR and PRC Engineering (now ECI) to estimate the missing data
for the years of 1956 through 1960 and 1968 through 1984. This report updates this information
by providing estimates for the period from 1985 through 2001.

Based on the historic data from gauge 1345, the mean and standard deviation (STD) of monthly
runoff rates are estimated and shown in Table 1.

Table 1
Monthly Runoff for Leroux Creek (Gauge 1345) in cfs for Historic Data

Month 1 2 3 4 5 6 7 8 9 10 11 12 !Annual
Mean 5.6 5.7 7.5 54.0 238.2 141.3 39.7 27.9 19.6 14.3 8.9 6.2 47.3
STD 1.2 1.2 1.8 32.2 71.9 82.7 12.0 7.0 7.0 11.6 4.2 1.4 13.8
CV 0.2 0.2 0.2 0.6 0.3 0.6 0.3 0.3 0.4 0.8 0.5 0.2 0.3

Table 1 shows that maximum monthly runoff rates are expected during May and June. It also
shows that the coefficient of variation (CV) for monthly flows range between 19 percent and 80
percent. This high variation in monthly discharge rates is an indication of the need to correlate
these rates with nearby stations.

The mean runoff rates in acre-feet, percentage of the total runoff and standard deviation for each
month at Leroux Creek are shown in Table 2.

Table 2
Monthly Runoff for Leroux Creek (Gauge 1345) in ac-ft for Historic Data

Month 1 2 3 4 5 6 7 8 9 10 11 12 Annual
Mean 343 317 461 3,107 14,195 8,113 2,422 1,729 1,181 897 524 376 33,664
Percent 1 1 1 9 42 24 7 5 4 3 2 1 100
STD 78 65 111 1,919 4,222 4,640 748 440 421 746 264 88 9,405

Table 2 indicates that 42 percent of the annual runoff occurs in May and another 24 percent in
June. This is due to the rapid snowmelt over the drainage basin of Leroux Creek. The slopes are
generally facing south, which causes the snow to melt quickly during May and early June.

In order to estimate missing monthly runoff data for Leroux Creek between the periods of 1985
and 2001, data from Surface Creek was collected and analyzed. The Surface Creek and Leroux
Creek basins and their proximity to each other can be referenced on Figure 1. As mentioned
earlier, Surface Creek above gauge 1430 has a basin area of28 square miles, which is 76 percent
of the Leroux Creek basin area. Table 3 below shows average monthly runoff for Surface Creek
at gauge 1430 for the period of 1941 through 2001. Surface Creek data at this gauge is complete

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