Page 21 - Beaumont Basin Watermaster

Beaumont Basin Watermaster
2013 Reevaluation of the Beaumont Basin Safe Yield 3-Apr-15

Average annual reference evapotranspiration (ET o) in the Study Area is relatively high. Average
annual ET o at the University of California, Riverside CIMIS station, located approximately
12 miles west of the Study Area, is 56.37 inches. Due to the relatively deep groundwater table
throughout most of the Study Area, only groundwater in the riparian area along San Timoteo
Creek is subject to ET (see Figure 10).

3.2.3 Surface Water Features

The primary surface water drainage features in the Study Area are Little San Gorgonio Creek,
Noble Creek, Marshall Creek, Smith Creek, Coopers Creek, and San Timoteo Creek (see
Figure 10). All of the streams and drainage channels in the Study Area are naturally ephemeral
and only flow water during precipitation events or after prolonged periods of precipitation.
Surface water flow in portions of Cooper’s Creek and San Timoteo Creek is sustained year-
round from a combination of wastewater discharges from the City of Beaumont’s WWTP and
shallow groundwater discharge into San Timoteo Creek.

During most minor precipitation events, natural surface water runoff in the Beaumont Basin does
not reach San Timoteo Creek. Runoff in Noble Creek typically infiltrates into the subsurface
before reaching the underpass beneath Interstate 10. Surface water flow in Marshall Creek does
periodically flow past the Interstate 10 Bridge but, likewise, does not typically reach San
Timoteo Creek except during flash flood events or during prolonged periods of precipitation
(Personal communication with BCVWD staff, 2013).

Data from two surface water monitoring stations is available from within or near the Study Area:
a stream gage in Little San Gorgonio Creek north of the Study Area and a stream monitoring
location in San Timoteo Creek (see Figure 10). Data from the Little San Gorgonio Creek stream
gage is available from 1949 through 1985 (see Figure 13). These data show a baseflow less than
1 cfs with spikes in runoff recorded during years of significant above-normal precipitation (e.g.
approximately 51 cfs in February 1969). Surface water flow in San Timoteo Creek at the San
Timoteo Canyon Road crossing has been recorded on a weekly basis since January 2002 (see
Figure 14). Baseflow in this creek ranges from approximately 2 to 4 cfs and is supported by a
combination of wastewater discharges from the City of Beaumont’s WWTP and rising
groundwater. The highest observed streamflow at this location was approximately 98 cfs, which
was recorded in January 2005, a period of significantly high precipitation and runoff.

It is noted that some baseflow in San Timoteo Creek is likely from shallow groundwater
discharging to the stream. Groundwater levels in a well located adjacent to the stream and
downstream of its confluence with Noble Creek have historically been above the land surface
(flowing artesian). Additionally, a water quality analysis of surface water in the San Timoteo

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