Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

groundwater recharge or underflow. Monthly discharge data for the three gauge stations
listed in Table 3a appear in Appendix 3 and are summarized as water year totals.
Figure 9 depicts the streamflow hydrograph for the entire record (1966 to current)
of Pilarcitos Creek near Half Moon Bay, for Purisima Creek (1959 to 1969), and for
Pilarcitos Creek below Stone Dam (1998 to 2001). The average monthly discharge for
Purisima Creek (201 acre-feet [AF]) and upper Pilarcitos Creek (195 AF) are
comparable, while the average monthly discharge for Pilarcitos Creek near Half Moon
Bay is nine times greater, reflecting its larger drainage area.
Figures 10 and 11 compare monthly discharge for the records of coincidence of
Pilarcitos Creek near Half Moon Bay with Pilarcitos Creek below Stone Dam and
Purisima Creek, respectively. Note that for the period of coincidence (Figure 10) the
baseflow (about 100 AF per month) measured for Pilarcitos Creek at the Half Moon Bay
is about ten times the baseflow for Pilarcitos Creek below Stone Dam (about 10 AF per
month). The baseflow indicated at the Half Moon Bay gauge reflects the inflow of surface
water and groundwater into Pilarcitos Creek in its upper canyon and into its major
tributaries above the gauge.
Table 3b summarizes the average discharges for the entire streamflow record
and for the period of coincidence for Pilarcitos Creek below Stone Dam (1997 to 2001)
and Purisima Creek (1966 to 1969).
Figure 12 shows average monthly discharge for all three stations. As shown,
January, February, and March are the months with the highest streamflow, contributing
more than 60 percent of the total yearly discharge. In contrast, less than 10 percent of
the streamflow is contributed during September, October, and November.
Geologic Setting
The upper Pilarcitos Creek watershed is composed primarily of igneous,
metamorphic, and sedimentary rocks, recent alluvium, and colluvium (Figure 13). The
Lower Pilarcitos Creek groundwater basin, situated on a westward sloping marine
terrace incised by Pilarcitos Creek (Lajoie et al., 1979), is composed of four important
hydrogeologic units. These are consolidated sedimentary rocks (Purisima Formation),
marine terrace, alluvial fan, and recent surficial deposits. The recent deposits include
recent alluvium deposited by Pilarcitos Creek, beach and dune sands, colluvium, and
artificial fill.
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