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in 1913, strearn deposits in the western portion of the Sylmar Basin Subarea provided an

avenue of exit of subsurface water from this area. However, since completion of this
reservoir and Upper Van Norman Reservoir in 1921, the naturally occurring subsurface
outflow has essentially been cut off for practical purposes (Brown, 1975). Further, according
to Brown (1975), a limited amount of subsurface outflow only takes place at Sylmar Notch,
near the Mission well field, and Pacoima Notch, near Lopez Dam. This outflow apparently

occurs as subsurface flow cascading over the fault trace and truncated impermeable units.

8.3.3 Water Level Fluctuations

Prior to about 1960, the CDWR (1964) reported that water level data from wells located
near Castaic Junction and in the downstream portion of the Newhall-Saugus area exhibited

only minor fluctuations over a 30-year time span. However, water levels at wells in the
upstream portion of the Santa Clara River valley were reported to have declined by 40 to
100 feet below previous levels (CDWR, 1964). Data for 1985 indicated water levels ranging
from 35 to 60 feet bgs in the area of the South Fork of the Santa Clara River, and between
10 and 25 feet bgs within the main river drainage in the lower portion of the subunit (Slade,
1988).

Hydrograph data after about 1960 for wells screened within alluvium, Saugus Formation,
and screened over both units (combination wells) were reviewed by Slade (1986) to assess
groundwater storage due to changes in basin-wide recharge and discharge within the upper
Santa Clara River Basin. Hydrographs indicated that changes in groundwater storage occur

both within the short-term (seasonal) and in the long-tenn (period of several years). Such
changes occurred rapidly and to a greater degree in wells screened in the alluvium (Slade,
1986). Groundwater level fluctuations were significantly less pronounced in the wells
screened in the Saugus Formation and the combination wells (Slade, 1986). According to
Slade (1988), it is possible that the less pronounced changes in the Saugus Formation

aquifer and combination wells may be because: 1) the aquifer is generally under confined
conditions; 2) presumably large quantities of groundwater are in storage; and 3) there are
a limited number of wells tapping this aquifer as a source of supply. According to Slade
(1988), water level fluctuation in response to infiltration of direct precipitation or runoff was
observed to be rapid regardless of the location of alluvium wells along the Santa Clara
River.

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