WSP08775

• File Number: 8056
• Description: Drought Preparedness
• State: CO
• Basin: Statewide
• Date: 1/1/1979
• Author: USGS
• Title: Hydrologic and Human aspects of the 1976-77 Drought
• Water Supply Pro - Doc Type: Report/Study

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CHRONOLOGY OF THE 1976-77 DROUGHT

69

of the San Francisco Bay system to encroach
farther upstream than usual. The point of
maximum encroachment is defined as the point
farthest upstream where the chloride con-
centration reaches 1 ,000 mg/L (milligrams per
liter), Water of this salinity, when used for
irrigation over a considerable period of time,
will severely inhibit the yield of most crops.
Comparative chloride concentrations are
19,000 mg/L for sea water and near 7 mg/L for
Sacramento River water.
Figure 29 is a map of the Delta showing the
maximum intrusion of salinity for selected
years, including 1976 and 1977, The encroach-
ment shown for 1931 and 1939, two dry years,
occurred prior to the completion of Shasta
Dam in 1944, Since then, fresh water has been
released from Shasta Lake to repel the salin-
ity. In 1952 and 1958, heavy flood runoff
occurred, and 1966 was a dry year, 75 percent
of average runoff, before the completion of
Oroville Dam in 1968. Runoff in 1975 was 113
percent of average.
Runoff into the Delta dropped to 60 per-
cent of average in 1976 and to 28 percent of
average in 1977, The salinity intrusion ad-
vanced about 7 mi in 1976 from the position in
1975 and about 4 mi more in 1977. To halt the
saltwater encroachment in parts of the Delta,
temporary earthfill barriers were erected in
1977 across several channels in the Delta, A
pumping plant was built to provide higher
quality water to the Contra Costa Canal, and
several diversions were changed on an interim
basis to points upstream to tap better quality
water for use within the Delta.
Operations of the upstream reservoirs of
the State Water Project and the Federal Cen-
tral Valley Project are coordinated to provide
water in the Delta for local consumptive use,
for exports by the State and Federal projects,
and to maintain water-quality standards set by
the State Water Resources Control Board.
Twice during 1977, once on February 8 and
again on June 2, the water-quality standards
were modified so that less water was required
for Delta outflow, thus conserving the short
supplies upstream so that they would be avail-
able later in the season to protect the water
quality in the Delta,
Seawater intrusion raised the chloride con-
centrations in wells near the mouth of the
pajaro River near Watsonville, Calif., about 70
mi south of San Francisco. The lack of

recharge and the increased pumping during the
drought caused water levels to decline an
average of 10 ft between November 1975 and
November 1977, Under normal conditions, the
water levels rise during the winter months to
elevations above sea level; but throughout the
drought, the water levels in most wells re-
mained below sea level.
This adverse gradient was not sufficient to
cause the intrusion to advance any farther
inland than approximately 1 mi which was also
observed in 1974, but the increases in chloride
concentrations were dramatic. In a well per-
forated in the 100- to 200-ft interval, the
chloride concentration rose from 114 mg/L in
November 1975 to 571 mg/L in November
1977. The maximum increase in a well tapping
the aquifer at the 300- to 600-ft level was 100
mg/L to a high of 229 mg/L,
The use in gardens, etc., of "grey water"
from bathing, dishwashing, and laundering was
approved in some localities and banned in
others as detrimental to public health. The
State Water Quality Control Board imposed
restrictions in April 1977 on several com-
munities discharging sewage effluent into the
Russian River because most of the fresh water
released from storage near the headwaters was
diverted and the waste discharged into the
river exceeded the controlled flow in the
downstream reaches. In southern California,
reclaimed water has been used for irrigation
and for recharge to aquifers, but the use of
less fresh water increased the salinity of
effluents enough to make it less desirable or
even unusuable for recycling.
High water temperatures associated with
low flows in September 1977 caused a fish kill
in the Trinity River below Clair En~e Lak8'
The water temperature rose almost 9 to 70 F
which is warm for trout,

Land Subsidence

Subsidence is a problem in the San Joaquin
and Santa Clara Valleys, It is directly related
to hydraulic stresses induced by ground-water
pumping and the resulting compaction of
water-bearing deposits. After three decades
of pumping overdraft and water-level declines,
the widespread subsidence in the San Joaquin
Valley was halted or significantly reduced
after 1968 when imported canal water re-
placed ground-water pumpage. The drought of