7409

• UCREFRP Catalog Number: 7409
• Author: U.S. Department of the Interior.
• Title: Quality of Water, Colorado River Basin.
• USFW Year: 1991.
• USFW - Doc Type: Progress Report No. 15,
• Copyright Material: NO

Chapter 5

Historical Salinity Conditions

METHODS

Salinity in the Colorado River Basin is
monitored at 20 key stations. The average
concentrations and loads were determined on a
flow weighted basis using daily data whenever
possible. Salt loads and concentrations were
generally calculated from daily conductivity and
flow records using methods developed jointly
between Bureau of Reclamation (Reclamation)
and U.S. Geological Survey (USGS) [8].

Historical streamflow, salinity concentrations,
and salt load data at 20 key stations for January
1941 through December 1989 are presented in
tables 1 through 20 at the end of this report.
Figure 1 shows the historical salinity at Imperial
Dam.

HISTORICAL SALINITY

Salinity concentrations at Imperial Dam
decreased steadily from 1970-79, dropped
notably in 1980, increased sharply in 1981-82,
and dropped again in 1983-84. The 1970-80
salinity concentrations Sh9w the buffering of
annual fluctuations in salinities due to the effect
of nearly 50 million acre-feet of reservoir
storage. With the reservoir storage in the
Colorado River at near capacity, discharges from
Hoover Dam increased from 7.7 million acre-feet
in 1979 to 11.1 million acre-feet in 1980 (see
tigure 2), diluting the salinity at Imperial Dam
temporarily. With more normal flows in 1981
an.:l1982, the salinity rebounded. Higher
rele.lSes from Hoover and Glen Canyon Dams in
1983 and 1984, combined with lower salinities in
storage, caused salinity at Imperial Dam to drop
again. With the nearly 50 million acre-feet of
high quality water in storage and the relatively
high runoffin the basin, salinities at Imperial
Dam remained low through 1987 but have begun
to increase due to below normal runoff in the

last few years. Under average hydrologic
conditions, salinity would have been about
873 mg'lL.

FACTORS IN SALINITY TRENDS

The downward fluctuation of salinity at Imperial
Dam during the 1970's is within the expected
range and was simulated using the Colorado
River Simulation System (CRSS), a computer
model of the Colorado River Basin. Several
factors complicate the analysis of the decline and
leveling off of salinity concentrations. Most of
these factors are modeled using CRSS and
include variations in runoff, reservoir storage,
reservoir operations, salt pickup, and depletions
due to development of the basin. These, and
other factors which may cause shifts in salinity,
are discussed in the following sections.

Hydrologic Conditions

The salinity in rivers generally decreases with
increased flow. Years of lower than average flow
are characterized by higher than average
salinity. This in combination with the time lag
caused by reservoir storage suggests the decline
of salinity concentrations may have been in part
caused by the transition from a relatively drier
period (1955-65) with an annual virgin
(undepleted) flow of 13.23 million acre-feet at
Lees Ferry, to a relatively wetter period
(1965-75) with an annual virgin flow of
14.76 million acre-feet. This increase in the flow
(10 percent) may be responsible for a portion of
the decrease in the salinity concentration
observed at Imperial Dam.

The 1983-87 period was even more extreme with
some flows well above those ever recorded. This
was probably the major cause of the near
200 mgIL drop in salinity during that period. A