8013

• UCREFRP Catalog Number: 8013
• Author: Pontius, D. and I. SWCA.
• Title: Colorado River Basin Study.
• USFW Year: 1997.
• USFW - Doc Type: Tucson, Arizona.
• Copyright Material: NO

Colorado River Basin Study

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

Principal tributaries to the Colorado River upstream of Glen Canyon Dam
include the Green, San Juan, Escalante, Gunnison, and Dolores rivers.
Principal tributaries between Glen Canyon and Hoover Dams include the
Paria, Virgin, and Little Colorado rivers. Downstream of Hoover Dam are
the Bill Williams and Gila rivers. Major tributaries to the Colorado River
are depicted in Figure 3.

Historically, the flow of the Colorado River, measured at Lee Ferry,
fluctuated widely from year to year, season to season, and over long periods
of time. Annual flows in excess of 24.0 million acre-feet (mat) and less than
6.0 mafhave been calculated (Harding et al., 1995). The long-term historical
average measured is 14.95 mafbut tree-ring studies covering hundreds of
years suggest a long-term average annual flow of about 13.5 maf (Stockton
and Jacoby, 1976). Most of the flow for the Colorado originates in the Upper
Basin, which encompasses some 109,800 square miles. About 86 percent of
the annual runoff originates within only 15 percent of the area, in the high
mountains of Colorado (Stockton et al., 1991).

The natural flow of the Colorado followed a distinct seasonal pattern, with
more than 70 percent occurring in the months of May, June, and July
(Harding et al., 1995). Historically, floods of May and June peaked at
greater than 86,000 cfs (Collier et al., 1996). Since flows are now regulated
by several major dams, peak flows are significantly lower. Over the last
20 years (with Glen Canyon Dam in place), peak daily discharges at Lee
Ferry averaged 20,005 cfs in May and 25,735 cfs in June (USGS, 1996).

Over the past century, the bulk of the highest flows occurred in a "wet"
period from 1896 to 1930, when the average annual natural flow during this
period at Lee Ferry was about 17 maf/yr. By contrast, the average flow from
1930 to 1996 was about 13.9 maf/yr (Merritt, 1997). In the early 1980s,
heavier than anticipated runoff led to releases from Lake Powell of over
90,000 CfS2 and the emergency installation offlashboards on top of the
spillway gates and Hoover Dam spilled for the first time since 1941. Flood
control releases totaling over 50 maf led to substantial flooding and damages
along the lower river. The wettest 10-year period on record (1914 to 1923)
saw an average annual flow of 18.8 maf. This period is especially significant

2 According to Dave Wegner, releases peaked at 97,300 cfs on June 23, 1983 (Wegner,
1997).

6