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Effects of Glen Canyon Dam on Water in the Colorado River

photo of rafters going through rough rapids

Before

Before Glen Canyon Dam was built, flow rates, sediment loads, and water temperatures of the Colorado River varied widely from year to year and season to season.

Heavy runoff caused by melting of the Rocky Mountain snowpack commonly produced flows greater than 100,000 cubic feet per second (cfs) during late spring and early summer. In contrast, flows less than 3,000 cfs were typical in late summer, fall, and winter.

Sediment load increased during the spring runoff and again in late summer from tributary floods. Water temperatures ranged from near freezing in winter to more than 80 degrees Fahrenheit in late summer.

After

Glen Canyon Dam replaced seasonal flow variations with daily fluctuations, used to maximize efficiency of power generation.

Mean daily flows have exceeded 30,000 cfs only about 3 percent of the time (compared to 18 percent before the dam) and have been less than 5,000 cfs only about 10 percent of the time (compared to 16 percent before the dam).

While seasonal flows became less variable, daily flows became much more variable, with the median difference between daily minimum and maximum releases ranging from about 12,000 to 16,000 cfs. This fluctuation could mean a difference of 10 feet between low and high water on a given day on the river.

(Since 1991, Interim Operating Criteria--adopted to protect downstream resources pending completion of environmental studies--have held maximum flows to 20,000 cfs and daily fluctuations to about 5,000 to 8,000 cfs.)

The dam greatly reduced the river's sediment load, now supplied only by downstream tributaries, and resulted in a nearly constant water temperature, averaging a cool 46 degrees Fahrenheit year-round.

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