7423 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I a I I I I I' ,I I I SECTION IV: CHANGES SINCE DAM CONSTRUCTION Dramatic changes have taken place in Glen and Grand Canyons since Glen Canyon Dam was completed in 1963. Construction of the dam and flow regulation have changed the magnitude and timing of river flows, the amount of sediment carried by the river, and the temperature of the water. This, in turn, has sub- stantially changed the downstream riverine environment and associated recreation. Pre-Dam River Flows Bad Wide Seasonal Variations In Magnitude, Sediment Load, And Temperature Pre-dam river flows were characterized by low flows in fall and winter and floods in spring and summer. Spring floods from snowmelt runoff reached a peak in June, and ranged from 25,300 to 300,000 cfs. From 1922 to 1962, the annual volume of flow past the USGS gaging station at Lees Ferry averaged 11.7 maf and ranged from 2.5 to 19.2 maf. In 1953, a typical low-water year (8.79 maf), flow was above 31,500 cfs from late May until the end of June, with a peak of about 70,000 cfs. For most of the rest of the year, flow was very low--typically in the range of 3,000 to 8,000 cfs (Fig- ure IV-1). In 1957, a typical high-runoff year (17.3 maf) , flow reached 126,000 cfs, and was above 31,500 cfs from the beginning of May until early August. Except for short periods of tributary flooding, flow was in the range of 5,000 to 10,000 cfs for the rest of the year. Change in discharge during any given day was small. Annual suspended sediment load past Lees Ferry averaged 65.4 million tons in the period 1948 to 1962, about four to five times the average annual suspended sediment load delivered to the river by the three major tributaries below Lees Ferry. The amount of sediment carried in the river increased during the high flows of snowmelt runoff, but typically reached highest values during tributary floods in the late summer. Sediment carried by the river wass~fficient to replenish beaches scoured by spring floods. 23