The influence of Lake Powell on the suspended sediment-phosphorus dynamics of the Colorado River inflow to Lake Mead

摘要

The Colorado River has been successively modified by the construction of several reservoirs, beginning in 1935 with the formation of Lake Mead by Hoover Dam. These reservoirs are located in a chain, and each one has an influence on the nutrient dynamics and productivity of the river and downstream reservoir. Lake Mead derives 98% of its annual inflow from the Colorado River. Historically, the Colorado River inflow was unregulated into Lake Mead. Regulation occurred in 1963, when Lake Powell was impounded by the construction of Glen Canyon Dam, approximately 450 km upstream. The formation of Lake Powell drastically altered the physical characteristics of the Colorado River inflow to Lake Mead. Regulated releases from Glen Canyon Dam have eliminated the spring discharge peaks that historically resulted from spring flooding in the Upper Colorado River drainage basin. Temperatures in the Colorado River below Lake Powell have been reduced 5-10°C during the spring and summer, due to cold hypolimnetic releases from Glen Canyon Dam. There were also marked reductions in the suspended sediment loads due to decreases in spring and summer discharge peaks. The turbid overflows that once extended across the Upper Basin of Lake Mead during spring were not evident in 1977-78. The Upper Basin of Lake Mead is now severely phosphorus deficient, and this appears to have been caused by reductions in suspended sediment loading.Phosphorus has been reported by many investigators as the most common nutrient limiting phytoplankton productivity. Phosphorus loading models are generally based on total phosphorus (total-P), but this fraction may not accurately reflect the amount of phosphorus available for biological uptake in turbid river systems. Total-P loading models greatly overestimate the trophic states in Lake Powell and Lake Mead.Little emphasis has been placed on the interaction between suspended sediments and dissolved inorganic phosphorus in rivers. The removal of inorganic phosphorus by suspended sediment, however, does appear to be a sorption rather than a precipitation process. Loosely bound phosphorus on suspended sediments is more readily available than precipitated phosphorus. Wang and Brabec, in their work on the Illinois River at Peoria Lake, found that dissolved inorganic phosphorus was actively adsorbed by suspended sediments. Other workers have also observed this process occurring in oxygenated rivers and lakes. Mayer and Gloss have shown that phosphorus buffering by suspended sediments in the turbid Colorado River is an important mechanism for sustaining the dissolved inorganic phosphorus pool in Lake Powell. It appears that this same mechanism occurred in Lake Mead when it received turbid inflows from the Colorado River.The intent of this paper is to discuss the possible effects that the formation of Lake Powell has had on the suspended sediment-phosphorus dynamics of the Colorado River inflow to Lake Mead. This is based on results from recent investigations and on preliminary results of research conducted in the late-summer and early-fall of 1981.