RAPID PROGRADATION OF THE COLORADO AND SAN JUAN DELTAS INTO LAKE POWELL RESERVOIR, JULY 2002 TO JANUARY 2004

Extended drought in the upper Colorado River Basin has reduced Lake Powell to approximately 50 percent of full pool capacity and has exposed the deltas of the upper Colorado and San Juan Rivers at the upstream limits of the reservoir. Over the past two years, flow in both rivers has been relatively low (except for a few brief periods of flooding, generally less than 50 percent of median daily streamflow for the period of record). Subaerial exposure has resulted in complex erosional modification of delta surfaces and rapid progradation of delta fronts. Channel incision has been the primary erosional response to lowering of the reservoir pool, and delta surfaces are incised to depths of up to several meters along most of their lengths. This incision has triggered large-scale slumping of water-saturated channel banks that in many areas involve delta surfaces from canyon wall to canyon wall. Repeat measurements of delta front positions and aerial extents of delta surfaces (derived from Aster and Landsat 7 ETM image data and verified by ground based observation) document rapid advance of the deltas of both rivers between July 2002 and January 2004. During this period, the exposed surface of the Colorado delta advanced at an average rate of approximately 2.5 km/yr and the exposed surface of the San Juan delta advanced at an average rate of approximately 7.5 km/yr. On the Colorado delta (in the vicinity of Hite Marina), comparison of delta surface elevations with bathymetric surveys conducted in 1986 and 2001 indicates that, during the past three years, the rate of deposition on the front of the Colorado delta has been at least 2.5 times higher than the average rate before 2001. On the San Juan delta, lack of correlation between the occurrence of flash flooding (up to 20,000 cfs) and the timing of delta advance suggests that delta front deposition is not markedly affected by such floods. Therefore, it would appear that incision and subsequent slumping of recently exposed, water-saturated deltaic deposits during periods of low to moderate streamflow has been the principal sediment source for the rapid progradaton of both deltas.