EROSIONAL RESPONSE OF SUBAERIALLY EXPOSED DELTAS, LAKE POWELL, UTAH

Extended drought in the upper Colorado River Basin has reduced the Lake Powell reservoir to approximately 50 percent of full pool capacity and has exposed the entire 53-km length of the delta of the upper Colorado River from Gypsum Canyon to Hite Marina (just downstream from the confluence of the Colorado and the Dirty Devil rivers). Subaerial exposure has resulted in the rapid and complex modification of delta surfaces. Small circular domes and mud volcanoes (marking the locations of gas-charged springs arising from decaying vegetation within the 50-m thickness of deltaic sediment) are widely distributed across most exposed surfaces. Along the perennially flowing Colorado River, channel incision has generally kept pace with continued lowering of the reservoir, and the river channel is smoothly and continuously incised along most of the length of the delta. As the reservoir continued to recede during the fall and winter of 2002 however, at least four stretches of rapids developed in the two-mile reach immediately upstream of the newly exposed delta front. In most areas upstream from the Dirty Devil confluence, channel incision has triggered large-scale slumping of water-saturated channel banks that fully involve the delta surface from canyon wall to canyon wall. Response along the delta of the intermittently flowing Dirty Devil has been more complex. Channel incision was delayed until early in the fall of 2002 when streamflows reached the newly exposed delta surface causing entrenchment of the upper delta and deposition across lower delta areas. Continued low levels of streamflow coupled with continued recession of the reservoir have initiated the upstream migration of a low irregular knickpoint across lower delta areas. Middle reaches of the delta remain undissected.