An Adaptive Environmental Assessment and Management workshop process was used to assist Grand Canyon scientists and managers in developing conceptual and simulation models for the Colorado ecosystem affected by Glen Canyon Dam. This model examines ecosystem variables and processes at multiple scales in space and time, ranging from feet and hours for benthic algal response to diurnal flow changes, to reaches and decades for sediment storage and dynamics of long-lived native fish species. Its aim is to help screen policy options ranging from changes in hourly variation in flow allowed from Glen Canyon Dam, to major structural changes for restoration of more natural temperature regimes. It appears that we can make fairly accurate predictions about some components of ecosystem response to policy change (e.g., autochthonous primary production, insect communities, riparian vegetation, rainbow trout population), but we are moderately or grossly uncertain about others (e.g., long-term sediment storage, response of native and non-native fishes to physical habitat restoration). Further, we do not believe that existing monitoring programs are adequate to detect responses of native fishes or vegetation to anything short of gross habitat changes. Some experimental manipulations (such as controlled floods for beach/habitat- building) should proceed, but most should await development of better monitoring programs and sound temporal baseline information from those programs.

Between 1999 and 2005, drought in the western United States led to a >44 m fall in the level of Lake Powell (Arizona-Utah), the nation's second-largest reservoir. River discharges to the reservoir were halved, yet the rivers still incised the tops of deltas left exposed along the rim of the reservoir by the lake-level fall. Erosion of the deltas enriched the rivers in sediment such that upon entering the reservoir they discharged plunging subaqueous gravity flows, one of which was imaged acoustically. Repeat bathymetric surveys of the reservoir show that the gravity flows overtopped rockfalls and formed small subaqueous fans, locally raising sediment accumulation rates 10–100-fold. The timing of deep-basin deposition differed regionally across the reservoir with respect to lake-level change. Total mass of sediment transferred from the lake perimeter to its bottom equates to ~22 yr of river input.