Challenges in Assessing the Effects of Experimental Flow Regimes from Glen Canyon Dam on Fine Sediment Storage and Native Fish Populations in the Colorado River in Grand Canyon

The Colorado River ecosystem between Glen Canyon Dam (GCD) and upper Lake Mead, Arizona, provides a unique opportunity to test various ideas about river management and the use of adaptive management experiments to help resolve scientific uncertainties about best management practices. Beginning in the early 1990's, a variety of experimental discharge regimes from GCD have been implemented including the well-publicized 1996 controlled flood and the costly 2000 steady flow experiment (ca. \$21 million in lost power revenues). The experimental flows and the extensive monitoring, research, and modeling efforts have focused on quantifying the effects of flow on the storage of fine sediment in the Marble and Grand Canyon reaches of the Colorado River and on the survival and growth of native fish, with an emphasis on the endangered humpback chub (Gila cypha). Analysis of sediment and flow discharge data from natural hydrologic events and experimental flows has been more helpful in formulating current flow management regimes focused on sediment retention than results from single and multi-dimensional sediment transport models. Inferences from historical analyses have been limited by the resolution of sediment transport data, while inferences from multi-dimensional models have been limited by difficulties in scaling-up site specific results to reaches that are 10's to 100's of km long. Evaluation of the status and trends of exotic and native fish populations in Grand Canyon is highly uncertain because of the difficulties of conducting representative catch-per-unit-effort sampling in a large and turbid river with very difficult access, and because of multi-year delays associated with mark-recapture data. Application of stock assessment modeling procedures, originally developed for managing commercial fisheries, has been helpful for estimating population trends from the historical fisheries data, but not sufficient to resolve whether declines in native fish populations have been caused by the increasing abundance of exotic fishes, dam operations, or a combination of the two. Our ability to detect fish population responses to future experimental flows is weak in spite of the lessons learned from stock assessment modeling and expanded monitoring efforts. In contrast, near-term experimental flows proposed for 2002 through 2004 will likely be highly informative for distinguishing among alternate hypotheses about the response of sediment storage to dam operations.