DEVELOPMENT OF A SEDIMENT BUDGET TO INFORM RIVER MANAGEMENT OF THE REGULATED SNAKE RIVER IN GRAND TETON NATIONAL PARK, WYOMING

The predominant contribution of geomorphologists to the environmental management of dams is the analysis of historical channel change. This approach provides insight into the drivers, mechanisms, and magnitude of channel change, and this information is essential in establishing targets of river restoration. However, analysis of channel change is not, in itself, a sufficient guide to establishing alternative dam release schedules to achieve restoration. The essential tool that geomorphologists can provide in guiding development of environmental management flows is a temporally and spatially explicit sediment budget keyed to alternative dam release schedules. In the case of the regulated Snake River in Grand Teton National Park, channel change analysis demonstrates that there have been periods of both channel narrowing and widening, and that there is no long-term progressive change. Nevertheless, accumulation of gravel near some tributary mouths is of concern to river navigation and accumulation of gravel in braided reaches drives channel avulsions and floodplain formation. In an effort to advise managers of Jackson Lake Dam, we have developed a sediment budget that can be tied to dam release schedules. This budget is based on calibrated gravel transport relations for Pacific Creek and Buffalo Fork, the major tributaries to the Snake River downstream from the dam, and a similar relation for the Snake River at the downstream end of the study reach. We measured gravel transport rates using a Toutle River sampler deployed from a raft during a wide range of flow conditions. We used the Parker (1978) relation, calibrated to our transport data, to estimate accumulation and evacuation of gravel in the Snake River during different flow contributions. Preliminary estimates suggest that only in years with large dam releases are Snake River flows capable of remobilizing all the sediment delivered by its tributaries.