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0025, 6 ,. DOWNSTREAM RESOURCE CRITERIA FOR SEDIMENT CONSERVATION FLOWS GLEN CANYON DAM, AZ FEBRUARY 1998 Introduction In the October 1996 Record of Decision (ROD) for the Glen Canyon Dam Environmental Impact Statement (EIS), the Secretary of the Interior selected the Modified Low Fluctuating Flow (MLFF) Alternative for operations of Glen Canyon Dam. The MLFF includes provisions for doing special springtime releases, either a habitat maintenance flow (HMF) or a beachlhabitat- building flow (BHBF). While sediment conservation is a primary and common objective of the two special flows, there are important differences between them (see Table 1). Habitat maintenance flows were to be within powerplant capacity (usually somewhere between 31,000 - 33,200 cfs) and were designed with the objective of maintaining existing camping beaches and fish habitat from year-to-year. In contrast, BHBFOs were defined as high sustained flows with the objectives of moving and depositing more sediment at higher elevations than HMFOs and completing a more extensive reforming of riparian and backwater fish habitats, while restoring some of the natural system dynamics along the river corridor. The frequency of BHBFOs was to be 1 in every 5 years. For the MLFF alternative, it works out that a BHBF would always be above powerplant capacity, (something in the range of 40,000 - 45,000 cfs). Both flows were restricted to years when the level of Lake Powell was 19 million acre-feet, or less, as of January 1. At levels above 19 maf, itOs more likely that scheduled releases would be at or above powerplant capacity, and increasing the releases even more would represent an undesirable bypass of the powerplant. Also, neither flow was to be scheduled in a year when there was concern about adverse impacts to sensitive downstream resources, such as sediment or endangered species. Recognizing uncertainies about the best time to do HMFOs, the best time, duration and magnitude for BHBFOs, the EISIROD left it to the Adaptive Management process to test and confirm those elements. In the EIS, antecedent criteria for BHBFOs included the requirement that there be enough sediment available to replenish sandbars to an elevation that makes them several feet above the water surface for use by wildlife and for camping purposes. Leopold (1969) and Andrews (1991) estimated the magnitude of sufficient discharge to be between 40,000 and 50,000 cfs to initiate movement of substantial amounts of sand and subsequently rebuild sandbars. A test of a BHBF took place in late March/early April of 1996, prior to completion of the EIS and ROD. The seven-day test flow of 45,000 cfs demonstrated that while sandbars were rebuilt, the other objectives were not met (GCMRC 1997). Research results indicated that flows of this magnitude conserve sediment by moving it from the channel and depositing it on the channel margins (Parnell et al1996), but flows of this magnitude were insufficient to scour and rejuvenate backwaters, or to reset successional clocks associated with marsh communities. Other resources showed no subsequent adverse affects (6 months later) to this discharge (Kearsley and Ayers 1996; AZGF 1996; Shannon et alI996).