WSP04917 - Laserfiche WebLink

Home Browse Search

of concern (e.g., humpback chub) may also require consideration, as may culturally significant sites. Runoff Scenario 3: This scenario is an extreme condition. It assumes high Lake Powell and Lake Mead reservoir conditions, and high inflows to Lake Powell. Since there would be little flexibility to accommodate additional inflow by storing or making releases within powerplant capacity, the risk of an unplanned spill would be quite high. Inflows to, and releases from, Lake Powell may remain very high for an extended period in 1999. Under extremely wet conditions, large-volume transfers from Lake Powell to Lake Mead may be unavoidable. Such transfers could require extended periods of high flows well above the 33,200 cfs powerplant capacity. This scenario would be generally considered an uncontrolled spill (act of God) that may not be fully anticipated under the FY99 Annual Operating Plan. Hence, a high degree of uncertainty exists under this scenario, and additional planning meetings among scientists and resource managers will be required to mitigate high flow impacts for both the Colorado River ecosystem and water and power needs. For sediment conservation, high releases should be conducted as early as possible to store sediments at the highest possible stage elevations. ADAPTIVE CONTINGENCY FLOWS Given the potential of the above three 1998 Water Year scenarios, GCMRC has drafted the following possible hydrographs. GCMRC anticipates Water Year 1998 conditions will fall somewhere between Scenario I and 2 (above), and therefore has provided a detailed description of these scenarios. Possible Hydrograph Design in Response to Scenario 1: Limit flows to less than 25,000 cfs and reduce sediment export and sand bar erosion to the extent possible. Flows below 20,000 cfs are consistent with sediment conservation objectives and data from the Interim Flows period. Such flows may help preserve sediments recently supplied by tributaries, as may help conserve other resources as well. Possible Hydrograph Design in Response to Scenario 2: This scenario allows a BHBF of up to 44,000 cfs for 2-4 days, followed by the lowest possible constant or fluctuating flows for the remainder of Water Year 1999. Conducting this BHBF during late March/early April may be most conservative of the most river resources. Unlike the 1996 BHBF no flows of 8,000 cfs are proposed. Rather, the preceding and subsequent flows would be held constant at the mean flow, or at the lowest regularly achieved minimum flow for three days to allow for photo- documentation. This hydro graph would provide a preliminary high flow to move sand up to the 44,000 cfs stage along the channel, protecting sediment from subsequent high, steady powerplant releases. This hydrograph would provide an additional scientific benefit by allowing researchers to test conclusions of the 1996 BHBF regarding modes and rates of deposition. Potential [WORKING DRAFT 2 . GCMRC: 12/04/98] 6