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0U2546 Water Year Scenarios. The Colorado River Basin is anticipated to be at or above nonnallevels during WY98, as both Lake Powell and Lake Mead are nearly full and a pronounced EI Nino/Southern Oscillation pattern may increase streamflow runoff throughout the Colorado River basin, Based on correlations with past historical data for similar EI Nino events, GCMRC believes there is an increased likelihood that WY98 may produce at or above nonnal late S pring runoff into an already full Lake Powell, Thus, the possibility exists that flows from GCD may exceed 25,000 cfs in 1998 and may include unplanned spills with flows of 45,000 cfs or higher. Proposed Plannine Process and Schedule. The AMWG may wish to implement the following planning process and schedule: 1. Develop "risk of spill" criteria (September-October 1997); 2. Draft a hydrograph for a BHBF (October - November 1997); 3, Refine the objectives ofa BHBF (i.e" mitigate the negative effects of unplanned spills or high steady flows vs. an objective to enhance specific resources ), evaluate the "draft BHBF hydrograph" for positive and negative resource effects, and revise the timing and shape of the draft BHBF hydrograph as appropriate (November - December 1997); 4, Determine compliance (environmental and cultural) requirements and initiate needed compliance activities (November 1997), and design 1998 releases to meet upstream storage needs and downstream obligations; 5. Evaluate forecast, winter snowpack development, and State-of-the-CoIorado River Ecosystem Resources (December 1997 - February 1998); 6, Conduct a BHBF, if appropriate (January - June 1998); 7, Continue to monitor the forecast and inflows to assess potential for an unplanned spill in the Spring/Summer (March - July 1998); 8, Assess the State-of-the- Colorado River Ecosystem Resources and WY 1999 (August _ December 1998). ' PLANNING SCENARIO Description of Flow. In spring 1996, an experimental BHBF from GCD was initially tested, The BHBF consisted of a controlled constant release from GCD of 45,000 cfs for seven days during late March and early April. These constant releases were immediately preceded and followed by three days of steady 8,000 cfs releases from GCD, The constant 45,000 cfs flow was achieved through a combination of releases from the powerplant and river outlet works, Neither of the two spillways at GCD was used during the BHBF, 2