Yellowtail Dam & Bighorn Lake Reservoir Operating Criteria Proposed Modifications Lovell, Wyoming July 29, 2008 RECLAMATION Managing Water in the West

Operational Goals Desired Lake Levels –Minimum Lake Level during recreation or above above 3620 –Optimum Lake Level during Recreation Season to 3640 Desired River Flow –3,500 cfs – Optimum Flow (only possible in well above normal runoff yrs.) –2,500 cfs – to Provide good spawning and rearing conditions in all major side channels –2,000 cfs - to Provide limited spawning and rearing conditions in most side channels –1,500 cfs – to Provide only main channel habitat and no side channel habitat –Maintain 1,500 cfs as a minimum during drought years General –Transparent method for establishing fall and winter river release –Improved method for estimating fall and winter gains

Proposed Modified Lake Level Targets Present Modified End of March Target (+29 KAF) End of October Target (+34 KAF)

Fall & Winter Release Rate Improve method for estimating fall and winter Gain Provide somewhat higher priority to Lake before setting river release above 2500 cfs Provide somewhat higher priority to river flows when releases are set below 2,000 cfs Develop Transparent method for determining fall and winter river release rate.

Fall and Winter Gain Gain declined between 1967 and 1990 Gain has been fairly stable since 1990 Reason for reduction in Fall and Winter Gains are Primarily due to reduced Groundwater recharge –Conversion from gravity to sprinkler systems –Conversion from open laterals to piped laterals –New Reservoirs in basin storing water during Fall & Winter (Greybull Reservoir)

Model Evaluation Period of Study End of March Target 3618 End of July Target 3640 End of Oct. Target 3638 Formal transparent method used to establish fall and winter river release Higher priority given to Lake Level when releases are above 2500 cfs Higher priority given to River Release when release is below 2000 cfs

Results Lake Level Summary Higher average Lake level 7.5 feet Higher End of May Lake Level 5.5 feet Percent of Time Historic Modified or above % 81% May- or above % 60%

Results River Flow Summary River Flow Percent of Time Historic Modified 2500 cfs or greater 45% 45% 2000 cfs or greater 59% 64% 1500 cfs or greater 94% 100% Less Than 1500 cfs 6% 0%

Results April Reservoir Drawdown April Drawdown Change in Elev. feet Historic Modified Diff Average