Georgetown Lake Update 8-7-2019

The lake level on July 31 was 6429.33 feet, which is above the normal of 6429.18 feet.  Outflow was around 45 cfs for most of July, but has been recently decreased to about 30 cfs.  Through July, the lake lost 0.25 feet (3 inches) and July outflow averaged 41.8 cfs.  Back-calculating for July shows lake inputs of 29.7 cfs.  The long-term average input is 43.9 cfs, therefore July 2019 inputs (groundwater, precip, surface inflows) were only 68% of normal.  Late July showed shorter 10-day periods averaging less than 50% of normal inputs.

 A quick glance through weather records shows that many days in July were warmer than normal.  The Climate Prediction Center forecast as of July 18 predicts elevated chances of above-normal air temperatures for the rest of the year, and near-average precipitation.  In the short term, the NWS weather forecast shows a cooling trend and chances of precipitation later this week.  At Peterson Meadows, July precipitation amounted to 1.7 inches which is above the 0.90 inch long-term normal.

Hopefully we get some more rain (no lightning) and things cool down a bit.  Over the past month, inputs have averaged 68% of normal, and this is currently the best estimate we have of future conditions.  So I ran the model using this 68% figure (see attached JPG).  The model output shows there is enough water to keep the reservoir at favorable levels through the end of the season.  If inputs continue at 68% of normal or less, there isn’t a lot of room for outflow adjustment in order to keep the lake in good shape for the November freeze.  If inputs improve, there could be a little more management flexibility for the next few months.

 Thanks, Dave

D. Amman, Montana DNRC, 444-6648


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