Georgetown Lake Update 8-31-2018

This morning the water surface elevation is 6429.22 feet, which is much higher than the median August 31 elevation of 6428.63 feet.  Over the last 10 days, the lake has only dropped 0.14 feet (about 1.7 inches). Outflow during that period averaged 56.8 cfs, and there was a storm that dropped about half an inch of rain at Peterson Meadows. Coupled with high humidity and typically cooler temperatures, a small storm like that can have a significant effect on lake inputs and reduced lake evaporative losses.

Over the 10 day period (August 21 through August 30), inputs averaged 35.8 cfs, which is 170% of the long term August average. This figure will fluctuate with air temperatures and precipitation, and will gradually recede until another significant storm event. Recall that earlier in the month, inputs averaged 134% of normal.

I ran the model using conservative inputs of 140% and 100% of normal going forward. See the attached model outputs. Either scenario shows a surplus water supply and elevated outflows.  Using the 140% figure and outflow of 56.7 cfs continuing through September still leaves the lake in very good shape through autumn and winter. 

Using the normal (100% inputs) figure shows that an elevated outflow may still be possible for September, but with less margin for adjustment.  At this time, the lake is in great shape for autumn and through the winter.

Lake levels and weather conditions will require vigilance and adjustments to outflow in order to get the lake at the appropriate level for the November freeze. Typical winter water surface elevation is around 6428.30 to 6428.50 feet

Thanks, Dave

D. Amman, Montana DNRC, 444-6648


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