GEORGETOWN LAKE UPDATE – August 12, 2024

​Current Lake Elevation 6428.70 (PROVISIONAL DATA from USGS gage 12325000)
The water surface elevation is approximately normal for this time of year, based on 80 years’ worth of lake elevation data.

Elevation and Inputs
Lake elevation in July and early August was lower than what it has been in recent years. The only years in the last decade lower were 2021 and 2015. Above average precipitation and inflows previously were contributing to higher lake levels since the end of March; however, this year’s forecasts for hotter, drier conditions were mostly correct. July saw a major drop in inflows, this led to a decrease in lake level from 6429.32 ft at the end of June to 6428.88 ft by July 31. Inflows for July were significantly below the long-term average. The lake lost 1309 acre-ft of storage last month. July average monthly outflows of 29.4 cfs were more than inflows making the average total lake inputs equal to 8.07 cfs. The inflows were 18% of normal for July.

Estimated daily inflows were consistently low in July getting so low that net inputs were zero (meaning evaporation and seepage were higher than surface inflows). The first two weeks of August have seen a slight improvement with lower temps than late July, the net inputs are estimated between 4 – 8 cfs. The daily, 1-week, and 2-week average inflows for August have all shown a similar pattern of WELL BELOW AVERAGE inflows at around 20% of normal.

Precipitation
Spring precipitation virtually halted at the end of June after a storm that improved water supply conditions momentarily. The Peterson Meadows SNOTEL site accumulated 3.2 inches of precipitation in June (which was 92% of normal for the month and mostly was from the storm at the end of the month). Peterson Meadows accumulated only 0.3 inches (38% of normal) in July. The Warm Springs SNOTEL site, which gives an idea of the higher elevation precipitation conditions, accumulated 2.6 inches of precipitation in June (57% of normal) and 0.2 inches (17% of normal) in July.

Climate Forecast
The latest CPC climate forecast (created July 18, 2024) shows above average air temperatures and below average precipitation for the 1-month outlook but approximately normal precipitation with hotter than average temperatures for the 3-month outlook. The 1-week forecast for Philipsburg shows milder August temps with highs in the 70’s and lows getting down to 40 degrees F. There is a chance of rain early in the week.

Model Forecast
The water surface elevation sits at 6428.70 feet as of today (8/12/2024). I used the end of July lake level of 6428.88 ft for this model run. Based on July inflows, NRCS June-September water supply forecasts, and DNRC’s stream gage data I am using 35% of normal for inflows. This value leaves room for slight improvement in inflows in the next month but otherwise keeps the trend that we’ve seen since the 4th of July. DNRC’s stream gage on the NF Flint Creek is showing significantly lower flows this year when compared to the historic record. With the lake not filling this year and the exceptionally low inflows, it is likely that following October 15, outflows will likely need to decrease to the minimum outlined in the FERC license (or lower) to maintain or fill the lake before freezing.
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Todd Blythe, Montana DNRC; Water Sciences Bureau; 406-438-0717; todd.blythe@mt.gov