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Water level update 10/7/20: Gray's Bay dam closed

Gray's Bay Dam Closed

Gray’s Bay dam was closed on Monday, October 5, as required by the dam’s operating plan when Lake Minnetonka falls below 928.60 feet above sea level (“Zone 6” in the graphic below). The dam will remain closed unless the watershed receives enough rain to increase the lake level above 928.60 feet. The current level of Lake Minnetonka is 928.58 feet.

The Gray's Bay dam operating plan (visually shown in the graphic above), developed over 10 years with MCWD’s communities and the Minnesota DNR, lays out six management goals to reduce flooding risk while also maintaining healthy water levels during dry periods to protect ecological health and provide for recreation. MCWD operates the dam within the bounds of the plan to maintain reasonable water levels on Lake Minnetonka and Minnehaha Creek.
Due to the dryer-than-average weather the second half of 2020, the dam has been discharging at the minimum allowed rate since June 12, and for 65 percent of the 202 days that the dam was open this season.

Minnehaha Creek

Lake Nokomis (Minneapolis)

  • Yesterday the Lake Nokomis water level reading was 814.88 feet, which is approximately 2.64 inches below the concrete sill on the Lake Nokomis weir (see photo below)
  • Lake Nokomis' water level fell below the concrete sill elevation of 815.10 feet around September 22, 2020
  • Thus far in 2020, Lake Nokomis has had water flowing out of the weir for 183 days
  • Minneapolis Park & Recreation Board and MCWD staff will continue to coordinate on the operation the Lake Nokomis weir and anticipate that the weir will remain open unless significant rain is forecasted

Wide Variation in 2020 Precipitation

Precipitation across the state of Minnesota has varied widely in 2020. Southwestern Minnesota has been exceptionally dry while some northwestern Minnesota weather stations have recorded the wettest year to date. Currently the Twin Cities area has received slightly below average precipitation with 25.40 inches of precipitation to date in 2020, which is 11.72 inches lower than the same timeframe in 2019. The past three months (July - September) all received below average precipitation and the month of September was the 16th driest on record with only 0.88 inches of rain. 

Shown below is the U.S. Drought Monitor map for Minnesota, which illustrates that approximately 45% of the state is demonstrating abnormally dry and moderate drought conditions, including Hennepin County and the Minnehaha Creek Watershed District. 

This variation in precipitation across the state has also been observed across the 178 square miles of the Minnehaha Creek Watershed District. The western part of the watershed or "upper watershed" has received less rain than the eastern part of the watershed or "lower watershed". This variation is visible on the DNR precipitation ranking map below which shows the western part of Hennepin County/MCWD ranks near the 20th percentile for precipitation ranking from April 1 - October 6, meanwhile the eastern part of Hennepin County/MCWD ranks near the 40th percentile for precipitation ranking during the same time period. 

Real-Time Measurements Inform Dam Operation Decisions

To better track the variability of precipitation and the response it creates across the watershed, MCWD is in the midst of a partnership with Hennepin County Emergency Management (HCEM) to install a real-time sensor network (RESNET) that includes over 20 new real-time water level and flow sensors across the watershed (locations shown below). Coupled with HCEM's Hennepin West Mesonet weather stations and tailored weather forecasts from the National Weather Service, this network of sensors and forecast data provides an unprecedented level of detail about how much precipitation has fallen across the watershed and how that precipitation flows through the watershed system. This allows MCWD to further fine-tune how it proactively operates Gray’s Bay dam ahead of storms or during dry periods in order to balance the needs of Lake Minnetonka and Minnehaha Creek.

For example, in 2020 MCWD leveraged this newly-available data on water levels and weather to identify that the dryer and warmer weather this past spring and summer would impact the water flowing into Lake Minnetonka from its many tributaries, and as a result the water coming into the lake wouldn't be enough to offset the loss of water to summer evaporation. As a result, MCWD proactively reduced dam discharge to the minimum-allowed rate at the end of April and into mid-May, and from June 12- October 4.

Lower Water Levels Driving Bumper Crop of Wild Rice at Minnehaha Creek Headwaters

Wild rice that periodically appears at the headwaters of Minnehaha Creek (where Lake Minnetonka discharges into the creek through Gray’s Bay dam) is out in full force in 2020. It is one of many water bodies across Minnesota and Wisconsin experiencing bumper crops of wild rice this year due to lower water levels. The picture below is from October 5 and shows the wild rice at the Minnehaha Creek headwaters after it had gone to seed.

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