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Water level update 12/4/20: Gray's Bay dam closed and winterized

The Gray's Bay Dam has officially been winterized and closed for the 2020 season. With assistance from the City of Minnetonka, crews put in aluminum stop logs that protect the dam's equipment until spring ice out. The dam has been closed since October 19, 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 final lake level reading for 2020 was taken today, December 3, and was 928.51 feet above sea level. 

The Gray's Bay dam operating plan, developed over 10 years with the communities across the Minnehaha Creek Watershed District (MCWD) 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 set out in the plan (illustrated below) to maintain reasonable water levels on Lake Minnetonka and Minnehaha Creek. 

Due to the dryer-than-average weather during the second half of 2020, the dam discharged at the minimum allowed rate from June 12 until it was closed October 5, and again between October 12-18. 

Winterizing the Dam

The dam uses three control gates to manage the flow of water out of Lake Minnetonka. Over winter, MCWD raises these gates out of the water to prevent damage from ice and replaces them with aluminum beams known as "stop logs." These stop logs prevent water from passing during the winter months. The stop logs will be removed in the spring as ice conditions allow. 

Preventing flow during the winter months is important for preventing ice jams from forming on Minnehaha Creek. Prior to the dam’s existence, ice jams caused flooding along the creek, including a significant flood in 1966 that was the impetus for installing the current adjustable structure. 

2020-21 Winter Outlook

On October 15, 2020 the National Oceanic and Atmospheric Administration (NOAA) released their 2020-21 winter forecast for the United States. NOAA’s drought monitor map identifies that this fall parts of Minnesota, including MCWD, were “abnormally dry” according the U.S. Drought Monitor. NOAA’s precipitation outlook for this winter (shown below) forecasts a 40% chance of wetter than normal precipitation across most of Minnesota, including MCWD. NOAA’s temperature outlook forecasts an equal chance of above or below normal temperatures this winter. 

Winter Water Level Monitoring 

MCWD monitors water levels on Lake Minnetonka throughout the winter season in order to prevent or reduce spring flooding. In 2019, for example, MCWD determined that the lake level had risen significantly over the winter and based on the snowpack that more storage capacity was needed in the lake to reduce the risk of spring flooding. With the help of Tonka Bay Marina, aerators were installed at Gray's Bay Dam in 2019 to break up the ice, allowing the dam gates to be opened and begin discharging before ice-out on the lake. This helped create room in Lake Minnetonka for spring snowmelt and rain and avoided flooding on Minnehaha Creek and Lake Minnetonka.

Over the winter as MCWD tracks water levels, we also coordinate with staff from the National Weather Service, U.S. Geological Survey, and Hennepin County Emergency Management to understand spring flooding predictions, water content of snow, current stream flows, and if necessary organize emergency coordination efforts.

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, tailored weather forecasts from the National Weather Service (NWS), and U.S. Geological Survey (USGS) sensors, 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, June 12- October 4, and from October 12-18. 

MCWD's real-time sensor network and partnership with the NWS, USGS, and HCEM was featured as a "Community Highlight" on page 39 in the recently adopted 2020 State Water Plan: Water and Climate prepared by the Environmental Quality Board. 

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