About Gray’s Bay Dam:

Gray’s Bay Dam is located at the outlet of Lake Minnetonka into Minnehaha Creek. The dam was built in 1979 to help control flooding on Minnehaha Creek and Lake Minnetonka.

The dam is managed by Minnehaha Creek Watershed District according to an operating plan:

• Developed over the course of 10 years with local, state, and federal partners and was approved by local municipalities and the Minnesota Department of Natural Resources
• Created using historical lake levels
• Designed to mimic natural conditions throughout the year and to account for changing weather
• Prescribes discharge zones depending on the time of year, the existing lake level, downstream capacity in Minnehaha Creek, and forecasted precipitation
• Operated spring through fall and closed during the winter season

The management goals of the dam are to:

• Reduce flooding
• Keep water in the creek during dry periods
• Enhance recreation, wildlife, and aquatic life
• Improve or maintain conditions on the lake and creek

The graph below visually represents the operating plan for the Gray’s Bay Dam. We aim to keep dam operations within Zone 4 as this is considered to be the desirable operation zone to achieve all management goals of the dam.

Learn more (video):

Record Flooding:

In 2014, the Twin Cities saw the wettest first half of the year since modern day record keeping began in 1871.

Coupled with a long winter and late snow melt, this extreme precipitation led to record water levels in 26 lakes and streams and more than $1 million worth of damages along the six main streams in the Minnehaha Creek Watershed District. For 83 consecutive days, Gray’s Bay Dam was unable to control water levels on Lake Minnetonka or Minnehaha Creek because the dam was underwater.

On June 23, 2014, Lake Minnetonka reached a record elevation of 931.11 feet above sea level, more than seven inches higher than the previous record, and Minnehaha Creek achieved a record flow of 889 cubic feet per second.

Since then, the Twin Cities has also broken a couple other precipitation records:

• 2016 was the second wettest full year on record
• August 2016 – July 2017 was the wettest 12-month period on record
• 2019 was the wettest year on record

You can view past lake levels from 1906 to 2017 either as an Excel spreadsheet or a PDF graph, and you can view lake levels from 2013 through the present. The Minnesota DNR has a list of all available ice out dates for Lake Minnetonka.

National Weather Service Partnership:

After the significant flooding in 2014 and facing climate change and increasingly unpredictable weather, we formed a formal partnership with the National Weather Service, Hennepin County, and the U.S. Geological Survey (USGS) with the goal of better anticipating rain events and understand their effects on water levels to more proactively manage the Gray’s Bay Dam. As part of the multi-agency partnership, the USGS, NWS, and Hennepin County provide us with:

• Real-time water level readings on Minnehaha Creek and Lake Minnetonka
• Seven-day precipitation forecasts, in six-hour increments, for the 125-square mile area that drains to Lake Minnetonka
• Hydrologic inflow model which predicts how the forecasted precipitation will flow into Lake Minnetonka from the upper watershed streams
• Lake level simulation model which forecasts how the predicted inflow will affect the Lake Minnetonka water elevation
• Real-time precipitation and weather attributes

This information allows us to plan and moderate dam discharge before large rain events based on the predictive Lake Minnetonka area forecast, which means:

• We proactively increase discharge before large rain events to create space in Lake Minnetonka for the forecasted precipitation
• We reduce discharge right before large rain events to provide capacity in Minnehaha Creek for the runoff. This results in less water flowing into the creek to try to prevent flooding in downstream communities
• During dry periods, dam discharge is lowered in an effort to keep water flowing into Minnehaha Creek so the creek doesn’t run dry

The success of this partnership was highlighted in 2016, which was the wettest year on record in the Minnehaha Creek watershed. Due to information provided by the National Weather Service, Hennepin County, and USGS we were able to operate the dam in such a way as to not experience any flooding on Lake Minnetonka or Minnehaha Creek, despite the record-breaking rainy weather.

Learn more (video):

How it All Works Together:

The USGS gauges provide real-time water level data that is used in conjunction with precipitation forecasts from NWS to populate the NWS Lake Minnetonka water level simulation model. Based on that data and the results of the simulation, we moderate dam discharge into Minnehaha Creek ahead of precipitation within the limits of the dam operating plan. After the rain, the USGS gauges help us track actual water level responses. Additionally, we receive actual rainfall reports from seven Hennepin West Mesonet weather stations from across the watershed which we use to calibrate the lake level simulation model to inform future rain events.

What Affects Creek Flow:

While dam discharge does affect the flow of Minnehaha Creek, the creek flow is also impacted by water running off impervious surfaces along the creek east of the dam. As a result, you may continue to see higher creek flows after a rain event, even if the dam discharge is lowered. As a general rule of thumb, a rainfall of about half an inch will result in approximately a 50 cfs (cubic feet per second) increase in creek flow. View real-time creek flows on the USGS website (note: the USGS refers to creek flow as “discharge” on their site).