Managing for a More Resilient Watershed

The Gray’s Bay Dam is operated through the spring and fall and is typically closed during the winter season. The operating plan prescribes discharge zones depending on the time of year, the existing lake level, downstream capacity in Minnehaha Creek, and forecasted precipitation.

The operating plan for the Gray’s Bay Dam was designed to mimic the hydrologic conditions of Minnehaha Creek before urbanization, but it also accounts for changing weather. As MCWD works with partners to build a more resilient watershed, more real-time and predictive water level and weather data inform dam operations.

Dam Operations

The Gray’s Bay Dam Operating Plan has seven “discharge zones” that dictate operating decisions based on the time of year, water level, and weather conditions. The figure below shows how dam discharge and Lake Minnetonka’s elevation change over time.

Hover over a point to read the elevation and flow at that time. You can compare Lake Minnetonka’s elevation to historic lake elevations, the dam’s runout elevation, and the dam spillway elevation by clicking the boxes above.

Discharge Zones:

Zone 1- Maximum Creek Capacity

When the Lake Minnetonka level is high (between 929.6 to 930.0), or is rising very quickly, MCWD releases discharge from the dam at the maximum allowable rate. The maximum allowable rate is calculated based on runoff and inflows to the creek to avoid exceeding the carrying capacity of the creek. Although MCWD aims to not exceed the maximum capacity of the creek, sometimes overflow is unavoidable due to water entering the creek from other sources downstream. If the lake level rises beyond 930.0 ft, MCWD must operate the dam in Zone 7.

Zone 2 Variable Outflow: Zero to Maximum Creek Capacity

In Zone 2, the operating plan prescribes different rates, ranging from zero to the maximum allowable discharge rate, depending on the lake level. When the lake level is below 929.1 ft, MCWD can let out a maximum discharge of 30 cubic feet per second (cfs). As the lake level approaches 929.6 ft, the maximum allowable discharge is allowed. When the lake level is between 928.6 and 929.1 ft, discharge may be less than 12 cfs to avoid adding water into the creek during the spring melt and to retain water for summer evaporation. When lake levels are predicted to rise rapidly and flooding is predicted, MCWD may start discharging the maximum allowed amount of water before the lake reaches 929.1 ft to reduce flooding. Any changes to the maximum allowable discharge rates must be approved by the MN Department of Natural Resources.

Zone 3 Major Outflow: 150 Cubic Feet per Second Minimum to Maximum Creek Capacity

In the late summer and fall, when the lake level is high, MCWD can release between 150 cfs and the maximum allowable discharge rate. This is done to increase the storage capacity of Lake Minnetonka in preparation for spring snowmelt.

Zone 4 – Optimal Outflow: 20 Cubic Feet per Second Minimum to 150 Cubic Feet Per Second Maximum

Zone 4 is considered the ideal zone for the operating structure, when MCWD is operating in this zone, we have the best conditions to meet the management goals of the dam. In this zone, dam discharge is determined to be between 20 and 150 cfs depending on the capacity of Minnehaha Creek. From May to July, if the lake level is at or below the Ordinary High Water (OHW) level of 929.4 ft, the dam can discharge water at a rate between 20 and 100 cfs. When the lake level rises above the OHW, then the dam may discharge water at a maximum of 150 cfs. Each season, between July and November, staff calculate how much water must be let out to reach the desired fall lake level of 928.6 ft based on normal weather conditions. MCWD will adjust the dam’s discharge periodically based on weather conditions.

Zone 5 Minor Outflow: Base Flow Approximately 12 Cubic Feet per Second

A base flow discharge rate is required within Zone 5 to reduce the detrimental effects of creek stagnation during dry periods. The base flow rate is designed to assure that the volume of water flowing out of the dam mimics historic conditions during low lake levels, therefore, a flow of 12 cfs is required in this zone.

Zone 6 No Outflow

When the lake level elevation is below 928.6 ft, no discharge is allowed during the open water season. The 928.6 level was determined based on extensive research from the Minnesota Department of Natural Resources (DNR) which concluded that 928.6 was the pre-settlement runout elevation of the lake. Below this level, water will not flow out of the lake, even if the dam is open.

Zone 7 Uncontrolled Outflow

When Lake Minnetonka’s level exceeds 930.0 ft, water flows unrestricted from the lake into the creek. In this zone, the discharge cannot be controlled by the Gray’s Bay Dam. When the lake level recedes to an elevation below 930.0 ft, outflow will again be limited to the carrying capacity of the creek (Zone 1).

MCWD adjusts the dam based on current and forecasted precipitation and water level data, in accordance with the dam’s operating plan. Changes to dam discharge are documented in the table below during the open water season.

Date Lake Minnetonka Level Dam Discharge Explanation
June 10, 2024 929.56 ft 150 cfs Gray's Bay Dam discharge was lowered to 150 cubic feet per second (cfs) to reduce flow in the creek from 155 cfs toward paddling conditions, while continuing to lower Lake Minnetonka toward its ordinary high water (OHW) level of 929.4 feet and create storage for future precipitation.
June 5, 2024 929.71 ft 200 cfs Gray's Bay Dam discharge was increased to 200 cubic feet per second (cfs) to regain storage on Lake Minnetonka while balancing high flows on Minnehaha Creek.
June 3, 2024 929.66 ft 150 cfs Gray's Bay Dam discharge was increased to 150 cubic feet per second (cfs) to regain storage on Lake Minnetonka while balancing high flows on Minnehaha Creek.
May 30, 2024 929.56 ft 75 cfs With over an inch of rain predicted for Sunday night and into Monday, Gray's Bay Dam discharge was decreased to 75 cubic feet per second (cfs) to create capacity in Minnehaha Creek ahead of the forecasted rain.
May 25, 2024 929.67 ft 200 cfs Gray's Bay Dam discharge was increased to 200 cubic feet per second (cfs) to regain storage on Lake Minnetonka while balancing high flows on Minnehaha Creek.
May 23, 2024 929.62 ft 150 cfs Gray's Bay Dam discharge was increased to 150 cubic feet per second (cfs) to regain storage on Lake Minnetonka while balancing high flows on Minnehaha Creek.
May 22, 2024 929.68 ft 125 cfs With heavy rain the watershed received on Tuesday, Gray’s Bay Dam discharge was increased to 125 cubic feet per second (cfs) to recapture storage in Lake Minnetonka while balancing high flows in Minnehaha Creek.
May 14, 2024 929.45 ft 75 cfs With little precipitation in the short-term forecast and the Lake Minnetonka level gradually decreasing, Gray's Bay Dam discharge was lowered to 75 cubic feet per second (cfs) to reduce flow in the creek to paddling conditions, while lowering Lake Minnetonka toward its ordinary high water (OHW) level of 929.4 ft.
May 13, 2024 929.49 ft 100 cfs With little precipitation in the short-term forecast and the Lake Minnetonka level decreasing gradually, Gray's Bay Dam discharge was lowered to 100 cubic feet per second (cfs) to reduce flow in the creek from 160 cfs toward paddling conditions, while continuing to lower Lake Minnetonka toward its ordinary high water (OHW) level of 929.4 ft.
May 9, 2024 929.54 ft 150 cfs With dry weather in the short-term forecast, Gray's Bay Dam discharge was lowered further to 150 cubic feet per second (cfs) to reduce flow in the creek from 221 cfs toward paddling conditions, while gradually lowering Lake Minnetonka toward its ordinary high water (OHW) level of 929.4 ft.
May 8, 2024 929.55 ft 175 cfs With dry weather in the short-term forecast, Gray’s Bay Dam discharge was lowered to 175 cubic feet per second (cfs) to reduce flow in the creek from 224 cfs toward paddling conditions, while slowly lowering Lake Minnetonka toward its ordinary high water (OHW) level of 929.4 ft.
May 7, 2024 929.61 ft 225 cfs Gray's Bay Dam discharge was adjusted to 225 cubic feet per second (cfs), in accordance with Zone 2 of the operating plan, based on current lake level, inflow to and capacity within Minnehaha Creek, and received precipitation and weather forecasts.
May 5, 2024 929.61 ft 275 cfs Gray’s Bay Dam discharge was adjusted to 275 cubic feet per second (cfs), in accordance with Zone 2 of the operating plan, based on inflows from the upper watershed, current lake level, inflow to and capacity within Minnehaha Creek, and weather forecasts.
May 3, 2024 929.65 ft 200 cfs Gray’s Bay Dam discharge was adjusted to 200 cubic feet per second (cfs), in accordance with Zone 2 of the operating plan, based on anticipated inflow from the upper watershed into Lake Minnetonka and weather forecasts.
May 1, 2024 929.52 ft 115 cfs Gray's Bay Dam was opened, in accordance with Zone 2 of the operating plan, after recent spring precipitation removed the watershed from drought classification.

Operating Plan

The dam is managed by Minnehaha Creek Watershed District according to an operating plan that was developed over the course of 10 years with local, state, and federal partners. The operating plan was approved by local municipalities and the MN Department of Natural Resources. The operating plan was designed to mimic the natural conditions of the lake and creek throughout the year, using historic lake levels. However, the operating plan also accounts for changing weather.

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