Weeks of scarce rainfall are leaving a visible mark on both Lake Minnetonka and Minnehaha Creek.
The Minnehaha Creek Watershed District (MCWD) closed Gray’s Bay Dam on August 24th after Lake Minnetonka fell below its designated natural runout elevation of 928.6 feet.
The flow rate of Minnehaha Creek had decreased to 3 cfs (Cubic Feet Per Second), reflecting the impact of ongoing dry conditions across the watershed. According to MCWD officials, the closure was required under the dam’s operating plan.
The dam will remain closed until enough rainfall raises Lake Minnetonka above the level required to resume operations.
Dry conditions have steadily reduced water levels
Water levels within the watershed are low because there has been a prolonged period of insufficient rainfall. According to an August 24 update released by the MCWD, the Twin Cities had received just 16.03 inches of precipitation year-to-date, approximately six inches below the seasonal average.
This represents the least amount of rainfall measured in the Twin Cities from January through August this century.
Up to that specific point in August, only 0.20 inches of rainfall had been recorded across the region, putting the month temporarily on pace to rival historic lows.
However, subsequent late-month rain brought the final August total up to 0.46 inches.
This ultimately caused the month to finish comfortably as the fourth-driest August on record instead of tying the legendary 0.20-inch record set back in 1925.
Moderate drought conditions have also developed across the region, the first such occurrence since October 2024.
The last time Gray’s Bay Dam was closed during the mid-season operating schedule was in July 2023. That year Minnesota experienced a particularly dry summer.
Falling below a key threshold triggered the closure
Gray’s Bay Dam is managed using an operating plan developed by the Minnehaha Creek Watershed District in cooperation with local, state, and federal agencies. The plan utilizes different discharge zones based upon lake levels, creek conditions, seasonal factors, and forecasted weather conditions.
The dam opened in late April and later transitioned between operating zones as rainfall events temporarily increased water levels before dry conditions returned.
As water levels in Lake Minnetonka continued to decline, operators further decreased dam discharges.
By early August, discharge had been reduced to its lowest allowable rate as defined by the operating plan in order to balance evaporation occurring on the lake surface while maintaining flow downstream in Minnehaha Creek. Since May, operators had made repeated adjustments to discharge rates as they responded to changes in lake elevations and forecasts of rainfall.
Those reductions allowed water to stay in Lake Minnetonka for as long as possible while still providing base flow downstream.
When Lake Minnetonka water levels reached 928.58 feet on August 24th, another report from the Minnehaha Creek Watershed District indicated that Zone 6 of the operating plan had been activated.
That zone required the dam to close once Lake Minnetonka fell below its natural runout elevation.
Rainfall will determine what happens next
Ultimately, how long the closure lasts is dependent upon how much precipitation occurs. MCWD states that Gray’s Bay Dam will remain closed until a sufficient amount of rainfall raises Lake Minnetonka’s water level above its natural runout elevation.
From that point forward, staff will continue to monitor both short-term and long-range forecasts for rainfall to assist them in making future operating decisions.
Current conditions suggest recovery may take time. Lake Minnetonka is approximately 10 inches below its Ordinary High Water Level (OHWL).
The situation illustrates how persistent rainfall deficits can affect both lake levels and downstream flows.
It will be up to future rainfall to determine when water levels recover and allow Gray’s Bay Dam to potentially open later in the season.
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