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REGIONAL-Two months of above average rainfall led to the worst flooding in the Rainy River Basin in history last year, and lake level management prior to the onset of the massive inflow of water had …
REGIONAL-Two months of above average rainfall led to the worst flooding in the Rainy River Basin in history last year, and lake level management prior to the onset of the massive inflow of water had little effect on the situation.
Those are the conclusions reached in a Draft 2022 Post Flood Report issued March 10 by the Water Levels Committee (WLC) of the International Rainy-Lake of the Woods Watershed Board, the body responsible for making recommendations to the basin’s dam operators for management of Rainy and Namakan Lake levels. The report presents several key pieces of information used by the committee for its decision to follow the normal lake level rule curve, including meteorological data documenting the unprecedented amount of precipitation, the actions taken by dam operators in response to the flooding, and computer simulations of numerous lake level management alternatives that could have been employed. The report also seeks to answer questions frequently asked by the public.
Rainy River basin property owners who experienced significant damage to shoreline properties are all too aware of the historic level of the floods. On June 14, Rainy Lake hit the highest level ever recorded at 1,113.28 feet, while Namakan’s level on May 31 of 1,122.69 feet was the third-highest on record.
Many blamed the WLC for not managing lake levels for high flood risk and not moving to full-open status of dam floodgates quickly enough, allegedly leaving the lakes unable to accommodate the massive inflows from snowmelt and rain. The WLC acknowledged this expression of public sentiment in its report.
But using sophisticated computer models of the basin and its dams, the WLC determined that the massive inflows of snowmelt and precipitation in 2022 would have overwhelmed any alternative management strategy they would have employed to try to minimize the flooding. If the WLC had followed a more aggressive scenario of holding Namakan to the bottom 25 percent of the 2018 rule curve and implemented the high flood risk rule curve for Rainy Lake, the peak level on Namakan would have been only one centimeter lower, while the peak on Rainy would have been four centimeters lower. Namakan would have receded below the All-Gates Open Level one day sooner than it did, and Rainy two days sooner.
For comparison, the WLC also ran alternative modeling scenarios for the 2014 and 1950 floods. For all three floods, the WLC concluded that, “The results indicate that regardless of which rule curve was used, or if no dams existed at all, the level of each lake rises above the All-Gates Open level during these flood events. Furthermore, using one set of rule curves over another has very little influence on the peak level attained by each lake.”
“Flooding has always occurred in the Rainy River watershed and will continue to do so in the future,” the report continued. “No rule curve changes will prevent high water levels in the face of extreme precipitation in the future.”
Each year in March the WLC is tasked with determining the rule curve that will be used to guide the management of lake levels headed into the spring. The committee considers current conditions such as the amount of snowpack and its water-equivalent value, base flows, temperatures and their effect on ground frost depth, and short and long-range weather forecasts and compares that information against similar historical date from previous years to make its choice. Once the choice is made on March 10, the WLC continues to monitor conditions in the basin and can modify its decision and practices as spring approaches.
Drought conditions in 2020 and 2021 created abnormally dry conditions in Canada and moderate to severe drought in the U.S. portions of the watershed, with base flow conditions in the low to normal range in early March of last year. The winter had seen above average snowfall, with snow depths in the 80 to 95 percentile range. The snow water equivalent was measured to range from four to 5.9 inches. A La Niña weather pattern, a pattern correlated with high water years since 1970, was forecasted to continue through May. The long-term weather forecast called for a 40 percent chance of above normal temperatures and equal chances of low, normal, or above normal precipitation.
The Rainy Lake Property Owners Association wrote a letter to the WLC urging caution in its operations, based on multiple factors. The extended forecast included night temperatures well below freezing, which could push ice-out for the lakes into mid-May, creating the possibility of the snow melt, and spring rains to enter the basin at the same time. The association also cited area forestry experts who indicated higher precipitation rates typically follow drought years. The letter also noted that rain and heavy wet snow in November created six inches of water that was retained in the bush and would enter the watershed in the spring.
On March 10, the WLC determined that “current and forecasted conditions did not support the use of the high flood risk rule curve at this time,” according to the report. The middle range of the normal rule curve would be targeted to balance the needs of the lakes’ fisheries. The WLC met again on March 31 to review basin conditions and decided to inform dam operators Boise Cascade and H2O Power to continue targeting the middle of the rule curve.
Then came more precipitation
Snow continued to fall across the basin through mid-April, adding to the snowpack. As temperatures plummeted in early April, a “Colorado Low” dropped an additional 12 to 28 inches of snow across the watershed, with the highest amounts accumulating over the Namakan sub-basin, according to the report.
Torrential rains the weekend of April 22-23 were the triggering event for the flooding, causing a rain-on-snow event that resulted in an almost instant depletion of the snowpack, sending tributary flows soaring to peak levels. The flows continued to be fed by more rain through May, with a combined precipitation total for April and May of 10.2 inches, twice as much as average and including the largest single-day precipitation event of the year at the end of May. The Vermilion River set a new peak flow record and many other tributaries ranked second or third for all-time high flows. The long duration of these high flows created the highest average inflows into Namakan and Rainy Lakes on record for April and May, and the inflows April through July were the second highest, eclipsed by only 1950. These inflows were the driving force behind the flooding, the report concluded.
In response, all logs from the sluices of the Namakan dams were pulled on April 26, and the 15 gates of the International Falls/Fort Frances Dam were progressively opened according to the outflow from Rainy Lake so that all gates were open by May 5. As the outflow from Rainy Lake is constrained by the natural opening that forms the head of the Rainy River, going to an All-Gates-Open position any earlier would not have let more water pass through the dam and lower the lake level.
When Namakan Lake fell below its prescribed All-Gates-Open level on June 30, the Rainy Lake Property Owners Association sent a request to WLC to hold Namakan at a steady level to speed the decline of Rainy Lake. Following a reported discussion between the WLC and Namakan Lake residents, the WLC received a temporary permission to target the upper level of the 1970 rule curve, which remains flat, as opposed to following the 2018 rule curve, which drops gradually. The shift resulted in Namakan Lake being approximately five to 15 centimeters higher than it otherwise would have been, the report says.
Rainy Lake fell below its All-Gates-Open level on July 26, and dam outflows were set to target the middle 50 percent of their rule curve bands. The lakes returned to their standard rule curve levels on Aug. 3.
The report also details the numerous activities the WLC conducted to provide ongoing flood information to the public and its ongoing communications with agencies involved in the flood response. The WLC also held two public listening sessions in early August to hear feedback and answer questions.
Feedback from the community touched on numerous issues. Some expressed concern about the accuracy of snow data across the basin, given that information presented did not reflect what individuals observed locally, especially in remote areas. Several comments related to concerns about the timing of gate openings for Rainy Lake. The report noted that the term “high flood risk rule curve” is misleading, as the curve is not designed to prevent flooding entirely but only to provide modest mitigation under minor to moderate flood risk scenarios. Concern was also expressed about the possible impact of proposed changes to the Canadian Northern railway bridge at the Rainy Lake outlet and how it could affect outflow, particularly during floods. There were also calls to help communities increase shoreline resiliency for future floods.
A public comment period on the Draft 2022 Post Flood Report is open through April 3. A PDF copy of the report and instructions on how to submit comments to the WLC are available at https://tinyurl.com/3nrrv68e.
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