Update supplied by the City of Prior Lake.
Although the City of Prior Lake and Prior Lake-Spring Lake Watershed District (PLSLWD) staff continually monitor local conditions such as winter snowpack, precipitation, and lake levels to assess flooding risk, early spring is one time of year when flooding potential receives a lot of attention. City staff performed an analysis last year and showed there isn’t a strong correlation between the lake level at the ice-out date and the eventual high water elevation for Prior Lake for any given year.
In 2014, the early spring level for Prior Lake was very close to the long-term average of about 901.6 on April 20, and the record flooding level of 906.17 recorded on June 30 was caused by above average rainfall events over a period of a few weeks in May and June. Other years, such as 2001, start out high (902.75 on 4/15/2001) and the high level for the year is recorded shortly thereafter (904.33 on 4/30/2001); in this case melting snow could have contributed to the high-water levels.
There is currently no snowpack in the watershed above Spring Lake and Prior Lake. Water levels for Prior Lake are above the long-term early spring average of 902.58 as of March 27 and the lake has been discharging through the Prior Lake Outlet Channel since last fall. Our region has been experiencing some very wet weather lately, leading to full storage areas upstream (ponds, wetlands, depressions) and therefore lots of runoff every time we get more precipitation.
Our average annual precipitation is about 31.1 inches and the 365-day precipitation sum (amount of water we’ve received since March 22, 2016) is 40.8 inches. Even with the lack of snow, we could still experience spring flooding, but lake levels will depend only on the precipitation we receive. The National Weather Service in Chanhassen, in their initial Spring 2017 Flood Outlook, agrees with this assessment, stating: “The main flood threat will be determined by the occurrence (or lack of) heavy snow events in March and/or heavy rain events in March and April.” We will continue to keep an eye on the situation.
City staff is currently working on finalizing a plan that includes flood trigger points (lake elevations) for flood response actions and the role the city will play in those actions. The plan will be presented to the City Council soon in the form of a City Policy.
The PLSLWD is also working on short-term flood solutions by looking at how to optimize the operation of the Prior Lake Outlet Structure, which also will involve action at certain lake elevations. Per the recently-completed joint Flood Study, these short-term flood solutions are needed, combined with a long-term strategy of upstream water storage, to help reduce flood impacts on our residents and businesses.