The City of Prior Lake and Watershed District staff continue to monitor the weather and lake levels. Due to an abundance of rain, Prior Lake levels increased by almost 9 inches over the last week and a half to an elevation of about 903.44’ (as of May 25) and Spring Lake rose by about a foot. The ordinary high water (OHW) and no-wake level on Prior Lake is 903.9’. Spring Lake level and flow has already started to drop and is currently 912.19’ (OHW and no-wake zone is 912.8’).
Prior Lake continues to rise because more water is coming into Prior Lake than can leave. Currently, approximately 68 cubic feet per second (cfs) is entering Prior Lake (from Spring Lake) and 50 cfs is leaving Prior Lake (through the Outlet Structure). Spring Lake cfs does not need to drop all the way to the Prior Lake cfs before Prior Lake level starts to drop, because some infiltration and evaporation will occur on Prior Lake. For example, Prior Lake may level off when Spring Lake is discharging 60 cfs and Prior Lake is discharging 50 cfs, but it is largely dependent on variables like sunshine and wind.
Large rains could increase the lake levels quickly due to the saturated conditions in the watershed and little upstream holding capacity. However, according to National Oceanic and Atmospheric Administration (NOAA) on May 25, there is no more than a 30% chance of rain through the Memorial Day weekend. Even though no immediate threat of a large rainfall is in the forecast, the City and Watershed are preparing to dam Spring Lake, if needed. The Watershed is currently in communication with DNR to get approval. City staff would be installing the dam and they have all the materials ready, if needed. If a dam is installed on the Spring Lake outlet, that would likely not occur until next week. Unless the forecast changes significantly, it does not appear Prior Lake would surpass the No-Wake Zone for the Memorial Day weekend.
If the dam does get installed, an emergency procedure is in place and the dam would be removed, or lowered, if the level of Spring Lake nears the OWH level of 912.8’ to prevent damage to structures on Spring Lake.
The Watershed has realized some relief from even higher lake levels by opening the low-flow gate on April 25. The Watershed continues to operate the structure with the gate open (visit https://www.plslwd.org/2017/04/low-flow-gate-opened for more information) and will close the low-flow gate when the lake level drops to 902.5’. At lake levels above 903.4’, the low-flow gate has no impact on discharge because the pipe is full and cannot pass more water through the pipe. However, as the lake rises, pressure will slowly push more water through the pipe.
Staff will continue to provide lake level updates through various media sources and the Public Works Department has initiated a few early flood response protocols in the event water levels reach concerning levels that could start to impact city infrastructure and residents. Stay up to date by watching the City of Prior Lake website (www.cityofpriorlake.com) or the Watershed’s website (www.plslwd.org). The easiest way to find the current lake level for Spring Lake or Prior Lake is to visit the Prior Lake-Spring Lake Watershed District website and click on the links near the top, listed after “Current Lake Levels”.