Little Rock Lake Total Maximum Daily Load (TMDL) Study

Little Rock Lake is located in western Benton County, just north of St. Cloud Minnesota. It is in the North Central Hardwood Forests ecoregion, within the Platte-Spunk sub-basin. Little Rock Lake is located near the bottom of the Little Rock Creek watershed. The 67,650 acre watershed is nearly evenly split between Benton (36,030 acres) and Morrison (31,620 acres) counties. Little Rock Creek is a cold-water trout stream, and is currently the subject of a biological TMDL study to determine the cause of an impaired fish community. The Little Rock Lake Nutrient TMDL Project combines the data collected from the ongoing Little Rock Creek Biological TMDL Phase II Study, which is scheduled to be completed June 2009 (started in June 2006).

Little Rock Lake has an approximate surface area of 1,270 acres; it is in the upper 25 percent of the lakes in the state, in terms of surface area. Little Rock Lake has a large watershed relative to its surface area – 53:1 watershed: surface area. It is a very shallow lake, with a mean depth of about eight feet, maximum depth is 19 feet. The fetch is approximately 2 miles. The estimated water residence time is 0.3 to 0.5 years (LAP). Little Rock Lake retains approximately 48 percent of the phosphorus that enters the lake.

In 2007, Little Rock Lake experienced a large blue-green algae bloom, raising the concerns of landowners in the area. Algae samples collected by MPCA determined that the algae toxin levels were considered to be high risk according to the World Health Organization standards (MPCA Office Memorandum). Historically, algae blooms occurred regularly in the lake, including toxic blue-green algae.

This project aims at identifying the sources of the excessive nutrient impairment of Little Rock Lake. Due to the complicated nature of this watershed, this project will require an additional monitoring plan to determine flow and loading from the input sources. We also will identify and quantify significant nutrient sources. We will conduct paleolimnological studies to determine historic sedimentation rates and lake conditions which will be useful to setting present day goals. Using this information and the monitoring data collected, the Benton SWCD will be contracting with William Walker to conduct quality modeling, development of allocations and reduction scenarios and reporting. The Benton SWCD will coordinate all reporting, subcontracting and public participation activities as well as compiling the final TMDL document for EPA approval and assisting the MPCA with responses and modifications to the document.

Map of Little Rock Lake Watershed