Using TMDLs to Guide Storm Water Management in Minnehaha Creek

Minnehaha Creek is the physical link that binds together a network of urban lakes, parks and open space that define the southwestern Twin Cities area and south Minneapolis. It flows from Lake Minnetonka eastward for 22 miles to the Mississippi River. The creek drains an area of 47.3 square miles below Lake Minnetonka including one lake, Lake Hiawatha, which is in-line to the creek.Development in the Minnehaha Creek watershed has significantly changed the hydrology, resulting in increased storm water volumes and flow peaks compounded by reduced infiltration and base flow. Wetlands and depression storage that naturally extend the period of flow have largely been eliminated in the Minnehaha Creek watershed. Large volumes of surface runoff are produced by the impervious area, but are discharged over a short period leaving the creek dry at times. Lake Hiawatha is heavily influenced by Minnehaha Creek from both a hydrologic and nutrient loading perspective.Minnehaha Creek appears on the State of Minnesota's §303(d) list of Impaired Waters due to its impaired biotic community. Lake Hiawatha appears due to excess nutrients. Fecal coliform and chloride have been included as additional causes of impairment for Minnehaha Creek. The Minnehaha Creek Watershed District (MCWD) has partnered with the Minnesota Pollution Control Agency (PCA) and EPA Region 5 to develop a TMDL that addresses all listed impairments. MCWD has an extensive hydrologic data program through which it collects and analyzes precipitation, water level, discharge, water quality, stream flow, and groundwater level data. This information has provided a solid foundation for TMDL development. In addition, this data has helped guide preparation of the MCWD “Comprehensive Water Resources Management Plan”.Flow plays a major role in affecting water quality in Minnehaha Creek. The high percentage of impervious surface in this urbanized subwatershed has reduced the amount of storm water that would naturally infiltrate and help sustain base flow. This storm water is efficiently conveyed to the creek through numerous storm sewer outfalls, which results in the flashy flows. The increased storm water volumes also convey nutrients, sediment, and other pollutants to Minnehaha Creek. For this reason, the TMDLs are being developed around a hydrology-based framework including the use of duration curves.The TMDL focus on hydrology enables a natural connection to storm water management activities in Minnehaha Creek. A major priority identified in the MCWD “Comprehensive Water Resources Management Plan” is the implementation of opportunities to reduce storm water volumes and nutrient loading. Storm water management will also address water quality impairments associated with bacteria and chloride. Priority subwatersheds have been identified where reduction of peak discharges will collectively result in the greatest reduction of storm water flow to Minnehaha Creek. MCWD has developed a list of Capital Improvement Projects that work towards this objective. The TMDL technical analyses are being developed to ensure these projects work to maximize water quality benefits to Minnehaha Creek.