Minnehaha Creek is the primary stream within the subwatershed. It is formed at the outlet of Grays Bay in Minnetonka and flows 22 miles to the Mississippi River. One lake, Lake Hiawatha, is in-line to the creek and heavily influenced by it.
As the outlet for Lake Minnetonka and the upper watershed, Minnehaha Creek must discharge large volumes of water during spring snowmelt runoff, summer and fall. During a typical year, 4-6 inches of runoff from the 122 square mile watershed tributary to the outlet are discharged to Minnehaha Creek. The typical average flow in the creek from this discharge is 60 to 90 cfs.
An operating plan was established for the Grays Bay dam headwaters control structure when it was put into service in 1980. The plan was intended to emulate the historical discharge hydrograph produced by previous controls and the natural outlet of Lake Minnetonka.
In drier periods Lake Minnetonka typically does not discharge water. Development in the lower watershed has changed subwatershed hydrology. Wetlands and depression storage that naturally extend the period of flow have been largely eliminated in the lower watershed. A large volume of surface runoff is produced by the impervious area but it is discharged over a short period of time, leaving the creek dry at times. It is also likely that development has decreased groundwater discharge to the creek so the base flow to the creek from that source has been diminished.
Minnehaha Creek was studied in-depth in 2003-4 as part of the District’s Minnehaha Creek Stream Assessment, which included a physical inventory, erosion survey, fluvial geomorphic assessment to determine channel stability, and a biological assessment.
Flow in the creek is controlled by numerous structures, including major weirs at the Grays Bay Dam outlet, the Browndale Dam, West 54th Street, and Hiawatha Avenue. There are more than 100 bridge crossings, many of which provide grade stabilization. Some of those crossings provide a grade control substantial enough to create an impoundment of hydraulically stagnant water upstream.
There are 178 identified storm sewer outfalls larger than eight inches in diameter along the length of the creek. Most of those outfalls are located downstream of the Browndale Dam in Edina and Minneapolis. Many are experiencing local bank erosion and scour. Eleven exposed utility crossings were identified along the creek, eight of which are in the city of Minneapolis.
Approximately 15 percent of the streambank is armored by concrete or masonry retaining walls, riprap, or other protection such as gabion baskets. These are generally for the purpose of controlling erosion and meandering to prevent loss of property, stabilizing steep banks, or protecting structures such as bridges and storm sewer outfalls. Many of these stream walls were presumably constructed by the Works Progress Administration (WPA) during or following the Great Depression.
The Stream Assessment identified 35 sites of significant erosion or bank failure - typically more than one to two feet in height - and lesser erosion damage and bank failures along much of the creek. Most of the significant failures were downstream of the Browndale Dam in the city of Minneapolis. Other significant features cataloged in the Stream Assessment include 26 sites with deltas and sandbars; 20 sites with small amounts of miscellaneous debris, 16 private docks, and numerous miscellaneous features such as islands, side channels, stairs, wetlands, inlet channels, and rock piles.
A fluvial geomorphic assessment was completed as part of the Stream Assessment to evaluate channel stability. In general the assessment determined that although erosion and incision has occurred in the creek, it now has a relatively stable channel profile because numerous bridge crossings and other structures will prevent further incising. However, the channel width is not stable in several locations and was observed to be expanding. Eighteen of the 30 reaches were assessed as stable, eight as stable with a tendency to aggrade or degrade, and six are aggrading (accumulating sediment). Aggradation is likely the result of the numerous vertical controls such as bridges that create impoundments.
Minnehaha Creek is included in the District’s Annual Hydrologic Data monitoring program. Water quality and flow in the creek is monitored at eight locations. Phosphorus and TSS concentrations in Minnehaha Creek are comparable to the North Central Hardwood Forest ecoregion mean, which is generally a result of the good quality of water discharged into the creek from Lake Minnetonka. In general nutrient and sediment loads increase upstream to downstream, although the impoundments at the major grade controls impact those concentrations. While the flow-weighted average chloride concentrations in the creek were lower than the state chronic standards of 230 ug/L, several individual grab samples did exceed that standard, although none exceeded the acute standard of 830 ug/L.
Grab samples from seven sites were tested for the presence of e. coli bacteria. While the acute standard was not violated in 2004 in the creek, the 30 day geometric mean standard was violated at five sites on the lower creek for the months of September and October 2004. In 2005 the District expanded the creek monitoring to additional sites and adjusted sampling frequency to assist in identifying the source or sources of e. coli. In addition, samples are being analyzed for traces of caffeine, which may indicate that human waste is one of the sources of e. coli. Results are pending. Dissolved oxygen was measured at eight locations and generally maintained levels greater than the 5 mg/L State of Minnesota standard for class 2B waters. Measurements did dip below the 5 mg/L standard periodically, depending on flow in the creek and on location relative to large riparian wetland complexes.
Minnehaha Creek is listed on the State of Minnesota’s 303(d) list of Impaired Waters for impaired biotic integrity. The most limiting factor for the ecology of Minnehaha Creek is its variability of flow, which as noted above ranges from intensive periods of high volume and velocity flow to periods of low or no flow. During those latter periods much of the channel runs dry, leaving few pools or backwaters to serve as refuge for fish and macroinvertebrates. The creek also has a lack of physical complexity. The channel is mostly of relatively constant dimensions, has very small amounts of woody debris, and little variation in depth and slope. These factors severely limit opportunities for aquatic life to sustain viable populations. Section 2.3.2 above discusses stream biologic integrity more fully.
The District in 2005 undertook a joint partnership with the United States Army Corps of Engineers to develop a large-scale, long-term Vision for Minnehaha Creek to serve as guidance for organizations that share Creek corridor management responsibilities. A Citizen Advisory Committee of community representatives and a Technical Advisory Committee of agency representatives through a lengthy community input process developed a common vision and management recommendations.
The 2005 MCWD Minnehaha Creek Visioning Partnership Final Report presents the results of that process and summarizes the Partnership’s recommendations for future Creek management. Erosion control and support of aquatic life were overall the highest ranked priorities for improvement. However, when considered reach by reach, support and maintenance of recreation were the highest priority for the reaches upstream of the Browndale dam, followed by improvement of aquatic life and erosion control. Erosion control and streambank stabilization was the highest priority for the reach downstream of the Browndale dam. The Partnership recommended the District consider bioengineered stabilization techniques over hard armoring where possible, and that habitat improvement be focused on the management of riparian vegetation and retention of large woody debris rather than on in-stream habitat management. The Partnership also recommended that water quality be improved through the reduction of peak stormwater flows; pretreatment of discharges; application of BMPs and good housekeeping practices in the subwatershed; and repair of existing erosion.
An important part of the visioning process was the discussion of several streamflow management scenarios developed by the Corps to model what would happen with changes to the operation of the Grays Bay dam. The dam is managed to discharge water from Lake Minnetonka into the Creek only when the DNR-established runout elevation of the lake is exceeded. During dry periods lake level falls and there is minimal discharge; flow in the creek falls to minimal flow-related aquatic habitat conditions and canoeing is not possible. The Corps developed a number of scenarios that would provide targeted releases for recreation or habitat purposes, and then modeled the resulting impact on water level in Lake Minnetonka; the percent of time creek flow fell within optimal conditions for aquatic habitat and recreation; the percent of time potentially erosive flows could be expected; and resulting estimated water quality. Each scenario attempted to balance these often competing interests; in the end the Partnership recommended that further study be completed to find a way to optimize and balance year round minimum flows and moderated extreme flows with recreational and lake uses.