03-24-2009 Council Work Session Packet

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CMP Part 3A. Environmental Protection Plan Lake Minnetonka is dependent upon watershed runoff for all replenishment. Lake Minnetonka is not fed by any spring or river inlet. All water is received from precipitation falling within the immediate watershed. Most of this water enters the Lake by direct runoff through a series of five major streams and their associated lakes, marshland and sub-watersheds. Since the level of the Lake is generally below the level of the ground water in the glacial deposits of the surrounding area, some additional replenishment occurs from underground seepage, principally from the many wetlands and marshy ponding areas along the shoreline. Lake Minnetonka's ecology makes it easily susceptible to pollution hazards. The combination of a single-source local water supply, relatively small watershed area, many channel restrictions between separate bays, and a single outlet, have resulted in Lake Minnetonka having an extremely long 25-year flush out period for complete water change. This is second only to Lake Michigan in the region. The hazard is that once pollutants enter the Lake, they tend to remain for a long time and they are not easily diluted. Lake Minnetonka has historically suffered from two problems which directly affect its environmental health and its value as a public amenity. One problem which has been worsening with urbanization trends is a deterioration in water quality caused by eutrophication. Eutrophication is the aging process by which natural waters are enriched with nutrients that may stimulate excessive plant growth. The plants may be floating algae, attached algae, or rooted weeds. Eutrophication may occur naturally, as sediments accumulate in lake basins over geologic time or it may be induced artificially as a result of human activities, as in Lake Minnetonka. Much of Lake Minnetonka has become more eutrophic during the past century, but sedimentation has been negligible. Instead, the accelerated eutrophication of the Lake, which is due to increased inputs of nutrient elements, particularly phospharus and nitrogen, is evidenced by the increased growth of algae and weeds. The other problem is hydrologic instability. Because the lake level is dependent upon precipitation, wide ranges in water elevation have occurred over the years. During the 1930's, a general drought left the lake below normal level for 11 years with a maximum decline of more than 6 feet. This low water condition caused the construction of deep wells for water augmentation and discussions about possible diversion of the Crow River. A somewhat lesser drought during the late 1980's again raised concern about the instability� of lake levels, yet the lake recovered from a 4-foot drop in just two years without augmentation. In recent years, however, more urbanization has increased the amounts of direct storm water runoff resulting in high water levels and in some cases flooding problems for low-lying properties. Management of runoff and protection of water quality have now become primary objectives of the Minnehaha Creek Watershed District. City of Orono Community Management Plan 2008-2030 Page 3A-2