Chapter 1 - Overview

Lake Minnetonka lies some 15 miles west of Minneapolis in an area of rapidly expanding population. Lake Minnetonka is actually a number, of lakes or "bays" of varying depths up to. 90 feet interconnected by narrows and channels, some very shallow. The lake is made up of a score of named bays with an aggregate area of 21.5 square miles. The waters of Lake Minnetonka's many bays occupy depressions created by the melting of a huge block, or blocks, of ice left behind by the retreating glacier.

The 123 square mile watershed which drains to the lake has gently rolling terrain. The watershed soils are of glacial origin. Basically two types are found: a red drift around most of the lower lake and its bays and a grey drift around the upper lake, the north and western arms, and their associated bays. Moyle (1950) indicates that differences in water chemistry are associated with the two soil types. From the viewpoint of the fisherman, the lower lake resembles the more northern waters of the state, whereas the' upper lake and its bays resemble southern Minnesota waters.

The undeveloped areas of the watershed are generally in mixed hardwoods, brush, and grass. Some marshes are to be found in poorly drained depressions and near the lake. Channels have been dug through some of the marshy areas to improve their drainage. For the mos t part, these channels feed native streams which discharge into Lake Minnetonka.

The watershed sustains some marginal agriculture. The farms are mostly dairy operations and they are typically small with no more dairy cattle than the owner can tend himself. Two large portions of the watershed have recently been acquired by the Hennepin County Park Department for use by the public and as wildlife preserves.

While the lake is only the tenth largest of Minnesota's inland lakes, its closeness to Minneapolis combined with its size have long given it a unique status among the lakes in Minnesota. It has been a favorite locale for summer and suburban homes for generations.

Of the roughly 100 miles of shoreline, almost all of them which are reasonably suitable for development are in private ownership for residential or recreational use. Several communities have grown up in proximity to the lake so that most of the watershed's 1970 population of 46,000 reside near the lake. The lake has substantial value both as a public recreational and environmental resource and as a setting for private residential development.

Problems of Lake Minnetonka

Lake Minnetonka is subject to two major problems. One is deterioration in water quality caused by eutrophication, and the other is hydrologic instability. These two problems are closely related and alternative measures for alleviating pollution have different effects on the stability of lake levels.

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 during 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 last several decades, but sedimentation has been negligible. Instead, the accelerated eutrophication of the lake, which is due to increased inputs of nutrient elements, particularly phosphorus and nitrogen, is evidenced by the increased growth of algae and weeds. It is interesting to note, however, that MacMillan took note of the complaints of the "cottagers" about the algae which grew profusely in the lake during the summers. ("Minnesota Plant Life," by Conway MacMillan, pub. University of Minnesota, October 20, 1899.)

By far the greatest impact on the quality of Lake Minnetonka is attributable to the people living in the watershed and their activities. up to the present time the domestic wastewater of the people living in the watershed has been treated and discharged to the lake or water courses leading to the lake. However, waste treatment processes presently being used are relatively ineffective for removal of phosphorus, which is concluded to be the critical nutrient in the case of Lake Minnetonka. Since the 1940's the quantity of phosphorus in domestic wastewater has about doubled due to the universal use of phosphate based detergents. The population growth in the watershed from 13,000 in 1940 to 46,000 in 1970 together with the increase in phosphorus per capita have resulted in a vast increase in nutrient input to the lake.

Not only have people been contributing nutrients to the lake in the wastewaters they discharge, but nutrients run off of their lawns and highways. Nutrients also run off of open land, wooded areas, and farms, of course. However, the consensus of opinion from several studies is that the nutrient contributions from urban land is about four times greater than the contributions of nutrients from rural lands.

Surface levels of Lake Minnetonka have fallen in many years, impairing the beauty of its shoreline and interfering with boating, swimming, and fishing recreation. The most serious decline in lake level was from 1929 to 1940 when the lake remained below normal level for 11 years with a maximum decline of more than six feet. It appears that the increased discharge of treated wastewater to the lake since 1940 has helped to stabilize its levels.

Investigations

Investigations described in this report were directed to evaluating the water quality of Lake Minnetonka, appraising its value as an environmental and recreational resource, and determining the most appropriate measures for arresting eutrophication of the lake.

Since most nutrient input to the lake is attributable to population in watershed, a projection of future population and land use was made as described in Chapter II. The monetary and intangible value of Lake Minnetonka was appraised, as described in Chapter III, to provide a basis for justification of the cost of remedial measures.

An analysis of nutrient inputs and outputs or "nutrient budget," described in Chapter IV, was prepared for present and future conditions to identify sources of nutrients and evaluate their magnitude as a basis for management plans. The biological and chemical reactions in the lake due to nutrient inflows were studied in an intensive field sampling and investigation program and are described in Chapter V.

Since the remedial measures considered include diversion of wastewater out of the lake's watershed, and the lake's hydrologic stability is known to be in delicate balance, a hydrologic budget, described in Chapter VI was prepared to evaluate the possible effect of diversion on lake levels in the future.

Two basic alternatives for improving and protecting the water quality of the lake were formulated and evaluated. These alternatives, compared in Chapter VII, are in provision of more intensive wastewater treatment to remove most of the nutrients or the collection of all wastewaters and conveyance out of the watershed to a treatment plant discharging to the Minnesota River instead of Lake Minnetonka. Diversion to an out-of-watershed sewage treatment plant is the recommended alternative, and the reasons for selection are discussed in more detail in Chapter VIII.

Chapters IX and X present commentary on water quality management policies related to Lake Minnetonka and the institutional framework within which lake water quality management is carried out.

Technical descriptions, more detailed than appropriate for the report are presented in appendices on Lake Related Land Values, the relationship between phosphorus input and phosphorus content of the lake, and the Hydrologic Budget.

An interim report, titled, "Lake Minnetonka Comprehensive study Interim Report - Data Inventory and Findings - June, 1969-May, 1970" which documents all field data collected in this program, was published in September, 1970.