Landscape formation of Minnetonka
More than most communities, Minnetonka retains a distinctive landscape that reflects a long process of landscape change. A brief summary of this process, largely drawn from Landscapes of Hennepin County (Soil Conservation Service, 1976), can help to explain the origins of certain elements of Minnetonka’s present-day landscape, such as the steep, wooded hillsides, stream valleys and wetlands that cover so much of the city.
Like most of Minnesota, the landscapes of the Twin Cities region were shaped by successive glaciations. In Hennepin County, the most significant of these was the Superior Lobe, an ice floe that advanced southwest from the Lake Superior region and stopped just south of the Minnesota River, blocking its lower reaches. The melting of the ice sheet formed a band of hills known as the St. Croix Moraine, which covers much of Minnetonka and extends as far east as western Minneapolis.
Fifteen to twenty thousand years ago, the next ice sheet, the Grantsburg Lobe, advanced from the southwest, burying much of the Superior drift. However, parts of the St. Croix Moraine acted as a barrier to this ice sheet, creating a landscape of steep ridges protruding above the more level outwash plains of the Grantsburg moraine. This landscape is known as the Minnetonka Highlands. It covers approximately the southern two-thirds of the city and is most visible along I-494 between Highway 7 and I-394. As described in the book Landscapes of Hennepin County:
The visual force of the Minnetonka Highlands is derived from its ridges and hills which in turn hide numerous wetlands tucked away in small cavities. This combination of rugged hills and wet holes is referred to as “Kettle-Moraine” topography. Oriented ridges ride the skyline like whalebacks one-fourth to one-half mile in length. The conical-shaped hills, in turn, are scattered like chocolate drops all over the landscape… The dominant slope gradient is between eight and twenty-five percent. The “kettles,” or small potholes nestled among the hills, are closed basins which trap water that runs off the hills that enclose them.
This landscape, still quite young in geological terms, has seen several eras of climate and vegetation change, even before the era of European settlement. About seven thousand years ago the climate shifted from a cool, humid one characterized by a spruce forest, to a hotter, drier one that supported short prairie grass. About four thousand years ago the climate shifted to its present pattern, and the “Big Woods” spread westward into Hennepin County, with hardwood trees taking root on moist upland soils. The predominant species were Oak, Ash, Sugar Maple, Elm, Basswood and Ironwood. The descendants of these woods still cover most of Minnetonka’s hillsides. The more level slopes were generally cleared for farming, while the steeper ones were left intact.
Subsequent residential development of the wooded hillsides took place, for the most part, with admirable sensitivity to natural amenities. “We can look at the Highlands today and see residential developments so well done that hardly a tree has been moved or a spade full of earth cast away. Trees stood sentinel on the hillslopes and guarded the land from the bulldozer” (Landscapes of Hennepin County). Commercial and industrial development generally occurred on the more level plains or followed valleys. The I-394 corridor, for example, is located on a more level landscape known as the Corcoran Plain, characterized by prime farmland soils interspersed with broad marshes.
The other primary feature of Minnetonka’s landscape is, of course, Lake Minnetonka, which runs for some 23 miles east to west. The lake has been the catalyst for the growth of many lakeshore villages and both resort and year-round residential development, ranging from modest cottages to substantial private estates. While only the Gray’s Bay section of the lake falls within city limits, Minnehaha Creek flows out of Gray’s Bay and continues west-to-east across the city, with extensive marshes at its western end and substantial wetlands and floodplains along much of its length.
Natural Features Affecting Development
As the previous section shows, Minnetonka’s most significant natural features include its slopes, its woodlands, wetlands and water bodies, including creeks, ponds and lakes. Water bodies, especially lakes and ponds, have attracted housing development along their shores, while the presence of wetlands and steep slopes have tended to limit or discourage development. Soils—related in their composition to slope, vegetation, and drainage systems—also differ greatly in their suitability for development.
The Natural Resources Restoration and Management Plan, completed in 1996, identified four ecological system categories that predominate in Minnetonka:
- Old fields (prairie remnants)
- Forested communities
- Lakes, ponds and creeks
The study found substantial deterioration in the vegetation types found in each of these systems, due to urban stormwater runoff, former agricultural practices, and other disturbances.
Old fields (prairie remnants)
Old fields are generally associated with previous agricultural uses, most of which ended a decade or more ago. Some were subject to agricultural drain tiles and ditching, which also changed their character. Dominant vegetation consists of introduced naturalized grasses and weeds. These areas are now experiencing an invasion by shrubs and saplings.
Minnetonka’s wetlands range in size from narrow linear wetlands along Minnehaha, Purgatory and Nine-Mile Creeks to expansive basins like those found in Big Willow and Purgatory Parks. Wetlands and water bodies make up approximately 3,000 acres, or 30 percent of the City’s land area. In many locations, wetlands have seriously deteriorated due to nutrient-enriched urban stormwater, causing monocultures of cattails and reed canary grass to dominate. Wetlands have also been significantly modified through ditching, tiling and changes in water levels.
The Natural Resources Plan identified several types of forested communities.
- Recently Developed Forest Systems are comprised of early invading species such as cottonwood, box elder, green ash and red elm. They are typically found on old fields, drained wetland soils, and areas along ditches and lake margins. Shading by young trees and European buckthorn has contributed to the decline of native ground cover species.
- Historic Oak Savanna Systems dominated by burr oak and northern pin oak are found on higher, drier ridge tops, often with southern and western exposure. On north-facing slopes, species such as red oak, basswood, paper birch and ironwood are found in protected areas. Most of the savanna systems have experienced substantial erosion of topsoil, and a tree canopy that is “overstocked” with young trees and shrubs, shading out the native ground cover vegetation.
- Mixed Oak-Hardwood Systems are found on protected north and east-facing slopes and more rugged terrain along creeks. White oak, red oak, white ash, box elder and iron wood are the dominant tree species. Some of these areas are still in good health, with diverse ground cover vegetation, while others have been closed in with buckthorn and other shrubs.
- Aspen and Oak Parkland Systems occur on higher ridge slopes at Lone Lake Park; these are dominated by patches of aspen and include burr and red oak, shrubs and ground cover species. As in the other communities, invasion by buckthorn and prickly ash is occurring in many places.
Lakes, Ponds and Creeks
These water bodies are usually included in wetlands, although isolated ponds exist. In these locations (such as Lone Lake Park), the lake system had high water clarity and diverse vegetation. However, erosion and nutrient loading occur along many shorelines, resulting in an invasion of the pond edge by a few species that thrive in enriched conditions.
From the 1999 Comprehensive Plan, Chapter 3, Land Use.