2.2.2 Topography and Drainage
The shape of the watershed’s surface formed during the period of the last glacial age, the Wisconsin, about 13,000 years ago. As the last ice sheet retreated it dumped unsorted glacial debris of all sizes and shapes called “drift” in various patterns. Five such patterns or geomorphic units dominate the District’s topography. Figure 3 gives the names and sites of the watershed’s five major geomorphic units.
Varying patterns of glacial drift distinguish each major landform. Thinly spread drift formed till plains. This pattern occurs in a semi-circular belt in the watershed’s western region. Called the Lonsdale-Lerdal Till Region, the area is characterized by circular level topped hills with smooth side slopes. Lacking a uniform drainage pattern, many small streams end in lakes or depressions. Low lying areas find closed basins with numerous lakes and peat bogs common. Water table levels normally occur within 3 to 10 feet of the surface but vary seasonally.
Glacial till dumped at the edges of glaciers formed belts of hills called moraines. Numerous rugged hills or “knobs” and deep irregular depressions called “kettles” dominate morainic landforms. Kettles are formed when isolated blocks of ice melted, often creating lakes and ponds. Three distinct areas show these traits in the watershed: the Emmons-Faribault Moraine, including Lake Minnetonka; a small part of the Eastern St. Croix Moraine visible along the south central watershed boundary; and the Waconia-Waseca Moraine found in the watershed's far western area.
Gently rolling to steep hilly landscapes marked by an abundance of lakes and ponds mark the Emmons-Faribault Moraine. Elevations are similar to the Lonsdale-Lerdal Region although irregularities in surface contours are much more pronounced. The maximum surface relief is 150 feet. Many lakes dot the area with Lake Minnetonka as the prominent landform. Partly submerged knobs and kettles made its many bays, points and islands. Groundwater levels in lower areas run from above the surface to a depth of about 6 feet. In hilly areas water tables are generally greater than 10 feet deep.
One of the sharpest moraines to occur in Minnesota, the Eastern St. Croix Moraine, ends along Lake Minnetonka's southeastern shore. Steep, rugged hills dotted with deep basins give this geomorphic unit its character. Its erratic surface relief varies from 50 to 200 feet from hill base to hilltop. Water normally fills depressed areas, forming numerous wetlands, ponds and small lakes. Its closed low areas limit drainage. Groundwater levels vary with relief, normally at or near the surface in low lying areas and at depths greater than 10 feet in hilly regions.
Circular, level-topped hills with smooth side slopes define the Waconia-Waseca Moraine. Located in the far western watershed, this landform unit has numerous lakes and wetland areas formed when glacial ice fell apart. Other forms include nearly uniform summit elevations and contour lines, broad lower levels and variable water tables. Groundwater levels range from 3 to 10 feet in upland and slope areas while in low lying areas water tables run from above the surface to depths of about 6 feet.
The Mississippi Valley Outwash Plain runs from Hopkins east to the Mississippi River. An outwash plain forms when glacial meltwater dropped sorted and stratified materials. When blocks of glacial ice melt they may form small shallow lakes. These forces have shaped the Mississippi Valley Outwash Plain. Landforms undulate and roll in gentle terraces and bottom lands along the Mississippi River. Scattered about the landscape, many dry hollows with short, steeper slopes often border the deeper basin areas. Usually however, elevations in this nearly level relief differ less than 30 feet between high and low areas. Surface water runoff collects in closed shallow basins due to the area's poorly developed natural drainage. Water table levels are generally greater than 10 feet deep on the terraces. In bottom land areas groundwater lies within 6 feet of the surface.
The watershed is divided into eleven principle subwatersheds (see Figure 2). The lower watershed, which drains to Minnehaha Creek, comprises one subwatershed. Some land area within the lower subwatershed does not drain directly or indirectly to Minnehaha Creek, but drains directly or indirectly to the Mississippi River. The central portion of the subwatershed drains to the Minneapolis Chain of Lakes, which in turn discharge to Minnehaha Creek.
The upper watershed is divided into one subwatershed comprised of Lake Minnetonka, the areas that drain directly into it, and some other small drainage areas; and nine other subwatersheds draining to principle watershed resources. These subwatersheds are drained by streams, channels, and stormsewer to Lake Minnetonka. Lake Minnetonka discharges by a control structure, the Grays Bay dam, into Minnehaha Creek. The dam is managed to discharge water from Lake Minnetonka into Minnehaha Creek when the DNR-established runout elevation of the lake is exceeded.