Water resource conditions are influenced in large part by the physical geography of the watershed. Within Dane County, differences between glaciated and unglaciated terrain influence stream gradients, water velocity, and stream temperatures.
Valley and Ridge or “Driftless” Area
- In the western third of Dane County, this area is the only part of the county that was not affected by the Wisconsin Glaciation. The hills are covered by an irregular layer of soil (quite thin in many places) overlying fractured dolomite or sandstone bedrock.
- This area is characterized by steep ridges and valleys drained by fast-flowing streams, generally without natural lakes or impoundments. Most of the streams are fed by springs and seeps flowing from water-bearing layers of bedrock exposed on hillsides.
- Because of the steep gradients, cool water temperatures, and high dissolved oxygen levels, most of the county's trout streams are located in this portion. These streams generally have fair to good water quality, but are commonly affected by nonpoint source pollution as well as streambank erosion problems.
- The Wisconsin River Valley, located in the northern portion of the valley and ridge region, is a large valley on the northwestern edge of the county and consists of deep, alluvial sand and gravel deposits and extensive marshes in the river's floodplain.
- To the east of the Driftless Area, this area has glacial end moraines that create a major drainage divide. Headwaters of many streams of the Wisconsin, Sugar, and Yahara River basins originate here. The moraines include the Johnstown terminal moraine at the western edge of the glaciated area, and the Milton recessional moraine farther east. The moraines include hills and mixed deposits of glacial till (including clay, silt and boulders with sand and gravel layers) which were deposited and left behind as the glaciers retreated. The moraines also included large blocks of remnant ice which formed potholes or kettles, some of which remain as small ponds, marshes, and bogs.
Yahara River Valley
- East of the moraines and in the center of the county, deep glacial deposits dammed up large valleys, forming a chain of large lakes and wetlands. The four largest and most heavily used lakes in the county - Lakes Mendota, Monona, Waubesa and Kegonsa - are found here connected by the Yahara River. These are fertile lakes that support abundant algae and rooted aquatic plant growth, as well as a diverse warmwater fishery including northern pike, bass, and panfish.
- Streams in the Yahara River valley are generally flatter and more sluggish than those in the Driftless Area and fewer are spring-fed. Extensive ditching and wetland drainage has occurred near some water bodies in this area, resulting in water quality impacts and aquatic habitat loss.
Drumlin and Marsh
- The eastern part of the county consists primarily of general glacial deposits with extensive areas of marsh deposits. This area includes many small drumlin hills interspersed with shallow glacial deposits, which created an extensive system of interconnected wetlands with poorly defined drainage.
- Small streams wind slowly through the lowlands, and there are few springs supplying streamflow. The only lakes in this area are small stream impoundments or shallow, marshy lakes. No trout streams are present. A warmwater fishery predominates in the two major streams in the area - Koshkonong Creek and the Maunesha River. Extensive ditching and wetland drainage in this area have also affected water quality and habitat conditions.