Habitat

Habitat of the St. Louis River

Several natural processes strongly influence the habitats of the lower St. Louis River. These include water depth, temperature, clarity, substrate composition (gravel, sand, silt, mud), and nutrient levels.

Freshwater Estuary

The lower river, from the Fond du Lac Dam to Lake Superior, forms a freshwater estuary. Here, the river widens and slows, mixing its waters with the lake. The resulting environment differs in temperature, clarity, and nutrient levels. Like saltwater estuaries, freshwater estuaries provide protective nurseries for fish, invertebrates, and migrating birds, buffer against extreme water events and storms, and act as natural filters for pollutants and runoff from surrounding areas.

Cyclical changes in water depth are strongly influenced by the seiche, which occurs when winds or atmospheric pressure cause oscillations in Lake Superior’s water level. Water level changes of a few inches to more than a foot can significantly affect river habitats. The seiche influence extends upstream to Fond du Lac, with areas closer to the harbor more affected than those farther upstream. Strong seiches can even reverse river flow as far upstream as Fond du Lac, a hallmark of estuarine systems. Seiches also promote water exchange between the harbor and lake and can lead to stratification, with colder lake water sinking and warmer river water rising to the surface.

Sediment and water clarity are critical to aquatic habitats. Sediment enters the river from tributaries and is most visible following heavy rains. Water clarity fluctuates with climate and rainfall patterns. The river’s upper reaches drain boggy areas; during wet years, these bogs release tannic acids (“bog stain”), darkening the water. Changes in clarity influence submerged vegetation by affecting light penetration and depth.

Human Impacts

Human activities also heavily influence the river’s habitats. Dams affect water flow, water levels, and sediment transport. They trap sediment, altering how the estuary is replenished. Over the past century, industrialization along the shoreline and dredging of the shipping channel have further altered habitats. Erosion from ship wakes and waves, combined with changes in the landscape, has accelerated sediment movement into the river. Pavement and storm drains funnel water quickly into streams and ditches, while early successional and deciduous forests do less to slow snowmelt than the original coniferous forests. Faster runoff leads to higher peak flows and increased stream bank erosion.

Plant Communities and Uplands

Plant communities along the lower St. Louis River include upland and wetland habitats. Upland areas are elevated regions that gather drainage. While not directly part of the river or estuary, upland vegetation influences wetland and aquatic habitats by controlling erosion and sediment movement.

Wetlands

Wetlands are areas where the water table is at or near the surface, or where land is covered by shallow water up to six feet deep. They are transitional zones between uplands and open water, and water is the primary factor controlling plant and animal life.

Globally, wetlands are highly productive ecosystems. In the lower St. Louis River, they provide food and shelter for unique fish and mussel assemblages, shorebirds, waterfowl, passerines, raptors, and many other species. Despite alterations from dredging and filling, many present wetlands likely retain species composition and patterns similar to pre-development conditions.

Important wetland types in the lower St. Louis River include marshes, wet meadows, wet shrublands, mudflats, seeps, and fens. Some of these wetlands are rare and occur nowhere else outside the Great Lakes region.