Wetlands
Big pond completely drained summer 2001
Waterfowl returning to the wetland as it refilled.
History
The St. Olaf Natural Lands are home to 15 wetlands and ponds. They range in size from small temporary wetlands to ponds with up to 9 acres of surface water. The ponds and wetlands were drained to use the land for agriculture. This occurred almost immediately after immigration and homesteading entered Southern Minnesota. Half of Minnesota’s wetlands were drained over the last 130 years. Between 1950 and 1970, the restoration of wetlands in the Northfield area started.¹ In 1992, St. Olaf began the process of restoring the campus wetlands to their original locations and functions. This project was part of the US Fish and Wildlife Service Wetland Restoration Program. The goals of wetland restoration were creating wildlife habitat, improving water quality, and providing flood protection.² Wetland sites were identified through surveying and topographic maps. They were restored by first destroying existing drainage tiles, which diverted naturally accumulating water away. Secondly, constructing dikes, embankments made of earth to keep water in newly created wetlands.³ The Ole wetlands are some of the 82,143 wetlands which are part of the Cannon River Watershed. Water that collects in the ponds flows east away from campus, through wetlands and ditches in the Northfield neighborhoods, and into the Cannon River. Water levels and the volume of water released are controlled with weirs. A weir is a structural barrier placed in the open drainage ditch of the pond to control wetland discharge flow.
The pictures to the left show the large wetland near Skoglund, Big Pond. The wetland weir was removed to drain the water in the late Summer, 2001. The weir was replaced in late Fall 2001, so the wetland filled during Spring of 2002.
Wetland Classifications
The types of wetlands in the natural lands can be classified into the broad categories of permanent and ephemeral wetlands. Both types are depressional wetlands, replenished either by surface water or groundwater. Surface water wetlands are formed when precipitation and overland water flow encounter a depression in the landscape and water accumulates. Groundwater wetlands are formed when a depression in the landscape intersects the underground water table and receives groundwater inflow as well as precipitation and surface water flow.⁴ Ephemeral wetlands proceed through an annual wet/dry cycle, filling in the spring/early summer and drying up at the end of summer early fall. Permanent wetlands are ponds that retain water throughout the whole year. These include Big Pond, Baseball Pond, Soccer Pond, and Duckweed Pond. Other ponds in the Natural Lands, such as James Farm Pond, Coyote Ponds, Eaves Ponds, and Dragonfly Pond are seasonally or temporarily flooded emergent wetlands.⁵ Learn more about the wetlands of the Natural Lands from the National Wetlands Inventory.
Northfield is in a region of the Upper Midwest called the Prairie Pothole Region. These are depressional wetlands in formerly glaciated regions. A pattern of rough concentric circles develops into a depression, which fills with water from precipitation or snowmelt. Submerged and floating aquatic plants live in the center while cattails grow closer to the shore and marshy sedges close to dryland.⁶ See Geologic History & Climate for more information on glaciation.
Wetland Functions
Habitat and Biodiversity
Wetlands are one of the most productive ecosystems on the planet. The St. Olaf wetlands provide important habitat for a variety of waterfowl and 9 species of amphibians, including frogs, toads, and salamanders. Ephemeral wetlands are important for landscape connectivity and support the biodiversity of amphibians and birds.⁷ The great volumes of vegetation attract immense numbers of other organisms and the broken down particles of plants, “detritus”, provide the basis of the food web. Detritus is eaten by insects and shellfish, which is a food source for amphibians, fish, birds, reptiles, and mammals.⁸ The importance of the wetlands in the agricultural area of Northfield is great. In agricultural areas, wetlands are necessary habitat for migratory waterfowl, providing stopping points for migration and breeding locations.⁹
American Bullfrog - St Olaf Natural Lands
Components of a wetland water budget - MN Wetland Restoration Guide
Seasonal Flood Control and Ground-Water Recharge
Wetlands are important in their ability to hold and slowly release large volumes of water. This is useful in the case of storms, holding stormwater and slowly releasing it, minimizing the chances and effects of flooding. The slow release of water also prevents erosion, and allows the water to remain in one place long enough to recharge groundwater sources. Wetlands act as a sponge and release water at the right moments, in the right direction. One of these directions is down. In this case, groundwater recharge occurs. In regions where water from aquifers is used for agriculture, the recharge from water seeping from wetlands down into the aquifer is essential.¹⁰
Water Quality
The water chemistry of a wetland is a reflection of its many water sources. These sources may include surface water recharge, precipitation, and groundwater discharge. Factors also include geologic setting, soil type, and human activity. However, a function of wetlands is water filtration and improvement of water quality. Excess nutrients or trace minerals which enter wetlands may be filtered out by plants absorbing them, particles sticking to plant roots, or microbial chemical transformations. The extended amount of time water spends in the system increases the opportunity for water filtration.¹¹
The maintenance and improvement of water quality are dependent on four things:¹²
Water: The ground and surface water transport material into the wetland and remove the by-products by transporting it out.
Substrate: The soil is where many chemical transformations are performed and stored.
Vegetation: Aquatic vegetation decreases the flow of water causing minerals and materials to be deposited or attached to plant roots. They are the surface of microbial colonization and store nutrients and other compounds in their tissue.
Microbes: The bacteria, algae, fungi, and protozoa which perform most of the chemical transformations.
Links of Interest
Works Cited
Allen, C., R. Gonzales, and L. Parrott. 2020. Modelling the contribution of ephemeral wetlands to landscape connectivity. Ecological Modelling 419:108944.
Knutson, M. G., W. B. Richardson, D. M. Reineke, B. R. Gray, J. R. Parmelee, and S. E. Weick. 2004. Agricultural ponds support amphibian populations. Ecological Applications 14:669–684.