Midwest Region


Union Slough National Wildlife Refuge
1710 360th Street
Titonka, IA 50480
(515) 928-2523, Fax (515) 928-2230

September 26, 2011

Union Slough Marsh Lowered to ‘Kick-Start’ Food Chain


Photo by Indiana DNR

The water-level of the 1300-acre marsh at Union Slough National Wildlife Refuge (NWR) has been lowered or ‘drawn down’ to improve habitat for wildlife. When wetlands are drawn down, mudflats are created.  Mudflats are used for resting areas by ducks and geese.  Shorebirds are frequently seen scurrying across these same mudflats as they search for food such as tiny worms and insects. The primary purpose Union Slough NWR is to provide a place of refuge and breeding ground for waterfowl and other migratory birds. Food is an essential part of this equation and lower water levels are key.

In marsh systems, long-term productivity is greatest when the wetland cycles through wet and dry phases over time. In effect, the food chain is ‘kick-started’ each time land moves through this cycle. With Iowa’s high precipitation rate, deeper wetlands like the Union Slough marsh tend to get ‘stuck’ in the wet phase for years (possibly decades) at a time. While some plants and animals like to “get their feet wet,” the kinds of aquatic insects and plants that migratory birds and other waterfowl like to eat do not take hold in stable high water levels. Add an increase in the fish population, most notably carp, bullheads and minnows, and this sort of food even less likely to grow.

Being stuck in the wet phase not only reduces food production, but also makes the food less available to the birds. The maximum feeding depth for the vast majority of waterfowl and wetland-dependent migratory birds is measured in inches rather than feet. Consequently, the high water in the wet phase commonly makes what little food is produced out of reach for most of the birds that need it.

The more shallow and varied the water levels, the more diverse the bird life. Deeper, wetter marsh areas, like those found along County Highway B-14 this summer illustrate this point. Other than a few pelicans, very few birds could be seen in this area at that time. This picture will be changing though, as the dry conditions of late summer have allowed the Refuge staff to open the water control structures on the north and south ends of the marsh to release water. The water level will remain low over the winter in order to freeze out the fish and set the stage for a boom of food-producing plants and aquatic insects when the marsh refills next spring. In the meantime, the shallow water and mud flats are making a smorgasbord of food resources available to a wide variety of waterfowl and other migratory birds this fall.

The importance of wetland cycling is widely recognized by the conservation community beyond Union Slough NWR. The Iowa Department of Natural Resources and Ducks Unlimited are currently designing and implementing water manipulation systems to boost production in deep marshes and shallow lakes throughout north-central Iowa.

Union Slough NWR is located 6 miles east of Bancroft on County Road A-42 in Kossuth County, IA. The Refuge is managed by the U. S. Fish and Wildlife Service as part of the National Wildlife Refuge System. For further information about the Refuge, please call (515) 928-2523.


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Last updated: June 15, 2016