Historically, the Mississippi River was an ever-changing system of sloughs, islands, sandbars, and water. Annual floods changed the course of the river, and created new wetlands, deposited nutrient-rich sediments on forests and prairies, and provided spawning habitat for fish. Summer low water enhanced the growth of wetland vegetation. These natural water level fluctuations provided a variety of rich habitat for native fish and wildlife species.
As European settlers arrived in the 19th century, large tracts of floodplain forest and prairie were lost the agriculture and other development. The Mississippi River became an increasingly important travel and trade route so Congress authorized a series of navigation improvements to be implemented by the Corps of Engineers. Thousands of wing dams and side channel closing dams were built to constrict the main channel and increase its depth. In 1930, construction began on a series of locks and dams to provide a 9-foot-deep navigation channel for heavy barge traffic. These dams created a series of 26 navigation pools extending from St. Paul, Minnesota to St. Louis, Missouri. The Corps was also given flood control responsibilities and assisted landowners in building levees that isolated the river from its historic floodplain except during extremely high floods.
These human-caused changes to the Upper Mississippi River have dramatically impacted fish and wildlife habitat. Over time, sediment from soil erosion has been filling wetlands, reducing water clarity, and destroying fish habitat. Increased water levels in the pools formed by the dams have accelerated island erosion and reduced the diversity of our bottomland forests. Many efforts are now underway to strike a better balance between human needs and the needs of fish and wildlife on the river.
Several divisions of Port Louisa (Big Timber, Keithsburg, and Louisa) were established in the 1940's and early 1950's following construction of the lock and dam system. These units were consolidated as the Wapello District of Mark Twain National Wildlife Refuge in 1958. Following the record-breaking flood of 1993, Horseshoe Bend and the Iowa River Corridor were purchased from floodplain landowners who decided farming was no longer economically feasible. In 2000, Mark Twain NWR was split into five separate refuges and the Wapello District became Port Louisa National Wildlife Refuge.
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