PROBLEMS ASSOCIATED WITH RE-CONNECTING A HIGHLY ALTERED RIVER SYSTEM TO ITS FORMER FLOODPLAIN WETLANDS

A 1600 ac (647.5 ha) leveed agricultural bottomland site along a major mid-western river is being considered for wetland restoration. One strategy would involve removal of the levee, thus re-connecting the site to the river. The difficulty is to determine if this option has the best chance of re-establishing the former floodplain’s benefits of floodwater storage, water quality improvement, and habitat creation. Pre-settlement, the site was likely covered by floodplain forest, interspersed with a network of shallow lakes, sloughs and moist-soil areas, all seasonally flooded. Internal agricultural alterations included the emplacement of levees, ditches, drainage tile, and a pumping station. Profound external changes to the river have also taken place, including a lock and dam system, diversion of water from another basin, decreased floodplain area due to levee construction, and increased flow volume and flashiness due to expanding agriculture and urban development. One result of all of these basin-wide changes is that the river stage shows a much larger amplitude than it once did, with higher minimum stages and higher peak flood elevations. Summer minimums are especially altered, as the lock and dam system artificially maintains a higher water level for barge navigation. Historical hydrographs from 1878 to present show an episodic increase in average annual stage totaling roughly 4.6 ft (1.4 m) and an increase in average summer low stage of about 7.5 ft (2.3 m). Flashy flows and agricultural development have also decreased water quality by adding sediment and nutrients to the river. If the site were opened to this altered hydrologic regime, comparison of stage records to site elevations shows that prolonged floods with significant water depths would occur in most years. This would have the combined effect of impacting wetland vegetation and possibly rendering large areas of the site as “deepwater habitat”, hence disqualifying these areas from meeting jurisdictional wetland status. Poor water quality and invasion of exotic species may also reduce the value of the wetland. Other strategies are under consideration, ranging from an engineered (or controlled) connection to the river, to complete isolation from the river and a reliance upon non-riverine water sources such as precipitation, runoff, and groundwater.