EFFECTS OF HISTORICAL SEDIMENT LOADING AT HALFWAY CREEK MARSH, UPPER MISSISSIPPI RIVER NATIONAL WILDLIFE AND FISH REFUGE, WISCONSIN

The sedimentation history of Halfway Creek Marsh near La Crosse, Wis. was examined during 2005-06 as part of a broader U.S. Geological Survey, U.S. EPA, U.S. Fish and Wildlife Service, and University of Wisconsin-Madison study of sediment and nutrient loadings to the Upper Mississippi River bottomlands. Historical (1860-2006) sedimentation patterns and rates were interpreted from topographic surveys and sediment cores collected from the marsh and upstream floodplains. Historical maps and aerial photographs were used to establish the timing of disturbances and to document channel changes after Euro-American settlement. Episodic sedimentation patterns and rates were linked to watershed agricultural activity, large floods, artificial levee construction, channel alterations, and dam failures. These activities and events affected sedimentation on and between levees, the development of alluvial fans and floodplain splays, and the general pattern of floodplain sedimentation through the marsh. Episodically deposited historical overbank deposits, mainly composed of silty fine sand, are 1.2–1.8 m thick in the marsh, representing a total volume of approximately 1.38 million cubic meters. Sedimentation rates were highest from 1919-1936 when an average rate of 20,560 m3yr-1 exceeded by about 30 times the 1846-1885 rate of 705 m3yr-1 and exceeded by seven times the 1994-2006 rate of 2,860 m3yr-1. The 1994-2006 sedimentation rate represents the lowest since Euro-American settlement, but sedimentation continues especially on natural levees along the channel of Halfway Creek through the lower marsh. Historical overbank deposits affect modern fluvial processes and wetland/fluvial and vegetation dynamics. System-wide adjustments to historical sediment loading will continue for decades and centuries, mainly during moderate and extreme floods that remobilize stored sediment from upstream channel margins and transport it downstream into the marsh and eventually to the Mississippi River. The impact of human activities, frequency and magnitude of overbank sediment loading events, and continued fluvial adjustments, observed at Halfway Creek Marsh are representative of numerous tributaries that flow into critical back-water marshes along the Upper Mississippi River National Wildlife and Fish Refuge.