Sedimentation History of Halfway Creek Marsh, Upper Mississippi River National Wildlife and Fish Refuge, Wisconsin, 1846-2006

The history of overbank sedimentation in the vicinity of Halfway Creek Marsh near La Crosse, Wis., was examined during 2005?06 by the U.S. Geological Survey and University of Wisconsin?Madison as part of a broader study of sediment and nutrient loadings to the Upper Mississippi River bottomlands by the U.S. Environmental Protection Agency, U.S. Fish and Wildlife Service, and U.S. Geological Survey. Historical sedimentation patterns and rates were interpreted from field-scale topographic surveys and sediment cores collected from the marsh and upstream flood plains. Historical maps and aerial photographs were used to establish the timing of disturbances and to document changes in channel patterns after Euro-American settlement (post 1846). Episodic overbank sedimentation patterns and rates were linked to watershed agricultural activity, large floods, artificial levee construction, channel alterations, and dam failures over the past 160 years. These forces affected sedimentation on and between levees, the development of alluvial fans and flood-plain splays, and the general pattern of flood-plain sedimentation through the upper and lower marsh. Historical overbank deposits, episodically deposited after about 1860, are as much as 6 feet thick in the upper marsh and as much as 4 feet thick in the lower marsh, representing a total volume of approximately 1.8 million cubic yards.

These stratified deposits consist of multiple layers of silt and clay, very fine to fine sand, and some medium to very coarse sand. Coarse-grained deposits are associated with flood-plain splays caused by breaches in artificial levees during large floods. Estimated sedimentation rates were highest from 1919 to 1936 [26,890 cubic yards per year (yd3/yr)] and exceeded by about 30 times the 1846?85 rate of 920 yd3/yr and exceeded by 7 times the 1994?2006 rate of 3,740 yd3/yr. The 1994?2006 sedimentation rate was the lowest since Euro-American settlement, but natural levees along the 1994?2006 channel of Halfway Creek through the lower marsh continued to form and are currently (2006) about 1 foot higher than the surrounding marsh. Natural levee building in the lower marsh from 1994?2006 was accentuated by the lack of overbank sediment storage in the upper marsh. The historical storage of sediment in the upper and lower marsh affects modern streamflow and sediment transport processes of Halfway Creek and Sand Lake Coulee through the marsh, and it also affects marsh vegetation and wildlife habitat. Results from this investigation will help improve the understanding of how past overbank sedimentation patterns continue to influence modern and future water quality, sediment transport, nutrient loads, and water-related resources in riparian habitats common to the Upper Mississippi River National Wildlife and Fish Refuge.