Historical Floodplain Sedimentation Along the Upper Mississippi River, Pool 11

Abstract

Many of the ecologically rich aquatic habitats created by the construction of locks and dams on the upper Mississippi River are now disappearing as a result of increased sedimentation. Sub-aqueous sediment cores were collected from two backwater lakes, one with a hydraulic connection to the main channel and the other with no low water connection to the main channel. Multiple lab analyses were combined with river stage data to provide a detailed history of sedimentation rates and processes in each lake. Estimated rates of sedimentation for the 1938-2003 post-lock and dam period range from 0.83-1.00 cm/yr in the backwater lakes. While the rates are much lower than rates found in most previous studies of upper Mississippi River backwaters, these rates of sedimentation are still high for the shallow backwaters and at least an order of magnitude above pre-Euro-American settlement rates. Analysis of a series of aerial photographs shows wide spread sediment deposition since dam operation, though there has been little terrestrial encroachment into the lakes. Large overbank floods contribute the majority of the sediment to the isolated backwater and a large percentage of sediment to the contiguous backwater. Terrestrial floodplain surveying and sediment sampling after the 2004 summer flood showed lower elevations generally had greater deposition and coarser sediments, though a large logjam also influenced flow direction and deposition patterns. Small floods on the upper Mississippi River are an important factor in floodplain evolution and have the ability to deliver sediment to backwaters on a near annual basis.