Environmental Impact of Dredged Material Disposal on the Upper Mississippi River at Crosby Slough: Fid-Bau Portal

Environmental Impact of Dredged Material Disposal on the Upper Mississippi River at Crosby Slough

J. W. Held

This study was designed to investigate the environmental effects of disposal of material from maintenance dredging on the aquatic habitat around Island 117, Pool 8, Upper Mississippi River. Dredging impacts of disposal of dredged material in the past have included the conversion of productive aquatic and semiaquatic habitat to sandy shoals and islands, as well as the impairment of backwater circulation. Disposal activity at Island 117 during 1974 produced no measurable effect on those variables considered. The reasons for the negligible impact may have been as follows: (1) a relatively small amount of material was deposited at the site in 1974 (11,144 cu yds compared to a 19-year average job size of 65,500 cu yds); (2) dredged material was not allowed to pass over the crest of Island 117 into backwater areas as it had in past years because of court-ordered dredged material placement limitations imposed by the State of Wisconsin; and (3) the variance of the baseline data, caused by annual, seasonal, and diel fluctuations, was significant for most variables. If 1974 disposal activities affected water quality in the study area, these effects were masked by the natural background variation in the variables considered. Turbidity and nitrite-nitrogen, which were increased by disposal of dredged material in 1973, were not significantly altered during 1974.