Influence of Wing Dam Notching on Fish, Aquatic Macroinvertebrates, Hydrographic Relief, Current Velocity, Substrate, Water Temperature, and Dissolved Oxygen in Pool 13, Upper Mississippi River

Abstract

Six wing dams and an adjacent side channel in Pool 13 of the upper Mississippi River were studied in 1978-1980 to determine effects of wing dam notching on fish, aquatic macroinvertebrates, hydrographic relief, current velocity, substrate, water temperature, and dissolved oxygen. Three wing dams were notched in May-July 1979. Water temperature and dissolved oxygen concentration did not appear to be affected by notching. No major changes in the configuration of river bottom adjacent to the wing dams or in the side channel were detected after notching, but changes in the structure of wing dams caused by notching and repair were evident. Current velocity increased significantly below notches. An increase in the proportion of sand in the substrate of the side channel appeared to be related to notching, but a general, significant increase in benthos populations in the main channel border (near wing dams) from the prenotching to postnotching period probably was not related to notching. Rather this increase was probably due to a severe reduction of Chironomidae (Diptera) and Potamyia flava (Hagen) (Trichoptera: Hydropsychidae) populations by unusually high discharge in July 1978 (prenotching period) and their subsequent recovery in postnotching 1979 and 1980. Benthos did not increase significantly in the side channel after notching probably because of the increase in sand which appeared to be caused by notching. High gravel content in the substrate and high benthic invertebrate densities below the notch in wing dam 26 in the fall of 1979 were probably a localized effect of increased flow caused by notching. Artificial substrates on the wing dams were colonized much more heavily than the predominantly sand river bed, which was sampled with a Ponar grab. A detrimental effect of notching may be the removal of productive substrate (wing dam rock) for aquatic organisms. There were no appreciable effects of notching on fish populations. The increased proportion of sand in the substrate of the side channel during the postnotching period may indicate the occurrence of a long-term increase in sediment deposition in the side channel, shortening its life. The predominantly submerged character of the wing dams in the study area may be more important than notching in reducing accretion of sediments between the dams and in the side channel.