ASSESSMENT OF NEW CHEVRON STRUCTURES FOR INCREASING PHYSICAL HABITAT DIVERSITY WITHIN THE MIDDLE MISSISSIPPI RIVER, USA

Habitat losses and degradation on the Mississippi River and its tributaries are a national concern, as are efforts at habitat rehabilitation. Habitat rehabilitation along the Middle Mississippi River (MMR) is being undertaken by construction of new type of river training structure, called blunt-nose chevrons, within the river channel. Although chevrons were initially designed to facilitate river navigation, these structures now are being justified as a tool for promoting physical habitat heterogeneity within the channel. To assess the ability of chevrons to increase habitat heterogeneity, we compiled pre- and post-construction multi-beam and acoustic surveys along a 2.0 km (~164 ha) reach of the MMR near St. Louis, MO. We used these data to construct detailed 2-D hydrodynamic models of pre- and post construction conditions to assess changes in depths and flow dynamics for a range of discharge conditions from 0.5 to 3 * MAF (mean annual flow). For assessment of physical habitat, we used three metrics: depth-velocity fields, Froude number, and wake metric. We also assessed reach-scale habitat patch complexity for habitat classes based on depth and velocity fields using spatial statistical tools within Fragstats. Changes in physical habitat area between the pre- and post chevron reference conditions were small (<4% of the reach area). Changes in shallow water habitat area varied little (<2 ha). Small increases (up to 1 ha) in wake zone area suggest the chevrons created more flow complexity in the local area of each structure. The reach-scale patch statistics revealed little change in habitat diversity for higher flows (>1.5 MAF). However for lower flows (<1.5 MAF), patch density, edge density, Simpsons Diversity Index all increased (SDI by up to 21%), suggesting increases in physical habitat diversity for these discharge conditions. This investigation shows that the use of chevrons can increase physical habitat heterogeneity for lower flow conditions, at least for very small areas in the immediate vicinity of these structures. However, more work needs to be undertaken to assess if these changes are ecologically meaningful, if the small area of habitat impacts justify use of these structures, and if habitat improvements are counterbalanced by chevron-induced increases in flood risk.