Floods, drying, habitat connectivity, and fish occupancy dynamics in restored and unrestored oxbows of West Central Iowa, USA

Abstract

Abstract In the agricultural landscape of the Midwestern USA, waterways are highly altered. Oxbows are among the few remaining off‐channel habitats associated with streams, supporting fish assemblages that include the endangered Topeka shiners Notropis topeka in portions of their remaining range. Oxbow restorations seek to increase the number and quality of oxbows for Topeka shiners. For oxbows to provide optimal habitat, periods of isolation from streams and connection with streams during floods are necessary. Water levels and patterns of drying and hydrological connectivity between 12 oxbows and their neighboring streams in West Central Iowa were continuously monitored from May to October 2011, and fish assemblages were assessed for responses to the differing hydrodynamics using dynamic occupancy modelling. The 12 oxbows exhibited varied hydrodynamics and connectivity with streams. Two oxbows never contained fish; these oxbows never flooded and were among the three oxbows that were dry for the longest periods. Occupancy modelling suggested that connection with the stream via floods significantly increased the probability of colonization, and low water level increased the probability of local extinction from oxbows. Thirteen of the 16 fish species encountered had detection probabilities over 60%, and eight had detection probabilities over 90%, including Topeka shiners. None of the five previously restored oxbows flooded; all five contained fish, but only one contained Topeka shiners. Three of the four oxbows containing Topeka shiners flooded and all four dried at least once. These results suggest that planning for future oxbow restorations should consider: (i) sites that flood frequently; and (ii) construction methods promoting alternating periods of isolation from and connection with streams.