Fish passage in a western Iowa stream modified by grade control structures

Grade control structures (GCSs) are commonly used in streams of western Iowa tocontrol bank erosion and channel headcutting but may be barriers to fish passage. From May2002 to May 2006, we used mark–recapture methods to evaluate fish passage over a total of fiveGCSs, ranging in slope (run : rise) from 13:1 to 18:1 in Turkey Creek, Cass County, Iowa. Threestructures, over which limited fish movement was documented from 2002 to 2004, were modifiedin the winter of 2004–2005 to facilitate fish passage. Before modification, the majority ofrecaptured fish were recaptured at the station where they were originally marked; only 1%displayed movement between sites and either upstream or downstream over a GCS. Aftermodification fish passage improved, 14% of recaptured fish displayed movement either upstreamor downstream over a GCS. Individuals of four target species—channel catfish (Ictaluruspunctatus), yellow bullhead (Ameiurus natalis), black bullhead (A. melas), and creek chub(Semotilus atromaculatus)—passed over at least one modified structure. The majority ofdocumented movements over GCSs were in the upstream direction and occurred in late springand early summer, when streamflow was relatively high. Although we documented low numbersof fish passing both upstream and downstream over GCSs, these structures are probably barriersto fish movement during periods of low flow and when there is a structural failure, such as inchannelmovement of riprap. Grade control structures are pervasive in western Iowa streams;nearly every low-order stream contains at least one instream structure. To sustain fishpopulations, management efforts should focus on constructing or modifying GCSs to allow fishpassage