Date of Award
Master of Science
Theses & dissertations (Interdisciplinary)
Nearly 400 riprap weirs, also termed grade control structures (GCS), have been placed in streams of western Iowa since the early 1990's to decrease erosion and protect bridge infrastructure and farmland. These structures consist of a 1.2 m high metal dam and a downstream apron of rock riprap, and are commonly placed downstream of bridges that are susceptible to bank erosion. Recently, concern has arisen that steeply sloped structures (i.e. 4:1 backslope) may be restricting fish passage and that sport fish populations, particularly channel catfish, may be declining in streams modified by GCS. However, increases in habitat heterogeneity provided by these structures may positively influence both fish and macroinvertebrate communities near GCS. Our research, beginning in 2004, used mark-recapture methods to evaluate fish passage over GCS in Turkey Creek, Cass County, Iowa, seasonal electrofishing to evaluate the effects of GCS on fish communities in Turkey Creek, and macroinvertebrate sampling from GCS riprap and non-GCS sites in Walnut Creek, Montgomery County, Iowa to evaluate the effects of GCS on macroinvertebrate assemblages. Baseline data from 2001-2003 indicates that channel catfish, flathead chub, and creek chub are able to pass over 20:1 sloped structures in Walnut Creek but have restricted passage over steeply sloped structures in Turkey Creek. Our results indicate that fish movement is restricted over steeply sloped structures (built with 4:1 to 10:1 original backslopes), and that fish are capable of passing over 13:1 to 18:1 sloped structures in Turkey Creek. In addition, we found that the modification of three steeply sloped GCS in Turkey Creek to more gradual slopes was successful in allowing fish passage and improving the overall stream fish community. Reaches downstream from GCS are characterized by greater maximum depth, pool habitat, and centrarchid abundance. In addition, GCS riprap supports greater macroinvertebrate abundance, biomass, and taxa richness than natural fine substrates. The presence of GCS in streams of western Iowa is pervasive, with nearly every low order stream containing at least one in-stream structure. Understanding the effects of GCS on stream ecosystems is essential in managing for the long-term sustainability of riverine life in this region.
Mary Emily Litvan
Litvan, Mary Emily, "Evaluation of fish passage and fish and macroinvertebrate communities in southwest Iowa streams modified by grade control structures" (2006). Retrospective Theses and Dissertations. 19323.