Degree Name

Master of Science (MS)

Semester of Degree Completion


Thesis Director

Robert E. Colombo


The natural flow regime of the majority of rivers in the United States has been altered by anthropogenic structures. This loss of connectivity plays a strong role in shaping river ecosystems by altering physical habitat characteristics and shaping fish community assemblages. Although the impacts of large dams on river systems are well documented and often easy to observe, there are fewer studies on the effects of smaller low-head dams. Additionally, the influences of low-head dams on the genetic structuring of small-bodied riverine fishes have yet to be fully explored. I assessed the effects of two low-head dams on the Vermilion and North Fork Vermilion Rivers in east-central Illinois to determine differences in river habitat quality in the vicinity of these dams and to investigate fish community assemblages driven by the presence of these structures. I also assessed how these structures affected genetic diversity and differentiation in two fish species (longear sunfish Lepomis megalotis and bluntnose minnow Pimephales notatus) using microsatellites. Data were collected at six sites on each river from 2012 to 2015; study sites included two sites below each dam, two sites in the pool above each dam, and two sites upstream of the pool extent. Fish communities were sampled using a multi-gear approach in spring and fall seasons. I used the environmental guild concept to aggregate fishes into habitat and reproductive guilds in order to ascertain dams' effects on groups of fishes that respond similarly to environmental variation. I found that habitat quality was significantly poorer in the artificial pools created above the dams compared to all other sampling sites. Fishes aggregated into habitat guilds reflected local habitat quality and habitat guild structure was associated with habitat type and location respective to the dams. Reproductive guilds were significantly driven by flow and also were associated with habitat characteristics. Genetic diversity and differentiation were estimated from 10 microsatellite loci for longear sunfish and 11 microsatellite loci for bluntnose minnow. Measures of genetic diversity suggested that there was no loss of genetic diversity in either species above the impoundments, nor did data reveal patterns in genetic diversity or FST values along a longitudinal river gradient. Data revealed that there is one relatively homogenous population of longear sunfish in the study area (global FST = 0.001, K = 1 as determined by the program STRUCTURE). In contrast, there were two genetically distinct populations of bluntnose minnow in the study area (global FST = 0.006, K = 2) separated by one of the two low-head dams in the river system. Overall, these low-head dams play a substantial role in shaping habitat, which appears to then shape fish community structure in the context of environmental guilds. On a genetic level, data suggested that these dams are impacting the two study species differently. Genetic differentiation in the bluntnose minnow appears to be impacted by only one dam, likely due to the poor quality, lentic habitat above this dam serving as a deterrent to fish movement.