Fish and Macroinvertebrate Assemblages Following the Removal of Low-Head Dams in Two Pennsylvania Streams
Open Access
- Author:
- Hutchison, Brianna
- Graduate Program:
- Wildlife and Fisheries Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- November 18, 2008
- Committee Members:
- C Paola Ferreri, Thesis Advisor/Co-Advisor
C Paola Ferreri, Thesis Advisor/Co-Advisor
Jay Richard Stauffer Jr., Thesis Advisor/Co-Advisor
Walter Matthew Tzilkowski, Thesis Advisor/Co-Advisor - Keywords:
- dam removal
biomonitoring
assemblage structure
freshwater fishes
benthic macroinvertebrates
impoundment
river restoration - Abstract:
- Following the industrial revolution and the urbanization of America, dams became prominent features of the country’s landscape in the 19th and 20th centuries. Dams interrupt the natural connectivity of lotic systems by severing the flow of water, sediment, nutrients, energy, and biota between the upstream and downstream segments of streams and rivers. Removing dams and restoring streams to a more natural, free-flowing state is assumed to benefit aquatic organisms inhabiting the system. Despite the large number of dams being removed across the United States, few of these projects have focused on the effects of dam removal on biotic assemblages. The Pennsylvania Fish and Boat Commission has developed an active dam removal program as part of their efforts to restore fish habitat and migration of American shad in the Susquehanna River basin. This program provided the unique opportunity to evaluate the effects of removing small, run-of-the-river dams on biotic assemblages. Three small dams in the Susquehanna River basin, two on the Conestoga River in Lancaster County, PA (Hellberg’s and Hinkletown Dams), and one on Middle Creek (Franklin Mills Dam) in Snyder County, PA, were monitored. The Pennsylvania Fish and Boat Commission removed Hellberg’s dam in December 1999 and Franklin Mills dam in April 2000; Hinkletown dam was removed by the Pennsylvania Department of Transportation in March 2000 as part of a bridge repair project. The overall goal of this study was to determine if the fish and benthic macroinvertebrate assemblages found in areas that were previously impounded were different from or became similar to the assemblages of free-flowing reaches that were not affected by the impoundment in the years following dam removal. Fish assemblages were sampled at Hellberg’s, Hinkletown, and Franklin Mills impoundments in October 1999 by making a single pass around the shoreline of each impoundment at night using boat electrofishing. At each study site, fishes were also collected from three 200 m, free-flowing sampling reaches that were unaffected by the dam and impoundment in upstream and downstream sections using a towboat electrofishing unit in 1999. Benthic macroinvertebrates were sampled in the impoundments by sweeping a 30 cm D-frame net with a 500 µm mesh for 1 m along the bottom and in the water column for 1 minute in wadeable regions of the impoundment in 1999. In the free-flowing reaches upstream and downstream of the dam, benthic macroinvertebrates were sampled from riffles or runs using the same D-frame kick net. Five samples were collected at each sampling reach. The kick net was held against the bottom substrate while an area approximately one meter long and the width of the net wide was disturbed upstream (~0.33 m2), allowing detritus and organisms to drift downstream into the net. After the dams were removed, two additional 200 m free-flowing reaches were established within the area that was previously impounded at the Hellberg’s and Franklin Mills study sites and one additional reach was established at Hinkletown (due to the short extent of the former impoundment). Following dam removal (in years 2000, 2001, 2005, and 2006), all free-flowing reaches, including those in the formerly impounded sections, were sampled in the same manner as free-flowing reaches sampled in 1999. Due to differences in sampling gear and methodology used to sample the impoundments versus the free-flowing reaches, I could not make direct comparisons between the samples collected in the impoundment and those collected from free-flowing reaches; however, results from this study contribute to the growing body of dam removal research by demonstrating that fish and macroinvertebrate assemblages in formerly impounded stream reaches become nearly indistinguishable from those in permanently free-flowing reaches following dam removal. Non-metric multi-dimensional scaling (NMDS) ordination did not identify groupings associated with location relative to the former dam site (i.e., upstream, former impoundment, downstream) for any of the assemblages evaluated in free-flowing reaches. Samples from the formerly impounded reaches (FIMP1 and FIMP2) were randomly interspersed among the upstream and downstream samples on the NMDS plots. Global R statistics generated by analysis of similarities (ANOSIM) were not significant when comparing assemblages by location, except in the case of the Hinkletown macroinvertebrate assemblage (Global R = 0.50). In other words, the fish and macroinvertebrate assemblages at upstream, former impoundment, and downstream reaches of the Franklin Mills and Hellberg’s sites, and the fish assemblages at upstream and former impoundments reaches at the Hinkletown site, were very similar in terms of taxonomic composition. Although fish and macroinvertebrate assemblage similarity did not appear to differ according to location relative to the dam site, NMDS ordination and ANOSIM results indicated differences related to year at all three study sites for both fish and macroinvertebrates. Samples from 2000 and 2001 grouped together in the NMDS ordination, as did samples from 2005 and 2006. The 1999 samples usually formed their own group or appeared closer to the 2000 and 2001 samples according to NMDS ordination, and pairwise comparisons between 1999 and other years generally yielded high ( > 0.75) R values. Differences in assemblage similarity among years may have resulted because of environmental factors such as precipitation and timing of sampling events, or differences in individual efforts between years. As indicated above, my results were confounded by differences in sampling gears used and I was unable to make direct comparisons between the impoundments and upstream, formerly impounded, and downstream free-flowing reaches. Future dam removal studies should develop sampling methodology that allows for direct comparison between the very different types of habitat found in small dam impoundments versus free-flowing reaches. Expanding monitoring efforts to include quarterly or at least biannual collections, as well as habitat and water quality monitoring, will greatly enhance understanding of the ways in which small dams and their removal affect stream ecosystems. With over 100 dam removals scheduled to take place in Pennsylvania over the next several years and an ever-increasing number of small dams reaching the end of their useful lifespan, dam removal is certain to remain in the scientific and public spotlight for many years to come. Data from improved monitoring efforts will help to ensure that the money spent on dam removal projects is invested in such a way as to enhance available water resources for the benefit of both wildlife and human users.