Small Dams: Research Programs
To predict how streams and watersheds are likely to respond to the potential removal of small dams, we believe that it is important to first understand the ecological effects of existing small dams. Accordingly, the central goal of our study is to quantify how dams of different size influence various physical, chemical, and biological properties of Pennsylvania streams. To gain an understanding of how dams of different size influence stream characteristics, we are studying how the ecological effects of dams vary along a continuum of dam characteristics, including dam height and storage volume.
Assessments of physical habitat, river channel structure, water quality, periphyton (attached algae), benthic macroinvertebrates, fish, and riparian vegetation have been made downstream of each dam and at an upstream reference location.
Habitat Assessment
Jamie Carr and the project leader,
Dr. Rebecca Brown, assessing habitat
below a dam.
The quantity and quality of available stream habitat has a significant effect on biological communities. Dams can potentially alter flow and sediment transport, and thus can impact channel shape and physical habitat. Visual habitat assessments were made following EPA rapid assessment protocols, and include estimates of biological cover, substrate embeddedness, water velocity and depth, pool variability, sediment deposition, riffle frequency, channel modifications, and bank stability. Quantitative measurements of physical habitat also included channel dimensions and bed substrate size distributions.
Geomorphology and Sediment Characterization
Katie Skalak setting up a transect line
prior to characterizing sediment
particle size.
Dams can cause significant changes in both the volume and size distribution of the sediment along a stream channel. Because dam removal can likely mobilize a substantial fraction of the sediment stored behind a dam, any risk assessment of the effects of dam removal must consider sediment-related impacts. To determine the effect of the dam on sediment size and distribution, pebble counts were conducted at both upstream and downstream reaches.
To determine the effect of the dam on channel morphology, surveys of each site were conducted using a Total Station surveyor. A mid-channel longitudinal profile was constructed along the entire length of both the upstream and downstream reaches to quantify potential differences in slope or pool/riffle structure, which may result from the presence of the dam. Three cross-sections of the channel were also surveyed at both the upstream and downstream reaches to determine the effect of the dam on channel width and depth. The size distribution and thickness of sediment fill behind a dam are being assessed at a select number of sites using a chirp sonar system, which can provide a high-resolution image of the sediment surface and subsurface material.
Water Quality
Jamie Carr using a YSI multi-probe
meter to record depth, temperature,
conductivity and dissolved oxygen
within a dam impoundment.
Water quality is an important aspect of stream ecological integrity. Samples were collected upstream and downstream of dams during low-flow periods to characterize dam-related water quality impacts. Water temperature, pH, dissolved oxygen and conductivity were measured at all study sites, including within the impoundment, using a YSI DM 6000 multiprobe meter. Water samples are being analyzed for total suspended matter, soluble reactive phosphorus (SRP), nitrite+nitrate (NO2+NO3- N), dissolved organic phosphorus (DOP), dissolved organic nitrogen (DON), total phosphorus (TP), total nitrogen (TN), dissolved organic carbon (DOC), and dissolved silicate. In addition, particles within each water sample are being analyzed for suspended chlorophyll a, particle size distribution, particle biochemistry, and bacteria cell concentration. Water quality data will be analyzed with respect to several potential effects of the dams including the type of dam release (surface or bottom) and residence time of the water within the impoundment. In addition, elemental carbon, nitrogen and phosphorus ratios will be used to assess within-impoundment transformations and the degree to which nutrient concentrations potentially limit primary productivity (i.e., the growth of algae, including diatoms).
Periphyton (attached algae)
Diane Winter scrubbing algae from
a rock.
Periphyton are the major primary producers in most streams, and are extremely useful in assessing their ecological condition. Because periphyton exhibit a wide range of sensitivities to stress and pollution, they are good diagnostic indicators of particular types of disturbance. Periphyton are sensitive to physical and chemical factors that are likely to be influenced by dams, including substrate type and stability, nutrients, and water temperature. Periphyton samples were collected from rock substrates below each dam and at upstream reference points and are being analyzed for measures of biomass (chlorophyll a and ash free dry mass) and species composition. In addition, the percent cover and thickness of algal growth were measured and samples of abundant filamentous algae present in the study reaches were collected for identification.
Macroinvertebrates
Tim Nightengale using a D-net to
collect macroinvertebrates.
The use of benthic macroinvertebrates (aquatic insects, snails, mussels, etc.) to assess ecological conditions in streams and rivers is widely accepted among state and federal water resource agencies. Benthic macroinvertebrates include species with a diverse array of habitat preferences, feeding types, and pollution tolerances, thus providing considerable information concerning the nature and cumulative effects of stress. More specifically, benthic macroinvertebrate communities are sensitive to variations in sediment and flow characteristics, concentrations of dissolved oxygen, temperature, quantity and quality of organic matter, and other environmental factors potentially affected by dams. Benthic macroinvertebrate assemblages were sampled at each study site using U.S. EPA rapid bioassessment protocols. The organisms in these samples are being enumerated and identified in the laboratory.
Fish
Amanda Kindt and Kevin O'Donnell,
surveying the fish population.
Fishes are commonly used in assessments of the biological integrity of streams. Stream fish communities respond to numerous environmental attributes, including water quality, hydrology, and habitat structure. The effects of dams on migratory fishes and other recreationally important species are of particular interest. Because dams can affect fishes in the free-flowing sections above their impoundment (e.g., by migratory blockage), the upstream reaches are not appropriate reference sites for all aspects of fish community structure. Instead, reference sites on similar streams, some sampled for other studies in the region, are being used as reference sites. Fishes have been sampled below each dam and at comparable reference streams with backpack or tow-barge electroshocking gear. Standard metrics of community structure (e.g., species richness, richness of habitat-specific groups, relative abundance of different trophic groups) are being calculated to compare below-dam and reference sites. In addition, fishes are being sampled above each dam and compared with those recorded downstream to determine whether the dam is blocking fish migration.
Riparian (streamside) Vegetation
Dr. Rebecca Brown surveying riparian
vegetation along one of the study
streams.
Riparian zones (the areas along rivers that are periodically flooded) contain highly diverse plant communities that are structured by flooding, which creates disturbance and acts as a dispersal mechanism for plants. Because dams potentially change the flood disturbance regime and block the downstream dispersal of plant propagules (i.e. seeds), they may cause a downstream change in native and exotic plant species diversity. Dams also drown the riparian zone within their impoundments and cause the riparian ecosystem to become fragmented. To assess the effects of dams on riparian plant communities, vegetation plots were established below each dam and at upstream reference points (unaffected by the dam) to compare native and exotic species diversity in the dammed sites and reference sites. In addition, vegetation plots were established adjacent to each impoundment to document the potential effects of inundation on riparian vegetation.