Manatawny Creek Dam Removal
Introduction
In recent years, concern over degradation of aquatic resources, issues of liability, and maintenance costs, has led to the removal of several small dams around the United States. Approximately 35 dams have already been removed from Pennsylvania rivers and streams, and an additional 30 are scheduled to be removed over the next one to two years.
Although dam removal clearly has great potential in river restoration, there is little scientific evidence documenting the scope and magnitude of the ecological changes associated with dam removal. As a result of Pennsylvania's leadership in dam removal, the PA Department of Environmental Protection (DEP), through the Growing Greener program, has funded several restoration projects including a few that focus on the ecological consequences of dam removal.
In May 2000, the Patrick Center for Environmental Research of the Academy of Natural Sciences, in collaboration with the Greater Pottstown Watershed Alliance, the Delaware Riverkeeper Network, and the Berks County Conservancy, began a two-year multi-disciplinary ecosystem study of the effects of removing the Manatawny Creek dam neat Pottstown, Pennsylvania. This project represents one of the first comprehensive studies attempting to document the large-scale physical, chemical, and biological changes in a river system following dam removal.
The detailed scientific study of the ecological impacts of dam removal had four basic components:
- removing the dam;
- restoring the newly established riparian corridor within the former impoundment area;
- conducting a comprehensive, interdisciplinary study of the physical, chemical, and biological changes in the Manatawny Creek watershed to measure and evaluate project success;
- educating the public and providing technology transfer throughout Pennsylvania concerning the effectiveness of dam removal as a watershed restoration method.
Information on the project partners, the study site and interdisciplinary study of physical, chemical and biological changes in Manatawny Creek are presented here. Information presentations, links and other information on dams and river restoration are also provided.
Partners
This project represents a partnership whereby the Patrick Center for Environmental Research will provide scientific expertise, the Greater Pottstown Watershed Alliance together with the Berks County Conservancy will provide public outreach and education expertise, and the Delaware Riverkeeper Network will provide stream corridor restoration and landscape design expertise.
Greater Pottstown Watershed Alliance
The Greater Pottstown Watershed Alliance (GPWA) is a citizen action volunteer organization based in Pottstown, PA. Its mission is to Secure Habitats and Defend Streams (S.H.A.D.S) within the Greater Pottstown area. To accomplish their watershed-based enhancement projects, GPWA seeks out and creates cooperative alliances with local, state and federal agencies. Without these valuable alliances, GPWA would not enjoy the success and accomplishments they have created to date.
Delaware Riverkeeper Network
Delaware Riverkeeper Network is a nonprofit, membership organization that has worked since 1988 to strengthen citizen protection of the Delaware River and its tributary watersheds. An affiliate of the American Littoral Society, a national conservation group, Riverkeeper works throughout the Delaware's entire 13,000 square mile watershed which includes portions of NY, NJ, PA and DE. Their programs include a watershed-wide advocacy program; taking a stance on regional and local isssues that threaten water quality; a tributary task force initiative designed to organize and strengthen local communities working to protect local streams; restoration projects organizing volunteers to restore eroded streambanks using bio-engineering techniques; a volunteer monitoring program with sites along the entire length of the River; pollution hotlines; an enforcement program; and student intern opportunities. For more information, please check out their web site at www.delawareriverkeeper.org.
Berks County Conservancy
Berks County Conservancy is a non-profit membership organization established in 1974, with a mission to preserve Berks County's unique cultural and environmental heritage for the benefit of future generations. They have focused their efforts on preserving agricultural land and open space; protecting the quality of streams and ground water; preserving historic landmarks and scenic landscapes and by promoting those values which contribute to a sustainable future for the community. For more information, go to their web site at www.berks-conservancy.org.
Study Site
The Manatawny Creek, located in southeastern Pennsylvania, has a drainage area of 91.6 square miles (238.3 square kilometers), and is a tributary to the Schuylkill River and Delaware Bay. The headwaters are located in the New England Physiographic province near Oley, PA while the lower reaches (dam location) are located in the Piedmont Physiographic province near Pottstown, PA. Pottstown, with an estimated population of 21,614 (1996) is the major urban center in the watershed. The major uses of land are forest and open (56%), agriculture (41%), and urban development (3%).
Study Sites
Current studies have selected several sites upstream and downstream of the dam. The reference site upstream is at Glasgow Bridge (MG) while there are three sites in the impoundment region: one above the island (MI), one near the baseball fields (MB) and one at the dam (MD). Two sites are downstream of the dam; the first is just downstream of the dam (MR) and the final site is further downstream near the local college (MC). In addition, there are sites at the confluence of the Schuylkill River (SC) and upstream and downstream of the confluence in the Schuylkill River (SU, SD1, SD2).
Manatawny Creek Dam
A low-head or run of the river dam (a small dam that allows continuous water movement over the top), was built on the Manatawny mainstem around 1850. It is located about 500 meters upstream of the confluence with the Schuylkill River. The dam is approximately 2.5 meters in height, 2 meters thick, and 30 meters across, and is constructed of stone and concrete. Water is impounded upstream of the dam for approximately 800 meters. No fish ladders or other structures were ever constructed to allow resident or migratory fishes to swim upstream of the dam. The land along both banks of the Manatawny from the Schuylkill River to the upper reaches of the impoundment area are owned by the city of Pottstown, and are managed as open space and public parks. As of July 1999, Pennsylvania DEP designated the dam an "orphan" with no legal owner.
Research Areas
Flowing water systems are critical links in the transport of inorganic and organic materials from terrestrial regions to the oceans accumulating and exchanging materials via runoff, groundwater, wind and direct inputs. The composition of transported materials is the result of several factors including geology, climate, basin vegetation, in-stream biological and chemical processes, and season. Due to the interplay of the physical, chemical,and biological components of stream ecosystems, scientists use a multi-disciplinary approach to study them. For the Manatawny Creek Dam Removal Ecosystem Study, the Patrick Center conducted experiments and analyzing several different components of the creek ecosystem. An experimental design has been developed which is based on surveying and conducting experiments before, during, and after dam removal at the various sites upstream and downstream of the dam to monitor short-term and long-term changes to the stream ecosystem.
Geomorphology
(Jim Pizzuto and Jennifer Egan of University of Delaware, Tom Johnson)
Understanding the physical dimensions of a stream is important to understanding the structure and function of a stream. Dams often alter the physical processes of streams and their removal affects the flow of water, movement of sediment and overall channel morphology. Our research goals are to:
- document the changes in the form of the stream after the dam is removed,
- determine the patterns of sediment movement that cause those changes
- evaluate existing methods for predicting changes that will be caused by future dam removal projects elsewhere.
Our work primarily involves surveying, sampling, and describing the sediments along the bottom of the stream, and measuring water current velocities and the rate of movement of sand and gravel along the bottom of the stream.
Streamwater Chemistry
(David Velinsky, Paul Kiry, Jennifer DeAlteris, and Faith Zerbe of the Delaware Riverkeeper Network)
The creation of impounded waters by dams is believed to degrade water quality as the result of several processes. For example, increased water residence times in impoundments can lead to higher water temperatures, which in turn cause reduced concentrations of dissolved oxygen. Water quality in the stream segments below impoundments is often lower than in segments above impoundments. Our objective is to monitor water quality through experimentation and analyses of several components including carbon, nitrogen and phosphorus compounds, biological oxygen demand, pH, temperature, conductivity, alkalinity, chlorine, and hardness.
Sediment Chemistry
(Jeffrey Ashley, Adam Boettner, Matt Wilhelm, and David Velinsky)
Sediments are known to accumulate behind dams and in urban watersheds. The potential for the accumulated sediments to have elevated levels of specific chemical contaminants (e.g., trace metals, petroleum hydrocarbons, pesticides) is high. Our objective is to determine the concentrations of various trace metals and organic contaminantsin the sediments before and after dam removal to determine how these concentrations change with sediment movement and water flow.
Food Webs
(Danielle Kreeger, David Velinsky, Karen Bushaw-Newton, Catherine Gatenby, Deborah Raksany, David Hart, and Richard Horwitz)
Healthy stream food webs comprise a complex array of plant-animal feeding relationships with strong links between certain groups. Dams break these linkages because biological communities are not allowed to move freely downstream or, as is the case with certain fishes, move upstream. The effects of dam removal on the overall community of organisms have not been well studied and are uncertain. Our objective is to determine how key aspects of the food web differ upstream and downstream of the dam, and how these aspects are altered by removal of the dam. Our approach will be to look at the breadth of food types available for animals that feed at the base of the food web and to compare these measures to the abundance and diversity of animals through the food web.
Dissolved Organic Matter
(Karen Bushaw-Newton and David Velinsky)
In lotic systems, particulate and dissolved organic matter (DOM) are important sources of carbon and energy to the microbial community. Fungi and bacteria utilize the varied components of OM and convert them to biomass, carbon dioxide, and other organic compounds. Dams may alter the residence times of organic matter in different reaches of a stream system and thereby affect processing of the organic matter. For example, in the impoundment region the flow of water is slower than in other reaches, and therefore there is opportunity for more organic matter to be processed. Our research objectives are to determine the organic matter dynamics of the system with respect to location of the dam.
Algae
(Don Charles, Roger Thomas, Michael Hoffman, and Erin Hagan)
From a management perspective, algae are important components ofthe river and impoundment habitats because they are a major energy source for the food web and because high levels of algal biomass can be an aesthetic problem. Removal of the Manatawny Dam will eliminate the algal nuisance conditions that exist in the impoundment duringat least parts of the summer, and may also affect algal biomass levels downstream from the dam as well. Downstream nutrient concentrations may be increased or decreased depending on the relative extent to which the impoundment was a nutrient sink and the newly exposed sediments may become a nutrient source. Dam removal may also influence algal assemblage composition and biomass by changing the nature and frequency of flows that scour algal growth, by altering populations of algal grazers, and by changing the temperature regime.
Freshwater Mussels
(Catherine Gatenby, Deborah Raksany, and Danielle Kreeger)
Where abundant, freshwater mussels play a signficant role in structuring trophic interactions and maintaining water quality because of the tremendous volume of water that is passed through their gills and transformed into products usable by other organisms; thus, they are considered to be an important organism in aquatic systems. Since freshwater mussels rely on fish to complete their life cycle, damsthat hinder fish migration can also impact mussel populations. The removal of dams, therefore, can have a very positive and significant impact to the future restoration of dwindling populations of mussels. Because they have limited mobility and are especially sensitive to degraded conditions, mussels also are of great value in assessing the health of local environmental conditions. Our research objectives are to determine the abundance and diversity of mussels prior to and following dam removal, to monitor changes in location and abundance of suitable habitat over time, and to monitor the overall health (condition, reproductive status) of two mussel populations, presently located above and below the dam, over time. Ultimately, we are interested in determining the ecological role of freshwater mussels in riverine systems.
Macroinvertebrates
(Tim Nightengale, Renata Malejki, Roger Thomas, David Hart)
Benthic (or bottom-dwelling) macroinvertebrates are important members of stream and river ecosystems. They have a diverse range of feeding adaptations, consume a wide array of food types, and are specialized for living in many different habitat types. Many fish species are also dependent on these invertebrates as food. Dams often change physical and chemical processes in ways that are likely to impact the abundance and diversity of benthic macroinvertebrates. Thus, one of the objectives of the Patrick Center's benthic macroinvertebrate studies is to determine how this assemblage responds to dam removal. Quantitative benthic samples are being collected at multiple locations (e.g., below the dam, within the present impoundment, and above the impoundment) prior to and following dam removal. Analysis of these data will be useful in developing models regarding the impacts of dams on ecosystem structure and function, as well as the recovery rates of and benefits to these valuable systems following dam removal.
Fish
(Richard Horwitz and Paul Overbeck )
In addition to their commercial and recreational importance, fish are important in ecosystems as predators on a variety of other groups and as prey of fish, birds and mammals. They are also important in nutrient cycling and movement, through excretion and through their upstream movements. A dam will affect fish by blocking movement up and down rivers, and by effects on habitat, temperature, and water quality upstream and downstream of the dam. We are studying the movements, abundance and community composition of fishes in various parts of the Manatawny Creek and adjacent Schuylkill River to assess different effects of the removal of the Manatawny Dam. A variety of techniques is used to collect samples, including boat electroshocking, backpack electroshocking, and trapping. Important questions include:
- How will effects of dam removal (sediment transfer, etc.) affect fishes in the Manatawny and Schuylkill rivers downstream of the dam?
- What is the change in community composition of fish in the current impoundment of the dam?
- Does the overall abundance of fishes increase or decrease in the current impoundment area as its habitat changes from pond-like conditions to stream conditions?
- The "scour hole" below the dam is an important fishing site in the area. What fishes occur inthis area? Is the loss of this hole compensated by formation of holes and pools in the reaches above and below the dam?
- Are there species of fish currently blocked from moving up the river by the dam? Will these species enter the upper Manatawny Creek after dam removal?
Riparian Vegetation
(Joy Lawrence of the Delaware Riverkeeper Network, Heidi Hertler, and Nichole Coulter)
Following removal of the dam, the width of the stream in the impoundment area will likely be decreased by up to several meters. This will expose potentially large areas of denuded, unstable sediments along one or both banks. If left to re-vegetate at a natural pace, these areas are likely to experience severe erosion and gully development, exacerbating sediment loads to the creek. Exposed banks will also be susceptible to colonization by invasive plant species; a large population of Canada thistle and purple loosestrife are already present in the park. In an effort to minimize these problems, the Delaware Riverkeeper Network has obtained funding from the Pennsylvania Department of Environmental Protection to implement a multi-phase plan for bank stabilization and the reintroduction of native riparian grasses, forbs, shrubs, and trees to newly exposed banks. In collaboration with these efforts, Heidi Hertler and Nichole Coulter of the Patrick Center are going to monitor changes in wetland and riparian vegetation using a combination of experimentation and surveys in upstream and downstream locations to assess whether dam removal results in a net increase or decrease in the vigor of the plant community.