Click here for an informational overview and statistics about the North American Beaver (Castor canadensis)
(Courtesy of NCpedia, an encyclopedia of North Carolina)
Impacts of Beavers on the Coharie Creek: A Review of the Literature
- Beaver dams and impoundments slow river flow, flood riverbanks, and turn rivers into more pond-like waterways (Pollock et al. 2003, Naiman et al. 1988)
- Beavers can indirectly cause increased sediment retention as well as increases in certain nutrients (Kroes & Bason 2015, Correll et al. 2000)
- Other potentially relevant forces like agricultural operation runoff also cause nutrient and other pollutant increases (Mallin et al. 2015)
Since beavers were reintroduced to North Carolina in 1939, the population has swelled to 500,000 (NC Wildlife Resources Commission). In 2002, the US Fish and Wildlife Service estimated that beavers would cause millions of dollars in damage to North Carolina infrastructure and natural resources. This is a particular problem in Sampson County, where beavers outnumber predators and professional trappers by a wide margin (WRAL). Tribal leaders involved in the Great Coharie River Initiative have suspected that beavers adversely affect the flow and water quality of the river. The Coharie Tribe has even sought permits from the state government to control the beaver population on the river. Still, the impact of beavers on the water quality of the river is mixed.
Our capstone team conducted a review of the literature seeking to summarize research on how beavers influence stream morphology and water quality. Our review could help inform recommendations for the tribe on how best to improve the quality and flow of the river.
As part of our review, we analyzed nine relevant articles, listed in the References section below. Our review of the literature found that beavers can have the following impacts on rivers and streams:
- Water quality – beavers can reduce water clarity and increase sediment retention, increasing the concentration of nitrates, ammonium, and total nitrogen, which can foster unwelcome growth of algae and invasive plants.
- Stream morphology – beavers can catalyze a shift from a free-flowing river to more pond-like hydrology by blocking the flow of water.
Each of these findings is discussed in more detail below.
Beaver impacts on water quality were somewhat ambiguous. The lower flow rates caused by beaver dams can lead to a buildup of nutrients in a waterway, particularly ammonium, nitrates and total nitrogen (Correll et al. 2000), (Kroes & Bason 2015). These additional nutrients can spur a large increase in plant growth, particularly invasive species such as Alligatorweed and Asian Spiderwort (TJCOG 2014). Some scientists suggest, however, that beaver dams could enhance water quality (Lazar et al. 2015). By holding back or ponding water, a beaver dam can allow microbes and plants to absorb and filter out excess nutrients and sediments from the river. Thus, the role of beavers on river water quality is complex. Overall, our review of the literature suggests that beavers can affect water quality, but other factors, such as runoff from farmland and nearby hog farms also can contribute to water quality issues.
The amount of nitrogen in a stream is influenced by factors such as fertilizer use on nearby farms, the amount of leaves falling into the river in autumn, and the number and type of plant species found on the riverbanks. Confined Animal Feeding Operations or CAFOs are present throughout Sampson County. “On the North Carolina Coastal Plain alone, an estimated 124,000 metric tons of nitrogen and 29,000 metric tons of phosphorus are generated annually by livestock. In general, CAFO wastes are either spread on fields as dry litter or pumped into waste lagoons and sprayed as liquid onto fields. Large amounts of nitrogen and phosphorus enter the environment through runoff, percolation into groundwater, and volatilization of ammonia. Many CAFOs are located in nutrient-sensitive watersheds where the wastes contribute to the eutrophication of streams, rivers, and estuaries” (Mallin et al. 2015). Additional factors such as plant detritus and downed trees in the river and deterioration of plant species near the riverbanks could also affect water quality (Kroes & Bason 2015).
Some scientists suggest that beaver dams could benefit water quality. Consider that a beaver dam near vibrant riverbank plants may actually help filter out sediments from the river (Lazar et al. 2015). As such, it would be a mistake to focus on the ‘beaver menace’ at the total expense of addressing these other factors. Our review of the literature suggests that beavers are certainly not the only factor of concern for the Great Coharie River Initiative, and perhaps some of these – most prominently discharged waste from nearby hog farms – have greater impacts on water quality than the beavers themselves.
Our review of the literature suggests that beavers affect stream morphology. In a 2003 study by the American Fisheries Society, researchers found that beaver dams slow the flow velocity of rivers causing sedimentation buildup (Pollock et al. 2003). This corroborated earlier research on the topic which claimed that beavers cause a defined transition from stream morphology into a “pond-like hydrologic environments” (Naiman et al. 1988).
Given the Tribe’s interest in restoring the river, several recommendations emerge from our review of the literature.
Improving the flow of the Great Coharie Creek – the tribe should work to restore the riverbanks, remove fallen trees from the river and control the beaver population. Controlling the beaver population could, however, adversely affect water quality.
Improving water quality – While beavers could impact water quality, it is unclear whether beavers or agricultural runoff pose the greatest threat. Further sampling and analysis would help identify the true cause of reduced or impaired water quality. We reiterate the recommendations of the TJCOG reports, which is to gather with farmers, landowners and community leaders to develop and implement strategies and practices to control discharges from farms.
Overall, rebuilding the banks of the river, reducing polluted runoff from drainage and irrigation ditches, re-planting native species, controlling the beaver population and removing downed trees will result in a more river-like flow and enhance the water quality of the Great Coharie Creek.
Correll, David L. (2005). Principles of Planning and Establishment of Buffer Zones. Ecological Engineering, 24(5), 433-439. doi:10.1016/.2005.01.007
Correll, David L., Jordan, Thomas E., & Weller, Donald E. (2000). Beaver Pond Biogeochemical Effects in the Maryland Coastal Plain. Biogeochemistry, 49(3), 217-239.
Kroes, Daniel E. & Bason, Christopher W. (2015). Sediment Trapping By Beaver Ponds in Streams of the Mid- Atlantic Piedmont and Coastal Plain, USA. Southeastern Naturalist, 14(3):577-595. doi: http://dx.doi.org/10.1656/058.014.0309
Lazar, J. G., Addy, K., Gold, A. J., Groffman, P. M., Mckinney, R. A., & Kellogg, D. Q. (2015). Beaver Ponds: Resurgent Nitrogen Sinks for Rural Watersheds in the Northeastern United States. Journal of Environment Quality,44(5), 1684. doi:10.2134/jeq2014.12.0540
Learning. (n.d.). Retrieved December 13, 2016, from http://www.ncwildlife.org/Learning/Species/Mammals/Beaver2
Mallin, Michael A., Matthew R. Mciver, Anna R. Robuck, and Amanda Kahn Dickens. (2015). Industrial Swine and Poultry Production Causes Chronic Nutrient and Fecal Microbial Stream Pollution. Water, Air, & Soil Pollution, 226.12
Naiman, R., Johnston, C., & Kelley, J. (1988). Alteration of North American Streams by Beaver. BioScience, 38(11), 753-762. doi:10.2307/1310784
Pollock, Michael M., Heim, Morgan, & Werner, Danielle. (2003). Hydrologic and Geomorphic Effects of Beaver Dams and Their Effect on Fishes. American Fisheries Society Symposium.
WRAL. (2002, December 18). Trappers Busy as Beavers In Sampson County: WRAL.com. Retrieved December 13, 2016, from http://www.wral.com/news/local/story/103671/