Triangle J Council of Governments Report
The Great Coharie Watershed in Sampson County, North Carolina is home to a wide variety of flora and fauna. A portion of the Coharie Creek has been designated a Natural Heritage Program Natural Area because it supports two relatively rare populations of Bluff Oak and Cypress Gum Swamp. It is also home to three rare freshwater mollusks: lampmussel, pod lance, eastern creekshell.
Recognizing the unique ecological value of the Great Coharie Creek Watershed, the Triangle J Council of Governments, in partnership with the NC Ecosystem Enhancement Program (EEP) and the NC DENR Division of Water Resources(DNR), conducted a detailed analysis of the Great Coharie Creek Watershed. The Triangle J Council of Governments, which is an intergovernmental organization that serves a seven county region in order to address challenges that transcend political boundaries, was uniquely positioned to spearhead the study. The study was conducted from January 2009 to October 2014 and included a series of reports which reflect findings from analysis of the watershed. The findings summarized here are primarily from two TJCOG reports:
- Final Great Coharie Creek Local Watershed Plan: Watershed Assessment Report, October 2014, and
- Final Great Coharie Creek Local Watershed Plan: Watershed Management Plan, October 2014
The key findings of the two TJCOG reports are summarized below. The summary focuses on four main areas: water quality, stream morphology and hydrology, invasive species and habitat. The summary also includes a few recommendations from the TJCOG reports.
Major Problem Areas
Nutrient enrichment and low dissolved oxygen are the two primary water quality concerns, although low dissolved oxygen seems to be a normal, natural condition for what TJCOG refers to as a blackwater swamp stream. The low dissolved oxygen observed is largely a natural phenomenon resulting form extremely low flow and impounding, possibly exacerbated by somewhat by dense growth of aquatic vegetation.
Nutrient pollution comes from agricultural field crops, turf grass production, and livestock operations, although within the study area, the greatest source of nutrient runoff was from agricultural crop land. According to TJCOG, surface runoff from agricultural and developed lands is quickly drained and transported to mainstem streams through a vast, interconnected network of ditches and waterways, which may account for nutrient issues observed in the waterways. (TJCOG Report, 2014:iii). Nearly every agricultural field has ditches along its edges. According to TJCOG, there are nearly 2 million linear feet of ditches within the Local Watershed Plan study area. Nutrients enter these ditches and waterways through various pathway including overland runoff, subsurface flows, and through airborne means when waste is spread as fertilizer during windy conditions.
Nutrient and sediment runoff is exacerbated by the lack of a vegetative buffer along the banks of these ditches and waterways. Buffers could help filter excess nutrients and reduce the impacts downstream. Without buffers, the likelihood of erosion is also increased. A vegetative buffer would also provide some shading, which would limit the direct sunlight on the creek, thus reducing water temperature and limiting algal growth.
Interestingly, TJCOG found that water quality at the outlet of the study area was better than in the headwaters, indicating that nutrient removal was occurring as water moved through the system.
Stream Morphology and Hydrology
In the Great Coharie Creek study area, the creek exists in a low slope, slow moving waterway with frequent flow obstructions such as beaver dams, downed trees and man-made impoundments. The low slope of the creek channel means that even a small fallen tree can become an obstruction that backs up water. In addition, in building dams, beavers create slower, warmer, open water conditions leading to lower dissolved oxygen. This can impact the survival rate of aquatic species. However, beaver dams and other obstructions such as downed trees provide some benefits to the creek. For example, obstructions slow the flow of water in the river and raise water levels along the floodplain, where excess nutrients can be taken up by plants. In its study of the Great Coharie Creek, TJCOG found that nutrient concentrations in the creek decreased as you move from headwaters downstream, indicating that nutrient removal occurs along the way.
Nutrient uptake and denitrification occurs as slow-moving water flows across active, forested floodplains. The primary nutrient processing mechanism occurs underground along the banks of the waterway. Under saturated conditions, bacteria on the roots of woody plants convert nitrogen in the water to nitrogen gas, which dissipates into the air. In addition, by spreading the flow of excess water during floods, beaver dams can reduce flooding downstream. In this way, the dams can provide a positive impact.
Invasive aquatic plant species, both native and non-native, were extensive throughout the study area. Low flow and impoundments, natural and man-made, in the creek create conditions conducive to excessive growth of algae and weedy invasive plants, which can rob the creek of dissolved oxygen and reduce the quality of habitat for native species. In areas where flooding has created open water and direct sunlight, aquatic plants can overrun small channels. According to TJCOG, this overabundant vegetation can “exacerbate flooding, reduce light penetration in the water column, harbor disease vectors (e.g., mosquitos), crowd out desirable vegetation, hinder recreational uses and consume dissolved oxygen as it dies and decays TJCOG, 2014:17). In other words, invasive plants are a problem.
Some common invasive species in the study area include Creeping Waterprimrose, Asian Spiderwort, and Alligatorweed. Most of the dense infestations are Asian spiderwort and a native species of smartweed.
As mentioned previously, the extensive network of agricultural ditches speeds the flow of excess nutrients (e.g., from fertilizers) and sediments into the smaller streams and eventually into the Great Choarie Creek. This sediment is deposited in the mainstem channel or in the floodplain, where it settles, degrading habitat for aquatic macroinvertebrates and fish. In addition, the extensive growth of invasive, aquatic plants chokes the stream channel, especially where flooding has created open water and direct sunlight.
Examining the Role of Agriculture
High nutrient concentrations can also be attributed partly to Concentrated Animal Feeding Operations (CAFOs) in the area. Sampson County and neighboring Duplin County have the highest densities of CAFOs in the state, and some of the highest nationwide.
CAFOs are well known to be detrimental to water quality, and have been relatively well studied in Eastern North Carolina. One variable that effects the impact a CAFO will have on a certain waterway is the distance to that waterway. Accordingly, we looked at the proximity of CAFOs to the 100 year flood plain in Sampson County in the form of a 500-foot buffer. We found that 63 CAFOs fall within this buffer zone and have a relatively high potential to negatively impact the water quality of the Great Coharie Creek.
Reduce runoff from agricultural lands through best management practices. These practices include proper nutrient management and fertilizer application and installation of vegetated buffers and waterways. Allowing ditches to become naturally vegetated with woody plants can help reduce agricultural runoff and erosion.
Protect the riparian floodplain. The seasonally flooded mainstem banks, or the riparian zone, serve the most important ecological functions in the Great Coharie Creek watershed. Floodplains can hold excess water during floods, reducing flooding downstream. They also provide crucial habitat for wildlife.
Remove obstructions to improve flow. As mentioned previously, downed trees, beaver dams and man-made impoundments can block the flow of water and change the morphology of the creek. Although these impoundments offer some benefits to water quality, their removal would improve river flow and make the creek more conducive to kayaking and canoeing. Careful, thoughtful management of beaver dams could help achieve the proper balance of improved water quality and improved stream flow.
The Great Coharie Creek watershed is a naturally slow-moving and low oxygen environment. Water moves slowly through the meandering creek, obstructed by beaver dams, downed trees, and man made impoundments. On balance, the impact of beavers is mixed. Beaver dams and other obstructions impede the flow of water and can convert a free-flowing creek into more of a swamp, obstructing the path of canoes and kayaks. However, slow moving water can help microbes and plants process excess nutrients, thus improving water quality.
Efforts to improve water quality should focus on reducing nutrient inputs into the Great Coharie Creek watershed, for example through the installation of vegetative buffers, especially woody plants, along the banks of agricultural ditches and small channels. In addition, encouraging landowners to engage in proper nutrient management and fertilizer application could also reduce nutrient run off.