Eno River Hydrilla Management Project

History of Hydrilla within the Eno River Watershed

The Eno River is part of the Neuse River Basin.  The Eno begins in the northwest reach of Orange County, flows eastward through Durham County and enters Falls Lake. From its origin to Falls Lake, the Eno River flows for approximately 28 miles and encompasses a ~150 square mile watershed area.

In 2005, Hydrilla was discovered in a section of the river within the Eno River State Park boundaries. Prior to 2005, the only known occurrence of Hydrilla in the Eno watershed was at Lake Orange. Lake Orange has a history of Hydrilla dating back to 1992. In 1994 and again in 1998, triploid grass carp were stocked to remove this invasive plant and Hydrilla was thought to have been eradicated from Lake Orange. 

Summary of Eno River Hydrilla management activities

During the years 2015-2019, a section of the Eno River was treated with an aquatic-use herbicide. The treatment was accomplished using a sophisticated metering system. The specific section of the Eno that was treated was modified over the 5 years, but generally, the treated section was defined by Lake Ben Johnson dam and Roxboro Rd. This marked the first time a flowing body of water was treated using this method in NC.  The annual treatments have been very successful.

The herbicide treatments did not prohibit or impact in any way, recreational activities such as wading and swimming nor did it impact fishing or fish consumption. The treated water remained fit for consumption by animals (livestock, domestic pets, wildlife, etc.).

Monitoring efforts were conducted throughout the project; no negative impacts to the environment have been observed/reported to date.   

Project planning years

During the 10 years leading up to the start of this management activity, there was a series of meetings, research and the development of the technology (the specific herbicide product and the metering system).

From 2005-2015 staff from the Eno River State Park monitored the river annually, documenting the presence and observing the spread of Hydrilla in the river.  More extensive survey work was also done (2013-2014).  Those surveys included sections of the Eno that were outside of the park boundaries.  The objective of that survey work was to delineate the infestation, providing environmental managers with a complete picture of where Hydrilla is within the Eno.   

Non-herbicide control methods were explored.  In 2011 a team of volunteers spent a day hand-pulling Hydrilla from the river.  This approach proved to be extremely challenging and ultimately unpractical.  In 2014 the NC Wildlife Resources Commission coordinated a release of tagged sterile grass carp in conjunction with the deployment of tracking equipment.  Grass carp are very effective as a biocontrol agent for Hydrilla management in ponds and lakes.  The outcome of that study showed that grass carp did not remain in the section of the river where they were released. 

Toxicology studies were performed at NC State University to determine native mussel and native snail sensitivity to the herbicides being considered.  This research showed that there would be no impact on the mussel and snail survivorship, growth or reproductive abilities at the concentration levels needed to suppress Hydrilla. 

Development of the Eno River Hydrilla Management Task Force

Since this project presented many challenges there was a real need to develop a partnership.  Challenges included an herbicide treatment that would span jurisdiction lines, the implementation of an herbicide treatment method that had not been previously used in NC, an herbicide treatment that could potentially include a section of the river which is used as a municipal water source and funding hurdles.

The Eno River Hydrilla Management Task Force was formed in 2013 to meet this need.  It is a partnership between government agencies, academia and non-profit organizations.  Representation includes aquatic vegetation management experts, environmental managers, stakeholders and environmental stewards. 

Partners

City of Durham

City of Raleigh

Durham County

Eno River State Park

NC Dept. of Agriculture & Consumer Services

NC Dept. of Health and Human Services

NC Division of Parks & Recreation

NC Division of Water Resources

NC Natural Heritage Program

NC State University

NC Wildlife Resources Commission

Orange County

Town of Hillsborough

Triangle Fly Fishers

US Army Corps of Engineers

US Fish & Wildlife Service

Details of the herbicide treatment

 

2015

A liquid formulation of the herbicide fluridone was metered into the water at a single location.  The treatment began in June and ended in August.  The treatment effectively controlled the growth of Hydrilla throughout the ~16-mile-long stretch.  The treatment area was defined as the Lawrence Road bridge (just downstream of Hillsborough) and down to N. Roxboro Street (north of Durham).

2016

The treatment area and herbicide product remained consistent with the 2015 treatment. The herbicide product was metered into the river using two injector systems. The addition of a second injector system (located approximately midway through the treatment area) allowed for better control of the target concentration and allowed for a reduction in the total quantity of herbicide needed.  During the 2015 treatment, moderate chlorosis was observed in Water Willow plants within the treatment area.  Those plants recovered, and no chlorosis was observed in 2016, likely due to the switch to the two-injector arrangement. 

2017

The herbicide product remained consistent with previous years.  The treatment started on June 30th and continued through August 15th.  Since the first two years of managing Hydrilla with herbicide had been very successful the ERHM Task Force decided to expand the treatment area in 2017.  The area that was treated in 2015-2016 was included in the treatment. In, 2017, the treatment area was expanded upstream to just below the Lake Ben Johnson dam.  Two injection systems were used to meter herbicide into the river.

2018

Essentially a repeat of what was done in 2017, the treatment started on May 24th and continued through August 15th.  The herbicide (fluridone) was applied by two injection systems.  Each system precisely metered the herbicide product and was routinely adjusted to match the flow of the river. 

2019

The treatment area, herbicide product and methods remained consistent with the 2017-2018 treatment. The duration of the treatment was from June 17th to August 15th.