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South Fork Rivanna Reservoir helped by trees and fish
by Tim Shea | Sunday, October 28, 2012 at 7:38 p.m.

The South Fork Rivanna Reservoir, which has been under recent scrutiny for both water supply and recreational uses, is getting some good news.

Upstream from the reservoir, stream banks are being restored and trees are being planted in an effort to reduce sedimentation. Lurking beneath the water, 635 grass carp have devoured a colony of invasive hydrilla plants that had grown to cover almost one-third of the reservoir.

While the Rivanna Water & Sewer Authority continues to review a dredging plan for the reservoir, other decisions made in support of the Charlottesville-Albemarle 50-year community water supply plan are intended to benefit the water impoundment built in 1966.

“I’m thrilled with our mitigation efforts,” said Andrea Terry, the RWSA’s water resources manager. “They are having a positive environmental benefit on our community.”

Required by the Clean Water Act, the RWSA must offset any damage that construction of the new Ragged Mountain dam will cause to the environment. As a result, RWSA officials developed a mitigation plan that focuses on stream bank enhancement, wetland construction and preservation of the existing riparian corridor.

“As part of our preservation efforts, we’re planting about 94 acres of trees along Buck Mountain Creek and Piney Creek,” Terry said. “We planted the first phase in May 2012, and the remaining 92 acres are scheduled to be planted between now and April 2013.”

While the 2006 conceptual mitigation plan identified a potential 200 acres for planting, in the final design, approximately 94 acres were selected.

In addition to plantings, the plan’s preservation component works to protect the riparian corridor under deed restrictions, which define in perpetuity the types of uses that can and cannot be performed on the land. In total, there are currently 590 acres in preservation, including the 94 soon-to-be-planted acres.

Just downstream from the RWSA’s initial planting site, stream bank enhancements have already begun.

“Some of these banks were completely eroded out, and some as high as six and seven feet,” Terry said. “We’ve graded back the slopes of streambanks and we’ve added ‘J-hooks’ (lines of large rocks in the shape of a J) in places to slow the water down.”

Terry pointed at clear water flowing along the repaired banks. Erosion is a major cause of increased sediment loads in streams and rivers, so water clarity is a good gauge for success.

“As part of building the Ragged Mountain Dam, there are impacts,” Terry said. “If we can do something upstream in our watershed to improve water quality in any of our reservoirs, that’s a good thing.”

In the South Fork Rivanna Reservoir itself, large mats of hydrilla are no longer hindering the reservoir’s boaters and fishermen, because carp have eaten all of the aquatic plant.

“It’s unbelievable,” the University of Virginia’s women’s rowing coach Kevin Sauer said. “RWSA put the carp in and we never saw the hydrilla again.”

During a 2008 survey by the South Fork Rivanna Stewardship Task Force, Sauer pointed out the hydrilla colonies and explained how they were creating problems for boaters.

“The hydrilla had choked off the entire reservoir from above our dock to the Reas Ford bridge (approximately 2.5 miles),” Sauer said. “It was affecting fishermen and recreational paddlers, and was really obstructing our practices.”

For some fisherman, moving through the reservoir is easier, but the catch is smaller.

“The hydrilla made the fishing better, but our boats did get stuck quite a bit,” James Fitzgerald, a fisherman from Orange County said.

“It was like a green mat on top of the water that would snag our oars and skegs,” Tom Jones, a local paddler, said. “It was a problem for boaters and fishermen, but the real issue was and is the health of the reservoir.”

Hydrilla is a submersed, rooted plant, and while it can serve useful purposes, such as food and habitat for forage fish, it is invasive in Virginia, and an overabundance of the plant can prove detrimental.

For example, a 2010 RWSA report on hydrilla noted that, when encompassing more than 25 percent of a body of water, hydrilla can cause large areas of low oxygen levels, daily pH swings and eventual disease to waterfowl.

As a result, the RWSA opted to use the grass carp, which have a life span of five to nine years, and can grow to weigh up to 30 pounds. The Virginia Department of Game and Inland Fisheries approved the fish for weed control in 1984.

In April 2011, RWSA officials released the grass carp into the reservoir, and no hydrilla sightings have been reported this year.

“When we did our boat survey last month, we didn’t see any hydrilla,” Terry said.

“It’s so much better,” Sauer added. “Now we can utilize the entire reservoir again. I commend the RWSA for figuring this out.”

“My hat is off to RWSA,” Jones said. “I’m delighted that it worked so well and so quickly.”

Over the next few years, the RWSA plans to continue monitoring both the carp and hydrilla populations to determine the efficacy of the carp. If needed, more fish may be added to the reservoir.

 

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