In 2003, hydrologist Stephen Bowler wrote a report that details the history of the watershed that supplies the South Fork Rivanna Reservoir. At the time, Bowler served as Watershed Manager for Albemarle County and the
Rivanna Water and Sewer Authority
(RWSA), and the report provided suggestions for how to further protect the watershed as the community began developing a 50-Year Community Water Supply plan.
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Five years later, Bowler is with the Federal Energy Regulatory Commission (FERC) and works on permitting for new hydroelectric dams. Specifically, Bowler studies whether fish and other aquatic life will be impacted by such structures.
On September 29, 2008, Bowler addressed the
South Fork Reservoir Stewardship Task Force
with a brief update of his report. He began his comments by disclosing that he was not representing FERC, and had not received any compensation for appearing before the task force. He also disclaimed that he at one point worked for the Nature Conservancy, an organization that contributed to the community water plan.
“While working for TNC, I helped with some of the ideas about the pump-storage concept to Ragged Mountain Reservoir,” Bowler said. Bowler added that he supports the adopted plan, but said there might be benefits to dredging. His comments to the task force mostly dealt with the reasons for why sedimentation has occurred since the SFRR was first filled in 1966.
But first, Bowler asked each member of the task force to give a reason why sedimentation affects their interest in the South Fork Rivanna Reservoir, and he supplied several possible answers. The following are responses from the Task Force.
“Reliable water access/quantity/quality to fill Ragged Mountain Reservoir”
“Water quality/quality supporting native, riverine fauna”
“Healthy and diverse wetlands”
“Water quality in general” (suggested by John Martin
“Diversity of habitat at Ivy Creek Natural Area”
“Still water lake aesthetic”
“Educational value of changing watershed”
“Convenient access to recreational rowing”
“Restoration of SFRR capacity to contribute to water supply” (added by Dede Smith)
* Tom Jones also took issue with the way the options were phrased. “[The reservoir is] a unique riparian environment for recreation, for flora, fauna and there’s nothing like it with flowing water of this scale,” Jones said. He asked to check all the boxes.
Bowler said the feedback from the task force would help him address the technical issues. After tallying their responses, he discussed the many conceptual models that can be developed to depict how sedimentation happens. After providing intricate details of how stream flows can affect channels, Bowler then turned to answering the question – what will happen to the South Fork Rivanna Reservoir in 50 years if nothing is done to address sedimentation?
Bowler identified two options for gathering information to make a decisions on how to proceed with dredging. First, he suggested an inexpensive approach could be conducted using conceptual models by sifting through existing data (aerial photographs, previous bathymetric data). Second, he identified several ways in which the community could gather additional data. Bowler cautioned that if the community takes the second approach, they should determine the areas of the reservoir where various stakeholder interests overlap the most.
“That might help you restrict [the modeling] to the fewest number of cross-sections you need to get the information you need to get on with some decision-making, because if you model the whole reservoir, it’s going to cost a lot of money,” Bowler said.
Bowler than provided a summary of his 2003 report, which he said still accurately describes the nature of the watershed. The reservoir is silting in at a rate of about 15.6 million gallons a year, or 1% of its storage capacity. However, he added that rate can range widely depending on how many powerful storms hit the area in a given year. While the sedimentation does relate to current development in the watershed, Bowler’s report also describes how much of the sediment can be traced to agricultural practices of the 19th century.
“We logged and farmed very intensely to the point where we were down to single-digit numbers in terms of percentage of forest cover in many of the watersheds in this region, and we basically lost all of our topsoil in a short time,” Bowler said. He said much of this “legacy sediment” filled in many of the stream valleys creating large flood plains. To give a sense of scale, Bowler said archaeologists have found evidence native settlements from the 16th century that are as much as 14 feet below the existing flood plain. He claimed those geologic processes are still at work today and that efforts to reduce sedimentation by further restricting development will likely not do much to stop the sedimentation. Bowler said it might be possible to stop some of that sediment using ponds upstream from the reservoir, but that might cause other environmental problems. He did suggest that creating ad hoc fore-bays by mining sand before it enters the reservoir could be a cost-effective solution.
Bowler said if nothing is done, the future of the South Fork Rivanna Reservoir will continue to look more like a wetland in certain areas. The inner bends will fill in faster, and the reservoir will take on characteristics that are more like a river. “Parts of it are going to very attractive wetland habitats rather than a mud-hole,” Bowler said.
Bowler concluded his presentation by offering his views on dredging. He said inland dredging with disposal is rare, because most dredging done inland is done for navigational purposes and the dredged sediment is deposited at other places in the river or lake. He claimed most dredging done for capacity is done for “boutique” reasons and do not generate a large volume of sediment. Bowler did not comment directly on whether or not dredging the South Fork Rivanna Reservoir was a good idea.
John Martin asked Bowler what he would do if he were the watershed manager in 1962, and if he would recommend building a reservoir at South Fork Rivanna Reservoir given what is known now about legacy sediment. Bowler responded that the South Fork Rivanna Reservoir is unique because it has a very high “watershed to reservoir ratio” of 424 to 1. Most reservoirs are in the 30 to 60 to 1 range. He said this is one reason why sediment load is so high in the South Fork Rivanna Reservoir, but said there were too many variables to give a good answer to Martin’s question.
After Bowler’s presentation, the task force spent some time discussing how to receive public comment, and revisited the question of why the task force was created. Charlottesville Tomorrow will post that podcast and summary in a future story.
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