Report: Population density affects Rivanna watershed

By Jeremy Borden / Daily Progress staff writer
October 13, 2006

The aquatic life in about half the streams in the Rivanna basin is impaired and the cause is strongly linked to population density around the streams, according to a study to be released today.

The study, undertaken by Stream-Watch, a combination of eight local government and environmental groups, shows that even when population density is relatively low, streams in the Rivanna watershed feel the effects. Fifty-two sites were monitored, but 24 were selected to be representative of the entire basin because of location and other factors.

The study began in the winter of 2003 and finished last fall, representing a large collaborative effort among 60 volunteers, a slew of technical and scientific advisers and Albemarle and Fluvanna counties. The Rivanna watershed or basin is about the size of Albemarle County but parts of it spill into Fluvanna and Orange counties.

The finding that around half the streams in the basin are impaired is in line with the rest of the Virginia, according to the Department of Environmental Quality. The state has studied about 30 percent of Virginia’s streams.

“We were able to link specific levels of disturbance with specific levels of stream degradation,” said John Murphy, the director of StreamWatch and the primary author of the report. While the study focused on several factors of stream impairment, such as forest cover and the effect of road pavement, it was able to show that such problems begin at around 55 people per square mile.

“That corresponds to one dwelling per 27 acres,” Murphy said. Once a stream becomes impaired, he said, it is impossible to reverse the effect without “extraordinary” measures such as changes in how government controls development or restoration efforts.

The study found that nine out of 10 times when population density hits 55 people per square mile, the stream is likely to be impaired. When it hits 210 people per square mile, the impairment was substantial.

Impairment was defined as those streams suffering from pollution and having clear signs of human-caused stressors such as excess sedimentation, among other factors. The group used DEQ standards to determine whether a stream was “impaired.”

StreamWatch published a study in April 2005 that showed seven of nine streams in developed areas in Albemarle were impaired.

But the current study, Murphy said, gives the bigger picture and points out that not only urban streams are affected.

“We haven’t up until now been too sure what’s going on in ex-urban areas” or areas that are just outside urban areas but not quite rural, Murphy said.

The study is unique in that it points out a specific level of population where impairment begins.

“I think it tells us that waters in Albemarle County aren’t quite as healthy as we might have expected and certainly as we would want them,” said Richard Parrish with the Southern Environmental Law Center.

“Business as usual isn’t working,” he said. The study’s findings do not indicate that the streams are harmful to humans, but that doesn’t mean Albemarle County isn’t approaching that critical point, he said.

“There must be something that we as humans are doing,” said Ridge Schuyler of the Nature Conservancy’s Piedmont program and a member of StreamWatch. “The next step really does need to be, ‘How do we take this statistical analysis and turn it into real science that we can use to make real decisions?’ … I want to dig deeper to understand.”

Greg Harper, the water resources manager for Albemarle County, said it shows the county’s land-use policy of directing development into certain areas – the county’s so-called “growth” areas where officials say they want to put the bulk of development – is a responsible planning tool that helps preserve natural resources.

“By concentrating land development, we can have an overall positive effect on the streams,” Harper said.

“The correlation between population and stream quality is intuitive really,” he said. “The surprising part is that impacts are seen at relatively low densities. … The answer is that people are messy. We don’t just build a small house in a large forest; we clear the forest [and] build roads. The way we live has a large effect on stream health.”

Contact Jeremy Borden at (434) 978-7263 or