THERE'S A MESSAGE for the future of Maryland's 9,000 miles of freshwater streams in Montgomery County's decision a couple of years ago to limit development along some 20 miles of the upper Paint Branch, where trout thrive not far from the Capital Beltway.
The suburban county allocated $13.5 million, its whole parks acquisition budget for six years, to preserve open space in the little stream's watershed.
Only then could they ensure that "impervious surfaces" would never amount to more than 10 percent of the lands draining runoff from rainfall to Paint Branch, a local icon of environmental quality.
Impervious surface is the sum of the rooftops, driveways, roadways, gutters, tennis courts and parking lots humans create when they move in - all the places where rain no longer soaks in, or trickles slowly to streams.
Where it falls on impervious surfaces, rain sheets off toward waterways with such force and volume that it can erode banks like a bulldozer, wash away insect and fish communities and strip stream channels down to bedrock.
In droughts, the water that used to seep underground, recharging streams through their bed and banks, is so depleted that a stream may dry up, or run so shallow that its water becomes too warm for aquatic life.
Impervious surface, another name for development, was noted last week as one of the major reasons that almost half Maryland's nontidal streams are in poor health, with a mere 10 percent qualifying as healthy. The survey by Maryland's Department of Natural Resources looked at the chemistry and biology of 1,000 stream sites throughout the state.
Once development hardens even 15 percent of a stream's watershed, aquatic health falls off sharply, the survey found. By 25 percent, degradation is severe. Native brook trout may disappear after as little as 2 percent.
"Around 10 percent, which is equal to single homes on 1- to-2-acre lots, is often the tipping point ... where you begin to see decline," said Tom Schueler, director of the Center for Watershed Protection, an Ellicott City-based group that provides consultation and education nationwide on preserving stream health. "You see the exact same impacts [from development] from Topeka, to Biloxi to Columbus Ohio."
Maryland has many streams on both sides of the 10 percent to 15 percent imperviousness divide: remote Sideling Hill Creek in Allegany County is 0.8 percent developed; Octoraro Creek in Harford County is 3.6 percent, and the Gunpowder Falls in Baltimore County is 7.5.
The Jones Falls and Gwynns Falls - the parts upstream of Baltimore City - are 19.4 percent and 32 percent, respectively; and the Anacostia River around Washington is 33.2 percent.
Too often, it is a polite fiction that development, done legally and caringly, can be balanced with a healthy environment. As the Maryland streams study shows, it can - up to a point. But past a certain point, we are kidding ourselves.
Ways exist to "cheat" on the harsh realities of imperviousness. Leaving forested buffers along waterways retards and cleanses runoff, but it's not required. Maryland's new storm-water management regulations, which require detention ponds and other techniques to mimic natural rainfall runoff after development, "are the best in the country now," said Richard D. Klein, an environmental consultant who was instrumental in pushing the first storm-water rules through the legislature years ago.
But the jury is still out on how this will play in terms of healthier streams, as sprawling development progresses rapidly.
"Can we tread more lightly? I don't think we're there yet," said Ron Klauda, a Department of Natural Resources official in charge of stream health assessment.
"We can't right now find any watershed in the country that is at 30 percent, acting as if it was at 10 percent," said Schueler. He said this might dictate a "triage" strategy, channeling development into watersheds already heavily paved, to save healthier streams.
Klein said that Maryland, except for a few counties, is sorely lacking enough inspectors to enforce storm-water and sediment pollution laws. Also the state has "tens of thousands" of existing storm-water detention ponds at developments that don't do much for stream quality.
Still, restoration can occur, even on streams like Sligo Creek, a 35 percent-impervious watershed inside the Capital Beltway in Silver Spring, said Cameron Wiegand, chief of watershed management for Montgomery County. At a cost of $2.2 million over 10 years, improved wetlands, storm-water controls and other techniques have returned 10 native fish species there.
It's no Paint Branch, but Wiegand, who grew up playing on Sligo Creek, said, "I am immensely proud of that."
Neither is lack of paving in a watershed a guarantee of stream health. The Maryland survey said other major sources of degradation are from ditching and channelizing for farm drainage and removal of dead trees that clog stream channels, but are vital habitat for aquatic insects and fish that feed on them.
We have the technology and science to develop land in ways that lessen the impact on streams. But until we employ it better, we'd do well to put conservative limits on paving in watersheds where good stream health, or prospects for recovering it, exist.