In the Chesapeake watershed, fish such as American shad and striped bass have more than 2,500 reasons why they don't migrate into streams where they historically spawned and fed. Barriers, many of them dams built so long ago its not even clear who owns them, block their path. But the fish are getting a helping hand: Pennsylvania leads the nation in dam removals, and efforts are being stepped up in Maryland and Virginia - where the large Embry Dam outside Fredericksburg was dynamited in 2004.

But from a fish-eye point of view, the problem posed by hundreds of thousands of little culverts is vaster and more insidious than the problem posed by dams. A poorly designed or installed road culvert can create obstructions or water velocities that stop a fish from swimming upstream as surely as a 100-foot dam. Unlike the blockages posed by dams, which are decreasing, the obstacles posed by culverts are increasing. As areas develop and roads spread, thousands more culverts slice the web of streams and creeks into tiny, isolated segments, like clipping the capillaries of the Chesapeake's circulatory system.

This partitioning of creeks reduces what biologists call "stream connectivity." It is a national problem for trout and salmon and a particular threat to the region's prized wild brook trout.

There is no official estimate of the number of culverts in the Mid-Atlantic, but the U.S. Forest Service has inventoried more than 150,000 culverts on its land in the East. It found that nearly half of all studied culverts at least partially blocked young brook trout. Out West, federal research found that more than half of culverts blocked migrating salmon.

An assessment by The Eastern Brook Trout Joint Venture, a collaborative effort among 17 states, six federal agencies, six nongovernmental organizations and two universities, considers stream fragmentation from road culverts one of the top 10 threats to wild brook trout across the states in the project. For example, Virginia fisheries biologists identified road culverts threatened brook trout populations in 60 percent of study areas.

With climate change looming on the horizon, this limit on movement poses new problems for brook trout, which need cool, clean water. Like a canary in a coal mine, brook trout are sensitive to their surroundings and provide a living indicator of water and stream habitat quality. Once the fish was found in all our coldwater streams, but degraded conditions have reduced their numbers and pushed the remainder into smaller headwater steams where good water remains.

If climate change occurs as predicted, an increase in droughts will dry up headwaters more frequently and more violent floods will more regularly scour out these small streams. In a healthy watershed, such events periodically eliminate trout populations, but fish surviving in larger valley stream sections, or in different streams in the watershed, quickly re-colonize these headwaters, returning populations to normal in a few years.

Unfortunately, warmer water temperatures caused by removal of riverside trees, degraded water quality and establishment of non-native fish have largely eliminated brook trout from crucial valley bottom habitats. As the climate warms, and access to colder headwater streams remains blocked by culverts, brook trout will be squeezed from below by warming water, and from above by drought and flooding. As the future unfolds, we are rolling the dice by expecting these headwater populations, often found in stream reaches only one to two miles in length, to survive in their isolation.

Mark Hudy, national aquatic ecologist for the Forest Service, knows the problem all too well. He spearheaded a project in the Shenandoah Valley to restore brook trout between headwater streams on National Forest lands to historic downstream habitats on farmlands. The cooperative program planted more than 13,000 trees and removed more than 250 cattle from the floodplain along a mile of an historic brook trout stream.

Hudy and his graduate students transplanted wild trout to the restored section of valley stream where, during the first year the fish doubled in size to 10-14 inches in the cool, spring-fed water. That fall they tracked some of the larger fish as they moved over two-and-a-half miles up to the base of the mountain, where a poorly designed culvert blocked them from reaching colder water and critical spawning grounds upstream.

Luckily, the culvert was to be replaced this summer with a simple span bridge. "Connectivity to a range of habitats is critically important for the long-term survival of brook trout and other fishes," notes Hudy.

Around the country, scientists have cited for years the largely unseen plague of impassable culverts as a major obstacle to restoring fisheries. For brook trout, whose list of threats is by no means simple, increasing the available habitat by ensuring free movement underneath roads may be the difference between hanging on in smaller streams - or not.