Thumbs up for streams' high-fiber diet.

Glade stream's wood habit is fed daily by the bark, twigs, branches, seeds, and leaves that fall from trees and shrubs along its banks. Every few months the stream binges on several tons of fresh wood as whole trees topple in.

I walk frequently in the Glade Stream Natural Area near my home in Reston, Virginia. I see the stream gnawing at its banks like a dog chews a bone, working a bit off one side, then turning to worry a few particles off the other. This process undermines streamside trees, washing soil away from roots until trees tilt and eventually fall over, crisscrossing the streambed with dams and diversions. Last year, in the four-mile stretch of stream valley where I generally walk, Glade Stream claimed five big trees - three 100-foot-tall tulip-poplars and two 80-foot red maples. At the same time, this section of stream also took in several dozen smaller trees and shrubs, and tons of leaves and woody debris dropped from the surrounding forest.

Is such a high-fiber diet good for life in a forest stream? U.S. laws say no. In most states loggers and landowners are prohibited from leaving felled trees in streams. Millions of dollars and hundred of laws have been focused on keeping wood out of waterways. Now, though, researchers tell us wood m the water in a critical source of shelter and food energy. As a matter of fact, they fear past programs and laws have kept many waterways too wood-free.

The tons of tree debris, mostly leaves, that fall into water annually can provide 75 percent of the food energy needed for most life m most forest streams. A brook trout's lunch of caddisflies or other insects is available only if organic material is in the water, making trout as dependent on trees as squirrels. Insects like caddisflies are organic shredders, extracting nutrients (and wood bits that they wear for protection) by shredding wood m the water. Other insects and food sources may be grazers, eating leaves that have fallen in the water or algae growing on decomposing organic matter. The grazers become fish food, too.

Large pieces of wood m the water provide some food energy, but most of their function is structural. The wood creates places to hide from predators and makes pool-forming dams, slowing the water so that sediment is deposited. Fish and other aquatic life can stay m place in pools and feed without expending excessive energy fighting swift currents. Waterfalls splash from the downstream side of the wood barriers, aerating the water and scouring out stairstep formations, which help stabilize stream channels. Tree stems laying at different angles sluice water off in new directions, chewing out meanders and backwaters - and finding new trees to undermine.

Are all attempts to clear wood from waterways bogus? No. Excessive amounts of wood can choke streams and block fish migrations. But zealous removal of wood from the water can also cause problems by creating a sterile waterway. The lesson I see in recent research publications and on Glade Stream Valley walks is that life in our forest streams depends on a regular and considerable diet of tree debris - and trees.

- LESTER A. DECOSTER