All the rivers respond to it


Dirt: Movement of sediment by the tributaries of the Chesapeake might not be exciting to watch, but the way it works is wondrous, and all part of nature's genius.

January 19, 2001|By Tom Horton | Tom Horton,SUN STAFF

IF WE SOARED as high as eagles, lived as long as redwoods, comprehending the bay as a whole through the centuries, we'd write poems to celebrate the dance of its rivers and streams.

From the perspectives of enough time and space, we'd appreciate how the estuary's arteries and capillaries shift and change as we change the lands they drain.

To the lumberman's mass shearing of their forested slopes and the steel plow's unzipping of virgin soils, the rivers responded.

Fattened with new sediments eroded from the land, rivers filled their old, deep gorges, built lush wetlands in their meanders, extended fertile floodplains to either side and extruded broad deltas from their mouths.

Conversely, where upstream paving or channel stabilization denies them a normal sediment diet, waterways cannibalize their banks, broadening and shallowing dramatically, or gnaw their bottoms down to bedrock.

From the human perspective, all this intrigue has seemed about as exciting as watching dirt move. But moving dirt is as much the artifice and genius of rivers as moving water.

Our ignorance of dirt's dance through the bay's vast network of tributaries has never been more apparent as Maryland, Virginia and Pennsylvania enter the second stage of restoring the health of the Chesapeake Bay.

The first stage, from 1987 to now, was meeting pollution-reduction goals. The second stage, from now to 2010, is meeting goals that will clean the place up, restoring vanished oxygen and underwater grasses.

The dirt part of that, largely ignored in the first-stage goals, seems deceptively simple.

About 5.7 million tons of sediment washes into the bay each year on average. It's believed that huge reductions in that - to 2 million to 3 million tons a year - are needed for a healthy, restored Chesapeake.

But to stop dirt, you need to know where it's coming from.

Of course, it all comes ultimately from the land, and generally, the further land departs from its original, forested state, the dirtier it makes the water.

Even soil conservation practices considered good by today's agricultural standards permit up to 30 to 40 times as much soil erosion per acre as a forest does.

And bulldozing for construction sites can be 10 times worse, per acre, than farmland.

But rivers, remember, are closer to living organisms than to mere conduits, pipes passing dirt from upland to bay. It is wondrous, all the things rivers do with their dirt.

A 1980s study of the Monocacy River, a tributary to the Potomac that drains extensive farmland in Carroll and Frederick counties, came to this astounding conclusion: About 90 percent of all the soil washed into the river in the last 200 years was still there, captured in its channels, banks, floodplains, deltas.

Eventually, maybe, it will all pollute the bay, but in the Monocacy's own sweet time.

Reducing erosion from farmland in the river basin is still a good idea, but it won't have immediate effects on the Chesapeake.

Consider Coon Creek in Wisconsin, a well-studied, agriculture-dominated tributary of the Mississippi.

Through 150 years, the lands drained by Coon Creek saw huge changes, from deforestation and plowing, to an era of increasingly more stringent soil conservation.

The typical yearly loads of sediment pollution from 1853 to 1938 were cut in half between 1938 and 1975, and by half again between 1975 and 1993.

But because of the creek's complex responses in storing and releasing sediment, the dirt delivered to the Mississippi virtually did not change in a century and a half.

The complications don't end there for bay managers, who are supposed to devise plans for reducing sediment runoff by the end of this year.

A recent study on the Susquehanna River, the bay's biggest tributary, indicates huge amounts of sediment are coming from the erosion of stream banks, maybe more than from farm fields.

But the runoff from fields tends to be finer, lighter soil particles, while that from stream banks tends to be coarser, heavier particles.

And it is the "fines," which may travel fastest to the bay and remain suspended in the water, that block sunlight and kill underwater grasses.

"We're in the dark ages on controlling sediment, still gathering basic information, but only 49 weeks from a deadline for setting baywide goals for reducing it," said Rich Batiuk, an official of the Environmental Protection Agency's Chesapeake Bay Program.

"To have any credibility in setting tough goals," he said, "there's a lot we've got to sort out."

It's not likely we can ignore any area for sediment control. Not only is the bay too dirty, but a huge, 70-year "free ride" it has been getting from the big hydroelectric dams on the Susquehanna will soon end.

Those dams have been trapping as much as 2 million tons of sediment a year, and millions of tons of polluting phosphorus as well.

By 2025, they will be full and will pass on most of the pollution; and this pass-through will begin to build long before 2025.

It seems unlikely the states and EPA will know enough to set out full-fledged, effective sediment reduction strategies by year's end.

Still, it's good news that sediment is finally beginning to get its due from scientists and managers.

It's one more step toward teaching humans to think more like rivers.

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