Acadia National Park - FLOW: Watersheds Part 3

From above, we see only the outermost layer of a watershed"s anatomy. We see ponds and streams, wetlands, forests, fields, towns, perhaps a few insects and birds. What we miss is the actual workings of a watershed, Earth"s underground organs and arteries.

Water travels through passages in the soil, picking up minerals and nutrients, carrying oxygen, removing waste. A watershed is the life support system for everything that lives within it. Surface waters make up only a small portion of that system. The bulk of it lies hidden within the confines of none-too-solid earth.

To appreciate how watersheds work, we have to employ something like radar, X-rays, or Superman"s ability to see through walls to the drama beyond. The hidden drama in a watershed is the story of life"s dependence on a reliable supply of water through the growing season despite unpredictable variations in the weather day-to-day, month-to-month. If plant roots are denied a steady supply of water, nutrients, minerals, and air--they die. When its roots die, a plant dies. And so does the pyramid of living beings founded upon it.

Soil, by definition, is porous. It consists of particles of rock and organic material nestled together (more closely in the case of clayey soils, less in gravely ones). Spaces between particles allow water to flow in and around them, picking up air and dissolved nutrients and minerals, which it conveys to thirsty roots and microbes on its relentless journey downslope toward the ocean. That journey does not take place across the land so much as within it, in tunnels of infinite complexity. The French peasant who held up a clod of soil from his field and exclaimed (in translation), "This is France!" had it almost right. He might have said, "This is life!"

Imagine slicing with a great blade across a watershed, ridge-to-ridge, down to bedrock at its lowest point, and even a little way into the crack-riddled rock itself. Draw a profile of the section exposed by that cut, showing bedrock rising to heights on either side, covered with sloping surficial deposits of coarse mineral soil, then up through ever-finer layers of increasingly organic soil, up to pure organic duff on the surface, with roots of trees, shrubs, and herbs rising out of the topmost layers, turning into stems, into branches, into leaves, noting the myriad rootless plants on the surface such as mosses and lichens, and the variety of minute species in upper layers of the soil, and the teeming diversity of life dependent on plants aboveground. There, now, is a glimpse of the innards of a watershed. Multiplying that glimpse by the watershed"s breadth and length gives some idea of what a watershed is in depth.

A stream, pond, or aquifer is an expression of the watershed that is its source. Dissolved nutrients feed aquatic and terrestrial algae and plants, which in turn feed everything else. The steeper the slope, the quicker water moves through the soil, and the fewer nutrients it picks up from its surroundings. Life in shaded headwater streams often relies more on plant matter fallen from streamside vegetation than on dissolved nutrients and aquatic vegetation itself. Here, for example, black fly larvae feed on bacteria-enriched detritus, and trout feed on the larvae. Lower down the stream, aquatic algae, mosses, phytoplankton, and rooted plants assume greater importance in sustaining stream life. Here invertebrates and zooplankton feed on aquatic vegetation, and are in turn eaten by, say, alewives, which are eaten by pickerel, herons, kingfishers, ospreys, and people.

We take it for granted that fish are expressions of the streams in which they live. It is similarly true that terrestrial animals are expressions of the water reaching the roots of plants on which they feed. Hares, squirrels, and deer are watershed products, just like alewives and trout. The same is true of beavers, foxes, and coyotes. And people. We all depend on plants grown in watersheds, on watershed soils, and on the flow of water through those soils, which comes from skywater, a portion of the water cycle intercepted by the local terrain.

In New England, the ground freezes in winter. It is not the soil itself that freezes, but the water it contains. Soil is like a great sponge. Plants draw water from it all summer long, even if it hasn"t rained for weeks. Passages in the soil allow root hairs to drink from the thin film of water clinging to each particle within reach. And seepage within the soil replenishes the supply. The damp layer of soil between bedrock and air forms the surface membrane within which earth springs to life.