Watersheds are often defined in simple terms as drainage basins supplying streams with water.
When first introduced into English in the early 1800s, the word "watershed" referred to the ridgeline dividing two valleys. We still use it in a metaphorical sense to describe events from which there is no turning back. By 1840 it came to mean the slope leading away from such a ridgeline or divide, and by the 1880s it was synonymous with "valley" itself.
Watersheds are natural water receiving, storing, and distributing systems; driven by solar energy and gravity; governed by climate and terrain; modified by human land use and development. Providing water and dissolved nutrients to absorptive tissues of plants and animals, they make life possible in both terrestrial and aquatic environments. Without a reliable supply of water throughout the growing season, photosynthesis would not take place, plants would not grow, browsers and carnivores would not eat, and both nature and the human economy would come to a standstill.
In Maine, watersheds divide the landscape into regions centering on major river systems including those of the Saint John, Saint Croix, Penobscot, Kennebec, Androscoggin, and Saco rivers, as well as a number of smaller coastal rivers and embayments. Since every acre of land in Maine is owned by some person or group, and is likely to be managed for a variety of uses, the functioning of Maine's natural watersheds is modified by the practice of forestry, fisheries, and agriculture; construction and operation of hydroelectric dams; roadbuilding; urban and rural development; and activities related to tourism and outdoor recreation, including hunting, trapping, fishing, hiking, camping, wildlife watching and photography, and boating.
The Maine landscape is managed by a partnership between people and nature. If that partnership sometimes fails to produce the results we desire, the cause is often due to our not fully understanding the influence we hold over natural systems, or the responsibility we have to uphold our end of the bargain. When, for example, the Mississippi River ignores levees people erect to keep it in its appointed channel, overFlowing into its natural floodplain, we tend to view the resulting flooding as a natural disaster, but in fact it is a disaster of our own making resulting from hundreds of years of land use based on less than a full understanding of how watersheds and aquatic systems function in nature.
The terrestrial phase of the water cycle is conducted in watersheds arrayed like puzzle pieces across the land. Here water either Flows across impervious (or saturated) surfaces or creeps micron-by-micron through complex passages between particles of organic and mineral soil. A watershed is is also a waterhold, a vast storage basin through which water molecules creep ever downward, though slowly, slowly, seeping, oozing, always on the move, but collectively making little progress, being more held than shed.
Even during a rainstorm, the water flooding into streams is likely to be old water from previous storms, not water from the current storm Flowing over the surface of the land. New water enters the watershed from the top, pushing old water out in seeps at the bottom. Only on impervious ground or saturated soil offering no entrance to falling drops does water actually run across the surface and Flow directly into streams. More commonly, rain runs through a watershed at a creeping pace, slipping grain to grain, molecule by molecule, holding on, then letting go, holding on, letting go, billions of times a second, hour after day after month after year.
Some water takes a very long time to pass through a watershed because it is captured as groundwater and held in the lower recesses of the soil where even tree roots cannot release it. Some enters cracks in the underlying bedrock and begins a journey taking perhaps hundreds of years before it can surface miles away in another watershed altogether. Given the uneven topography of most bedrock formations, or beds of clay impervious to water--with basins here and there, low points and highpoint, steep slopes and gentle grades--some water settles into pockets cut off from the general downward Flow, and stays put (as much as water can stay put) as groundwater in a kind of underground lake or aquifer that is not open water in any sense but water actively held by--and interacting with--soil.
If the groundwater surface (water table) is close enough to the upper layer of soil, in places it forms a perennially wet area where roots are immersed in inundated (hydric) soil for weeks or months at a time, creating a wetland where plants which like wet roots can thrive, and those that don't stay away. Wetlands are a stabilizing kind of habitat, taking water in when they have storage capacity to spare, sharing it when they are full up and can't hold any more. Like erosion, wetlands are sometimes viewed suspiciously by the public--swamps, bogs, fens, marshes, vernal pools, floodplains. Not realizing the role they play in the local storage and release of life's primary resource, people sometimes drain wetlands and put them to use as farms, golf courses, subdivisions, and industrial parks. But because wetlands are places where groundwater is stored near the surface, they are missed by frogs, turtles, ducks, snails, blackbirds, alligators, snakes, muskrats, and beavers, among countless others, depending on the locale.
If a water molecule lands directly in a stream--falling as precipitation (as is has perhaps a one-in-a-hundred chance of doing), seeping into it, or running off the land--then it has a straight shot at the ocean. If, that is, it doesn't evaporate before it gets there, or become absorbed by aquatic plant or animal life, or replenish a wetland along the way. Streams and ponds are the glamorous watershed features, diverting the public eye from wetlands and deep woods. This is where the fishing is, the boating, rafting, camping, swimming. We care for aspects of nature we use and enjoy, tending to neglect those we don't. Trout and canoe trips take precedence in our minds over impassable bogs and alder thickets, though pitcher plants and stands of spruce are as much watershed products as black flies and bass.
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.
Photosynthesis produces sugar by combining carbon dioxide and water in the presence of sunlight. It is the biochemical engine that runs almost all life on earth. Geothermal energy and fermentation power other living systems, but the world we know depends on the capture of solar energy in green plants. To sustain life, the process must persist over time, not be a sporadic affair. Carbon dioxide is reliably available in the air, but water comes and goes with the weather and seasons. It is most reliably to be found in the soil. The wonder of green plants is that they form a bridge between air and soil, tapping one world through their leaves, the other through their roots. The genius of plants is in bringing the two worlds together through the Flow of sap up and down their stems, creating an entirely different realm based on energy held in sugar and starch.
The vascular (circulatory) system of plants is an extension of the watersheds in which they grow. Powered by evaporation through the surface of leaves, a lifting force draws water taken in from damp soil upward into the presence of chlorophyll, where it intercepts energy from the sun, ionizes, and frees a hydrogen ion that triggers the process leading to the production of glucose--containing energy in a form plants can use for maintenance, growth, repair, reproduction, and defense.
With roots in the soil, leaves in the air, vascular plants have the best of both worlds. If they were not able to rise above the earth to spread their leaves in the sun, or able to draw water up to those leaves, plants would exist only in areas where water, air, and sunlight come together at ground level--shady and humid places such as where nonvascular plants like mosses and liverworts grow in glens and at the bases of cliffs, or in shallow streams and ponds.
But by enabling the aerial, sunlit world of wind and leaves to combine with the watery world of soil and roots, plants bring two aspects of a watershed together, the upper and lower, in a way that radically expands Earth's potential for growth.
Thinking in terms of watersheds on Mount Desert Island opens the way to considering the biointegrity of the island as a whole natural system. Though we are used to thinking of the island as being divided into four towns plus a national park, as far as the natural functioning of the island ecosystem, that is an artificial breakdown. Though the watershed of Long Pond, for instance, lies half in the town of Southwest Harbor and half in Mount Desert, or from another perspective, half in Acadia National Park and half in local towns, the water Flowing through that watershed is unaware when it crosses boundaries between human jurisdictions. Thinking in terms of the biointegrity of the watershed of Long Pond as a whole allows us to transcend our everyday thinking in terms of private or public property to understand how it makes possible the island landscape that it does.
Watersheds are integral systems that receive precipitation, store it, and distribute it through the growing season for the benefit of all local residents (plant, animal, and human). Watersheds nourish every segment of the island landscape, both in and outside the park. Since all life depends on water, watersheds are basic units of natural function on Mount Desert Island. They affect the health, scenic beauty, quality of life, and economy of the region as a whole.