The key to the life in a creek is dead things. Energy flows through every aquatic ecosystem, whether creek, pond, lake, or ocean. It begins with green plants, which serve as primary producers able to manufacture nutrients from sunlight, water, and carbon dioxide through photosynthesis. In ponds, lakes, and oceans, the most important primary producers are algae. But most algae are plankton (from the Greek planktos, "wandering") that drift with wind and current, and are therefore quickly washed from streams.

In running water, algae called diatoms grow on the surfaces of rocks, but the most significant primary producers are leafy plants in the water or on the banks beside it. They provide the foundation for the entire network of plants and animals in the stream by supplying fallen leaves, bits of crumbling wood, and pieces of dead aquatic weeds.

The importance of such raw material can be seen in mountain streams that flow directly from snow-filled valleys high above the tree line. Though pristine and gorgeous such streams are often nearly devoid of life. If mountain streams originate in forests or wooded ravines, however, where fallen trees crisscross the water and a bounty of leaves fall every autumn, they are probably inhabited by aquatic communities as diverse as those of marshy and woodland creeks.

Big trees are especially important to the life in streams. Small debris is flushed quickly from a stream, but when a sizable tree falls across the water it stays there for decades. It is colonized soon after it dies by boring and tunneling insects that open passages for further invasion by other insects, algae, and microbes. Moisture penetrates the bark and begins softening the inner wood. Fungi take hold and begin the slow work of decomposing tough fibers. Eventually oxygen penetrates the softened wood, followed by earthworms and larvae usually associated with soil. Indeed, at some point the tree becomes soil.

As the tree is slowly consumed, a steady rain of organic material falls to the stream below. If the stream's flow is strong and unobstructed the organic material is quickly flushed away downstream. But fallen trees and large rocks create tiny dams and pools that slow the stream, allowing bits of wood and fallen leaves to settle to the bottom, where they release nitrogen that feeds algae growing on the bottom, and become a feast for bacteria, aquatic fungi, and larvae of such insects as black flies, caddisflies, mayflies, and stoneflies.

Some bottom-dwelling insects are "shredders" that tear bits of softened wood into minute pieces and ingest them, and others are "raspers" that scrape nutrients from the surface of the wood. Together they succeed in crumbling the wood and leaves on the bottom into drifting compost called detritus that provides food for other organisms downstream.

Organisms that eat vegetable matter are the primary consumers of an aquatic ecosystem. They in turn are preyed on by secondary consumers such as fierce-looking aquatic beetles and the larvae of dragonflies, dobsonflies, certain stoneflies, and caddisflies. These predators prowl among the stones on the bottom in search of prey, but often become prey themselves to tertiary consumers such as trout and other fish. For an angler, the secret to finding fish in a stream is to find the creatures it depends on for food.

In northern Michigan we have both marshy creeks and rocky creeks, originating either in boggy, spring-fed ponds or from surface springs seeping through the porous, sandy soil. I knew from studying topographical maps that the creek I followed that May afternoon began in springs. As I walked I imagined some dramatic origin, a bubbling pool cradled among cedars, with the creek gushing full-grown from it. I was only a little disappointed to discover that, like most streams, this one springs from the ground in a less dramatic way.

My shoes and pantlegs were soaked by the time I reached the source of the creek, if it can truly be called the source. The creek, I discovered, is formed by the confluence of a dozen minor tributaries, each dividing and dividing again until it becomes so small it is scarcely noticeable--lidded with leaning grass, slipping out of sight beneath the moss-covered humps of old trees in the swamp.

At each confluence I took the strongest tributary, following first one, then another, then another until finally the one I chose had no more tributaries. It became too shallow to hide even a fingerling trout, barely deep enough to wet a shoe, and ended finally in a narrow, dark, aromatic gully among the cedars, where the ground was so wet I left a trail of slowly filling footprints. There in that tiny valley, with sides so close that trees leaned across and touched overhead, I found the springs that form the creek.

It begins with whispers and tears--gentle, nearly inaudible trickles seeping from the banks like wounds on a tree, each oozing enough water to nourish patches of swamp buttercups and wild peppermint and hummocks of moss thick as couch cushions. The seeps trickle downhill and gather at the bottom of the gully. It is there that the creek begins: six inches wide, half an inch deep, a rivulet trickling over rust-colored pebbles in a skinny bed lined both sides with moss. A hundred feet downstream the rivulet gathers the flow from a dozen other seeps and the water begins to fill with life.

Only big things get easily noticed. If we value just what is worthy of mention in newspapers and on television it is easy to believe that nothing much matters unless it is large enough to shake the earth. But standing in that gully crowded with growth and dampness, with newborn water leaking from the banks around me, I was reminded that great things often come from humble beginnings. In many ways it is the creek that makes the river.