Water is ageless, both ancient and young at once, but the places where it gathers can show their years. The oceans are old, and look it. Big rivers often have the dirty and timeworn look of roads rutted by years of hard use. Bogs with their stained and stagnant water are old lakes, bearded and decrepit, already with something of the grave about them.

But creeks and brooks are young. They're the kicking colts of the aquatic world, newborn and clean and fresh, tripping down mountain slopes, frolicking across meadows and woods, galloping ahead to meet whatever the future brings. Cross paths with a creek and it's likely to bring out the kid in you. You can't help getting your feet wet.

Depending on where you are, a small stream can be a rill, gill, rillet, runnel, rivulet, creek, crick, brook, branch, sike, beck, or burn. Random House Unabridged says brook is the smallest natural stream of freshwater, creek is a stream larger than a brook and smaller than a river, river is a fairly large stream flowing in a definite channel, and stream is any flowing body of water, whether it be brook, creek, or river. In practice those definitions are rarely absolute. Often a name is a matter of whim or regional convention. Idaho's Silver Creek, for instance, is considerably larger than Michigan's Boardman River.

Small streams are among the most inviting forms of surface water. They can be easily explored on foot and with equipment no more complicated than a pair of hip waders and a specimen net, and they can be fished with the simplest of gear. Some headwater streams flow for miles, but many are short enough to be explored in a single day. And though it can require much time and effort to discover the source of a river--it took 2,000 years to find the Nile's--the place where a creek is born is often just a short hike uphill.

Not long ago I fished a portion of a stream I already knew well in its lower reaches, where it flows through a valley of cedars and hardwoods before merging with a famous Michigan trout river. That lower water is strong and cold and dark, too wide to jump but still small enough to justify calling it a creek.

It is brook trout water, and I was fishing it with a fly rod rigged with a short leader, a splitshot, and a wire hook baited with a worm. Faint paths had been worn along the banks by other anglers, human and animal. I fished slowly upstream to unfamiliar water, passing though cedar groves and small hidden meadows, and around a long stretch of unfishable water choked with tag alders.

The creek grew smaller and colder, the pools shallower and less promising. I caught trout--brilliant seven-inchers that came out of the water gyrating so madly I could not get a hand on them--but they were fewer than downstream and smaller. When the creek grew too shallow for even small trout I stopped being a fisherman and became an explorer.

The valleys where creeks flow tend to be damp places with rich soil and much vegetation. Typically, the bottomland is filled with tag alders, willows, osiers, and other moisture-loving shrubs and small trees growing in dense and intertwined thickets. Anglers tolerate those thickets with fondness because they make effective guardians of some of our most productive trout streams.

Not every stream, of course, is well guarded. Mountain streams flow through high meadows bright with wildflowers or glissade down corridors of rock as wide and flat as paved roads. Desert streams flow through open canyons and sandy arroyos, exposed to sun and wind. Elsewhere, it is possible to find woodland creeks weaving among mature cedars or pines that have crowded out the lesser trees to create an open, canopied environment as accessible as a city park, with mossy fallen trees to serve as benches and game trails paralleling the creek like footpaths.

Creeks are likely to be predominately rocky or marshy. Rocky creeks are fed by springs, runoff, melting snowpack, glaciers, or a combination of them. They can originate in springs so consistent that their flow varies only a few inches from highest water to lowest, or they can be seasonal and intermittent, flooding after storms and reduced to trickles between rains. They are often steep and fast, with few pools, and follow a relatively straight course through young, V-shaped valleys. The water in these fast-flowing creeks is clear if fed by springs, milky when fed by glacial melt, or dark with sediment if there is much erosion upstream.

Marshy creeks usually flow through low-lying areas and are spring fed. Their bottoms are of mud, clay, soft sand, or organic debris and often support dense growths of aquatic vegetation. They meander slowly through grassy marshes or through lowlands thick with alders and other marsh-loving trees--vegetation that, as it dies, stains the water the color of tea. They will be cold if fed by frequent springs, but their slow current and dark color allows them to be warmed quickly by the sun and they may be home only to warmwater species of fish, whereas rocky creeks are more likely to support trout and other cold-water species.

Most creeks contain a wide variety of habitats--riffles and pools, slow water and fast water, silt bottom and gravel bottom--which allows them to support a diversity of life unrivaled by many still-water environments. Never mind the microscopic myriads, just those creatures you can hold in your hand and observe with naked eyes make up a community of such complexity and variety it could (it does) inspire volumes of research.

Tomorrow, Part Two of "A Natural History of Brooks and Creeks."