Come along and walk the right bank with me. (That's the river's right, not yours or mine, and don't forget that the river likes to look where it's going. It always faces downstream.) We'll go with the flow today, starting upriver and following the run of the water.

Right across the river from us is a sweeper. You don't want to find yourself just upstream of a sweeper, but each submerged branch creates a weak eddy on its downstream side. Since the branches ensnare much of what comes their way — including luckless paddlers! — a substantial barrier may form over time, and the eddy will grow with it. Such eddies can make useful staging points from which to rescue a trapped paddler. They're no place for a novice boater, however.

Now look downriver. See the "slick" in the middle? Yep, it's that curiously quiet area in midstream. It could be a boil — a place where water surges up from below and then oozes out in all directions. I don't think it is, though. Watch the leaves being swept along on the river's surface. Some of them drift into the slick near its foot. And what happens then? They're carried upstream toward the head. Most make several circuits before escaping to continue their downriver journey. Get the idea? Upstream flow. Circular movement. Right! The slick is a weak eddy. You can bet that there's a submerged bar or boulder just upriver. If the water level dropped till the obstruction broke the surface, the eddy might well be more powerful. It would certainly be more obvious.

When the river's in flood, however, the obstruction at the head of the eddy will be buried so deep that the slick may disappear altogether. It'll be "drowned out," in other words. It's a sort of paradox. Sometimes eddies are stronger in low water than in flood. (But don't think that high water is safe water. New and powerful eddies always appear when a river rises over its banks, and dangers multiply. The dynamics of floodwater are best appreciated from shore.)

Back on the riverbank, our walk has brought us to a bay. As the river opens out, part of it doubles back on itself. See the leaves sailing round and round? Another eddy. In effect, the riverbank acts as an obstruction. Then, when the river is released from its constraining force, a new eddy is born.

Downstream of the bay, a rock spine juts out into the current. And below it? Yet another eddy. Still further downriver, we come to a "rock garden," a covey of boulders clustered together in midriver. Each boulder has an eddy below it, and the eddies meld and overlap. The resulting water music is a joyful symphony for experienced paddlers, but novice boaters may have trouble finding the beat. It's best to begin a paddling apprenticeship by taking on eddies one at a time. A rock garden can give a beginner more hard knocks than she can cope with.

Next, look across the river to the other bank. See any obvious signs of an eddy? No? Neither do I. But it's a good bet that the shallow bay that we can see is home to one. Call it a shore eddy. And I'll bet it would be a good place to park your boat.

Our riverside walk is coming to an end. On the right bank, we find another shore eddy. (What's the difference between a "shore eddy" and a "bay eddy"? Scale and proportion. If it's a pretty good size, and if it's nearly as deep as it is wide, it's a bay eddy. If it's not, it's a shore eddy.) There's a long eddy below each bridge pier, too.

What lies ahead? The sea.

Coastwise

Rivers always run downhill, but the sea's nearshore currents change direction with each tide. And the tides aren't constant. They vary from place to place and time to time. The first rule of coastal navigation, then, is "Know your tides." (Strictly speaking, "tides" are the sea's rise and fall in response to the gravitational tug of the moon and sun. The resulting currents are "tidal currents" and not "tides." But even professional mariners sometimes nod, referring to the "set [i.e., direction] of the tide." And so will I.)

To the experienced eye, the clues to the tide's direction aren't hard to find. Unless the wind's blowing a gale or some lubberly skipper's left his sails up, boats on single moorings swing with the tide, turning round so their bows always face into the current. Buoys, too, are tugged by the tide. They lean downcurrent, pointing the way toward the "set of the tide." (Confused? I'm not surprised. While winds are named by the direction from which they blow — a north wind blows out the north — tidal currents are named according to the direction toward which they flow. The tide is said to "set south" when it flows from north to south, for example. Still confused? OK. Winds look back at where they've been. Tidal currents, like rivers, look ahead to where they're going.)

Now it's time to take a closer look at an arm of the sea. It's too far to walk, so what do you say to hitching a ride with a passing hot-air balloon? Up, up, and away!

First, let's get our bearings. The open ocean is to the east, and the tide is setting west into a deep bay. As the tidal current strikes the big island, it forms a large eddy to the west. Large eddies also form west of the promontories to the north and south, and behind the tip of the sand spit. A small eddy will form downstream of each buoy, too. (The eddies below buoys are not good places to park your kayak, by the way. The Coast Guard and Harbor Patrol take a decidedly dim view of such practices.)

Farther west, a small river empties into the bay, and a delta has grown up at its mouth. As the tide flows up the bay, eddies will form behind the shoals and islands in the delta. But the picture will seldom be this straightforward. The exact location of the eddies will depend on the relative strengths of the tidal and river currents. You'll need local knowledge if you want to do any exploring, and you'll want to be particularly careful whenever the river is in spate or the tidal currents are unusually strong. (As they might be, for example, if a storm surge coincides with the flood tide.)

Today, however, the river's low and slow, and the tide's calling the shots, forming a large, diffuse eddy complex to the nor'west. Later, as the tide slackens, these eddies will weaken. Then, when the tide begins to ebb, the tidal eddies will re-form to the east. Learn to anticipate these changes, and you'll be able to "work the tides" to get where you're going. It's one of the hallmarks of the expert waterman.

* * *

To less expert eyes, though, many eddies are invisible. But what if you could see them clearly outlined? Well, sometimes you can. This was brought home to me many years ago, on a late autumn day near North River on the upper Hudson. High clouds veiled the sun, and the temperature hovered just below the freezing point. The river was moving too fast for ice to form in the main channel, but every eddy was starkly delineated by a frosty glaze, gleaming alabaster in the watery light. The surface and shoreline were dotted with these jewel-like plaques. I didn't have my camera, but no matter. The image was burned into the emulsion of my mind.

Eddies are everywhere. You don't have to travel to the upper Hudson to find them, and you don't need to be in your boat. Do you walk past a neighborhood stream on weekends? Does your Saturday morning bicycle ride follow a canal? Do you eat lunch in a park overlooking a river? Then take a few moments to study the sinuous script of the current. Whenever you drive along the seacoast or a riverbank, pull off the road from time to time and watch the water's never-ending tale unfold. Make reading water a habit, and pretty soon its secrets will be an open book. You only need to look.

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