And then there are the tides. The sun and moon tug on the earth's oceans. Curiously, the moon's by far the stronger partner here. Though the sun is much bigger, it's also much farther away, and attraction is largely a matter of proximity. So the moon's pull creates a pair of humps in the sea, one just beneath it and the other on the opposite side of the globe. As the earth rotates on its axis, these humps—you can call them "tidal waves," if you want, but please don't confuse them with tsunamis—circle the world in a little more than 24 hours.

The result? High and low tides, corresponding to the crest and trough of each tidal wave. Along every marine shore, the sea rises and falls, sometimes twice a day, sometimes only once. Of course, it's not really as simple as I've suggested. Topography enters into the equation, as do winds and complex harmonics. The gravitational influence of the sun can't be neglected, either. But these are all subjects for another time. For now, it's enough to know that while some places experience extreme tides, with the difference between high and low water measured in tens of feet, in others the change is measured in inches. And even the largest fresh water lakes have no tides at all. Tides are an ocean phenomenon.

But what happens where rivers meet the sea? As the sea level rises with the tide, ocean waters flood into river mouths and bays. The flood tide pushes its way upriver. How far it gets depends upon many things, including the shape of the river mouth. But whenever a river butts heads with a flooding tide, waves grow taller and become steeper. Toss in a stiff upriver breeze, and the stage is set for some rough, breaking seas. It's a good time to be someplace else.

Once the tide begins to ebb (or fall), its opposition to the river's flow diminishes. At low water, when the tide has fallen as far as it's going to, the river runs unhindered out to sea, scouring the foreshore channel and carrying its burden of sediment farther away from shore.

Season, too, affects the pace of change at the river mouth. Beaches sometimes disappear altogether in winter. Sediments accumulate around river mouths during the sultry summer months, but winter storms quickly undo the summer's work. Only when the storms subside and the rivers replenish the lost sediments will the beach be rebuilt.

But what if the winter storms don't wash everything away? What if more sediment is deposited at a river mouth than is carried out to sea? Then a delta forms. The name tells you what to look for on the map. Generally triangular or broadly fan-shaped (just like the Greek letter ), and with its apex pointing upstream, a delta is a gently sloping wedge of sediment, thicker near land and dissected by many channels. Deltas aren't exclusively marine, either. They can also be found where rivers flow into lakes.

To be sure, not every delta is as perfectly shaped as this. Currents and rates of deposition, even compaction and subsequent depression of the earth's crust—each of these forces helps to sculpt a delta. But the basic formula remains the same everywhere. A delta's a sort of savings account. When deposits exceed losses, the delta grows. On the other hand, if losses outstrip deposits, it shrinks. The classic delta is the one found at the mouth of the Nile. It's seen better days, however. Since the construction of the Aswan High Dam, much of the sediment that once nourished the Nile delta has been trapped in a huge impoundment. Now the delta is wasting away, spending down capital accumulated over thousands of years. Someday it will cease to exist altogether.

Fortunately, other marine deltas have fared better. As the tide rises and falls, the slosh of water back and forth cuts so-called "distributary channels." The resulting interfingering of water and earth creates areas of extraordinary biological productivity. The Mississippi delta is just such a place. It's described by geologists as a "bird's foot" delta, and until quite recently it was expanding seaward. Now, though, the Mississippi is also spending down its deposit account, imperiling both the delta and the wildlife it supports.

Not all rivers form deltas. Some are simply too sediment-poor to set up a "savings account" in the first place. Others empty into the ocean through deep, long-drowned river valleys. These are called estuaries, relatively narrow, funnel-like incursions of the sea, creating a near-shore environment that extends well inland. Estuaries are special places. Marine and fresh waters vie for dominance in an estuary, creating a highly variegated physical and biological environment. With every flood of the tide, salt water invades the river valley and pushes upstream. How far? New York's Hudson River is an estuary as far north as Albany, nearly 150 miles upriver from New York City. The Chesapeake Bay is an estuary, too, as is the River Thames in England. And these are only a few of many examples.