If you watch carefully, youíll notice that waves usually donít approach the shore head-on, but rather at an angle. One effect of this is to create a slow and subtle movement of water parallel to the shore. This is called the longshore current. You've experienced the power of the longshore current if you've ever been out in the water for awhile, looked back at the shore, and found yourself 50 yards down the beach from where you planted your umbrella.

In addition to moving swimmers, the longshore current moves sand, particles, and other material parallel to the beach. The amount of this drift can be staggering, on the order of 200,000 cubic yards of sand per year at Cape Henlopen, for example. That's enough sand to build a one-foot thick wall a mile high and a thousand feet wide.

No wonder that beaches are sometimes called "rivers of sand." Material at the water's edge can move hundreds of feet or more each day. The fact is, at any point near the shore, most of what you see came from somewhere else.