Walking toward the estuary from Malibu Pier, I was surprised to see the beach so transformed.

Rather than the usual meandering stream that gives passage into and out of the lagoon, there was a coursing river surging around two large peninsulas of sand. The island that prominently stood in the center of the lagoon had vanished, and cormorants now clung to an old snag, mostly submerged.

Usually one would only have to wade across shallow water to get to the lagoon from the pier, but on this day the water was at least thighdeep, stopping all but the most adventurous beach walkers.

It was high tide, and the water coursing into the estuary was so strong that it reminded me of a white water river begging to be ridden. Watching, I was struck by the interface between water and land. Seawater would melt away a sandbank, and the new shape of the bank would change the course of the water. The entire shore was fluid, in dynamic flux. The edges between ocean and land blurred.

I remembered the interaction I'd just witnessed between friends of mine, a mother and daughter. The daughter was graduating from high school and soon leaving home. In her new-found freedom, she was constantly battering the shores of her parents, shaping and altering their relationship. Like the beach, the mother was dynamic, taking all the changes in stride. But, as with the beach, I was unsure which force was shaping which.

Beaches in flux

A beach is a geologic formation along a body of water, usually consisting of loose particles of rock or shell such as sand, cobble, pebbles or gravel. Although seemingly dense materials, hard and having substance, day after day the particles lose their firmness to the greater power of coastal erosion.

Beaches are fluid systems, rivers of sand moving laterally up and down shorelines as well as inshore and offshore, depending on the time of year and corresponding waves.

Although there are many types of waves, the most common we see on our beaches are wind generated. They occur from wind blowing across a fluid surface.

Several factors contribute to the breakers we see: wind speed, the distance of open water the wind has blown over, the duration the wind has blown over a given area and the water depth. The greater any of these variables are, the greater the size of the wave.

As a wave moves toward the shoreline, the ocean bottom will also affect its direction and strength. Submarine features such as ridges, canyons and sandbars influence waves, much like winds directed by mountains and valleys.

Closer to the shoreline, the continental shelf rises, and the water becomes shallow. When arriving in these shallow areas, the swell becomes steeper, ultimately taking on the traditional wave. As the wave rises in height, the base can no longer support the top, causing a collapse, and therefore "breaking."

It is this motion that is responsible for coastal erosion, the wearing away of land or removal of beaches by wave action, coastal currents or drainage.

Water versus rock

There are four different types of wave action. Hydraulic is when waves strike a cliff and compress the air in the cracks; the air then expands, forcing the rock to break. Attrition is the grinding of rocks together, making them smoother and smaller. There is the slow erosion caused by the constant breaking of waves on cliff faces, called corrosion or abrasion. The last type of wave action is corrosion; where the pH of the ocean is lower than 7.0, the rock is corroded by the water's acidity.

All of these forces break down the beach. Then the currents resulting from the breaking waves pick up the particles and move them, creating beaches in perpetual motion. Interestingly, while waves and currents affect the movement of sediments, the changes in sediment shape and depth affect the waves and currents.

For example, during winter storms, sand is eroded and moved offshore, forming a sandbar. This sandbar created by the waves then changes the nature of the waves on that particular beach, causing them to break farther offshore, therefore protecting the beach from further erosion.

Like a beach, it is difficult to determine who is shaping whom in our human relationships. But maybe it doesn't matter. Rather, the importance comes when we realize that we have both a core and a dynamic fluid surface, each part equally essential when interfacing with one another.

Meghan Walla-Murphy can be reached at the following e-mail: mwallamurphy@yahoo.com.