Building Beaches

Although every ripple has energy to shape the shoreline, the most dramatic effects are produced when storms lash the coast.

The swash from each break wave carries sand and gravel up onto the beach. The more powerful the wave, the larger the pieces of sediment it can carry. Each surge of swash brings new sediment and carries some sediment back into the surf zone. If more sediment is deposited by waves than is carried away, a beach builds up. If more sediment is removed than is deposited, the beach erodes.

October, 1997. Bird's eye view of Point Reyes National Seashore. Before the El-Niño storms hit, a narrow spit extended across the entry to a small estuary. Image from USGS Center for Coastal Geology.

April, 1998. Same location seen at left. The El-Niño storms piled huge amounts of sediment onto the spit and into the channel leading to the estuary. Image from USGS Center for Coastal Geology.

October, 1997. View of the mouth of Tomales Bay at Point Reyes National Seashore. Image from USGS Center for Coastal Geology.

April, 1998. Same location seen at left. El-Niño storms left behind greatly enlarged beach here too. Image from USGS Center for Coastal Geology.

The pair of images above show that El-Niño storms produced the same effects farther up the coast. Often we find that more gentle summer waves build up beaches, while wilder winter surf carries sand and gravel offshore, leaving a narrower beach behind. What happened here at Point Reyes? Where did all this sediment piled up by the storms come from?

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This page was last updated on 01/13/04