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USGS Geology in the Parks

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Building Beaches

Where did the sediment piled up by the El-NiƱo storms come from?

Streams supply some

While storm winds are whipping up waves out on the open ocean, we're often pelted with rain and snow on land. Runoff heads downhill, picking up sediment on its way. Streams and rivers fill with runoff and carrying the dirty load downstream. Whenever the slows down it deposits some of its sediment load. Most streams eventually reach the coast. Here they dump their load of sediment into the surf zone where it is moved down the beach by longshore transport.

View of Mobile Bay, Alabama area seen from Skylab 4.

View of Mobile Bay, Alabama area seen from Skylab 4. The sediment-laden Mobile River runs toward the sea. Where the brown fresh water reaches the surf zone sediment is transported by nearshore currents toward the west.

Where would you expect a sandy beach to be deposited?

Image from NASA's Earth Landscapes

Cliffs are culprits too

Along much of the west coast of North America, cliffs rise behind the shore. Storm waves can hurl their energy at these cliffs, undercutting their bases and making them unstable. Cliffs may simply collapse bit by bit into the churning surf. Sometimes the cliff material becomes so saturated with storm water that it weakens and slides catastrophically into the sea.

Landslide: Point Reyes National Seashore Point Reyes National Seashore. Steep, bowl-shaped slope is the scar caused by a landslide. Landslide deposits lie downslope of the erosional area where the failed material came to rest. Image from USGS Center for Coastal Geology. Landslide: Point Reyes National Seashore Shaded relief image of the Point Reyes landslide shown at left. Image from USGS Center for Coastal Geology.

Streams and coastal landslides contribute many, many tons of sediment to the surf zone during and after a storm. Then it's up to waves to do the work of moving the sediment onward.

On to Beaches: Sand on the move!
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