Breakwater.

Breakwaters create safe harbors but can also trap sediment moving along the coast. Long Beach Harbor, April, 1998

From: USGS

A more accurate description: The sand buildup shown is on the southeast end of the Long Beach Peninsula, with the jetty blocking the southward movement of sand being the west side of the entrance to Alamitos Bay. In the background, to the right of the center, is the Alamitos Bay Landing commercial area of the Long Beach Marina. To the far right in the back is the w:San Gabriel River. In the back top left is the sotheast end of w:Naples, a residential island in Alamitos Bay. The ocean shown in the foreground is the very southeast end of the Long Beach Outer Harbor.

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