Humboldt Bay began to form when a river valley drowned about 10,000 to 15,000 years ago during a period of rapid sea level rise.
Bay sediments also contain buried salt marsh deposits showing that areas of the bay have subsided during episodic large magnitude subduction earthquakes.

Three rivers, the Mad, Elk, and Eel, drained into Humboldt Bay during the mid-Pleistocene.
 Subsequently, the Mad River cut a new outlet to the sea, and the flow of the Eel was diverted by tectonic uplift of Table Bluff at the southern end of the bay, but Elk River continues to drain into Humboldt Bay.

In the 21st century, the bay is considered to have three regions:
the North Bay to the north of Samoa Bridge;
the Entrance Bay from Samoa Bridge to South Jetty; and
the South Bay, which is the remainder of the bay to the south.

Daby, Woodley, and Indian islands are in the North Bay, and all three are within the City of Eureka.
Low tides reveal two more islands: Sand Island, which was formed from dredge spoils left in the early 20th century, and Bird Island.
 A large eelgrass bed in the South Bay, which may be exposed at low tides, is locally known as Clam Island.
Humboldt Bay began to form when a river valley drowned about 10,000 to 15,000 years ago during a period of rapid sea level rise. Bay sediments also contain buried salt marsh deposits showing that areas of the bay have subsided during episodic large magnitude subduction earthquakes.
Three rivers, the Mad, Elk, and Eel, drained into Humboldt Bay during the mid-Pleistocene.  Subsequently, the Mad River cut a new outlet to the sea, and the flow of the Eel was diverted by tectonic uplift of Table Bluff at the southern end of the bay, but Elk River continues to drain into Humboldt Bay.
In the 21st century, the bay is considered to have three regions:
the North Bay to the north of Samoa Bridge;
the Entrance Bay from Samoa Bridge to South Jetty; and
the South Bay, which is the remainder of the bay to the south.
Daby, Woodley, and Indian islands are in the North Bay, and all three are within the City of Eureka. Low tides reveal two more islands: Sand Island, which was formed from dredge spoils left in the early 20th century, and Bird Island.  A large eelgrass bed in the South Bay, which may be exposed at low tides, is locally known as Clam Island.
Humboldt Bay