 Each tidal cycle replaces 41% of the water in Humboldt Bay, although exchange in small channels and sloughs of the bay can take up to three weeks.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.