 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