The Santa Barbara Channel was formed tectonically, and not by a glacier. Many of the layers of rocks that our homes are built on here in Santa Barbara and Goleta, and the mountains that we see to the north, were created under the ocean. Faults pushed these layers of rocks up and out of the water, but in the Santa Barbara Channel, those layers just weren't pushed up above the ocean. Today, the Santa Barbara Channel is about 600 meters (or about 2,000 feet) deep at its deepest point. That isn't very deep compared with the average depth of the whole ocean, which is over 3700 meters (or over 12,000 feet).
During the last glacial period, the Santa Barbara Channel looked different. The sea level around the world is lower during glacial periods because some of the water normally stored in the ocean is instead stored in glaciers during those cold times. With a lower sea level, our relatively shallow Santa Barbara channel was even shallower, and was shallow enough for large animals like mammoths to swim across to the Channel Islands. In fact, because the sea level was so low, the northern Channels Islands that we can see from Goleta and Santa Barbara were all part of one big island.
The Santa Clara and other rivers draining into the Santa Barbara Channel carry sediment with them into the ocean, so they're actually helping to form new rock layers in the Santa Barbara Channel, making it more shallow (over millions of years), and not deeper.
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