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The major canyons of the region did not start to form until about five to six million years ago when the Gulf of California opened up and thus lowered the river 's base level ( its lowest point ) .
 Gulf of California
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Gulf of California