The Southwest is a uniformly arid region with a variety of landscapes. Waters from rugged mountains and forested plateaus follow ancient channels through vast expanses of desert on their way to the gulfs of Mexico and California. The amount of water has fluctuated over time, but securing water has always been a challenge for southwestern peoples. In spite of this challenge, some of them became farmers.

The word “fluctuate” in the passage is closest in meaning to?

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