Crossword Genius

The tallest one in the U.S. is California's Oroville (3)

Ross

I believe the answer is:

dam

(Other definitions for dam that I've seen before include "River blockage" , "Wall restraining water" , "Water retainer" , "Retaining barrier" , "A mad sort of mum" .)

I've seen this clue in The New York Times.
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