Wordorigins.org

The state name Minnesota comes from the name of the Minnesota River, which in turn is from the Dakota name for the river, minisota, meaning something along the lines of white water.

The mini- portion of the name indisputably means water, but the sense of the -sota portion is somewhat uncertain. It could mean white or cloudy white, or it could mean reflecting a cloudy sky. In any case, the name comes from the reflected color of the water, not from rapids as is sometimes thought.

(Sources: Illustrated Dictionary of Place Names; Webster’s Third New International Dictionary)