River bottoms are the flat areas of land found at the bottom of a river. They are typically composed of sedimentary material such as sand, silt, and clay, and are often used for agricultural purposes. The term 'river bottoms' can also refer to the BEDS of rivers, which are the parts of the river that contain the most water. The crossword solution for this term is BEDS, which is an accurate description of the physical characteristics of river bottoms.