Sentence:  sea <WSD>bed</WSD>;  river <WSD>bed</WSD>;  lake <WSD>bed</WSD>;  There's a lot of trash on the <WSD>bed</WSD> of the river. 
Meaning of the word marked with <WSD> tags:  The bottom of a body of water, such as an ocean, sea, lake, or river. (from later 16thc.)