The river is deep around there, so be careful.
There's a lot of sand on the bed in this river.
"What did you do during Golden Week, Salmon?" "I lost my wedding ring in a river, so I was looking for it. It took me three days, but I finally found it buried in the riverbed."
The river bottom is the river bottom, also known as the riverbed.
The readings are the readings you learned with the kanji. Just don't forget the rendaku for 底!