Since I was asked the meaning of "river bottom" or "river bottoms", I looked up the meaning in an online dictionary and came up with this: "a channel occupied (or formerly occupied) by a river". The way I used the term, it would mean "formerly occupied.
It's the low-lying ground that is the first to flood when the river rises out of its banks. It's very fertile ground, since flooding that comes along every three or four years deposits rich silt and topsoil as the water retreats.
Wikipedia has information about the Missouri River bottoms HERE.