If I could have a river filled with anything, I would want a normal river.
The river is a valuable resource. Our most basic necessities are food and water. The river provides water in the form of running, fresh water. It also provides various fishes that I can possibly catch with my bare hands. If the river is the only thing I am allowed to have, then unfortunately, I will have to eat it raw. If it is the only thing though, there hopefully wouldn't be anything dangerous to me such as parasites and diseases. The rocks that flow with the water can be used however I want. They can provide shelter, or I could make tools out of them. For my vegetable supply, I hope I could get seaweed or some water vegetation.
Rivers or bodies of water are habitats in themselves. Although a river wouldn't be as resourceful as an ocean, I'm pretty sure I could live off a river.
I guess if I could only have a river of something, I would want a living ecosystem I could adapt to, and reasonably, a river would fit perfectly as a river.