The grasslands only had one river. Its source came from the rocky mountains, and the river itself went on for miles, through, and past the grasslands, past the west hills also. Gravity dragged it towards lower elevations, so there was no telling how long it stretched. The water could accumulate and form a lake, hundreds of miles away - it was possible, as the fresh water had to end up somewhere.
The inhabitants of the grasslands didn't really care how far the river stretched. The river was mostly useless to them, especially since the tribe hadn't practiced fishing in a while.
Likewise, they grew increasingly ignorant by the day in regards to what animals, what species resided below the surface of the water. When people saw the river, they automatically thought about fish, dinosaurs, and crocodiles, but their subconscious minds didn't budge past that point, they didn't explore further possibilities.
