"A lot of water is in the rivers."
Translation:A folyókban sok víz van.
I don't know, "A lot of water is in the rivers" sounds quite weird to me, so maybe "Sok víz a folyókban van." is an equally weird translation, thus, a good match.
Otherwise, I would use these, more natural versions:
Sok víz van a folyókban.
A folyókban sok víz van.