For muddy rivers, try a nice, light to medium, chocolate brown color to your liking. Paint a gloss-coat over it.
I haven't seen too many green rivers, but there are some, if the water is clearer, and there's some algae growing on the bottom.
For the latter, I'd start off with the brown mentioned above, and/or a sand color, and then do a bit of a wash with the green, in some spots. In others, you want the sand and brown to show through, to represent the bed of the river in the shallower areas. Try several shades of the same green color, mixed randomly (darker for deeper pools), preferably on a wet pallet, so you get a nice mixture, wherever you like.
Again, a glosscoat is needed to make it look like water.
If you don't want to glosscoat your rivers, you could get some clear acrylic, or rippled acrylic shower door, and/or light covers, to represent the shiny surface of the water, instead.