It really depends on the type of river : fast and clean? slow and muddy? shallow? deep?
If it's clean and relatively shallow i.e. you can see to the bottom, possibly go with dark green , even blackish or brownish green in the deep parts, and maybe creamy-grey for the shallows. But really it depends on the surrounding landscape.
Modelling the UK in 00, and New England - MEC, B&M, D&H and Guilford - in H0