I used Envirotex Lite for my river, and I like the effect of depth that can be obtained with its use. However, as you must have noticed, it will leave you with an ice skating rink look. Modge Podge should work fine to add "movement" to the river after the Envirotex cures. I have always intended to use Gloss Medium for the same purpose, but I have held off as I consider pouring one more layer of Envirotex to bring the level of my river up a tad to cover more of the rocks under the plate girder deck bridge in this picture:
Hope this helps,
Dave