I don;t have much experience with anything beyond casting regular clear pools of standing water, but I would think that if you want to show "flow" you would have ot get something that doesn't necessairly self-level?
I know that Woodland Scenics has some product that is specifically used to model falling or flowing water effects. I would think that you'd have to pour the water first, and then texture it, and then quickly sink in the reeds.
You'd have to read up on the usage of that product first of course, and see what the working time is, and then tailor your technique to that.