I've tried pouring gloss medium, and it does dry... sooner or later. It stays real runny, though, so make sure your dam is solid. Also, like the water you are modeling, seeks its own level, so a steep stream will end up with a lot of deep puddles at the bottom, and next to nothing at the top.
Given the high relief you've created with your stream bed rocks, you'll also get a lot of air bubbles. Not a big deal with gloss medium, as you can stand there and tease them out as they come up. But they'll keep coming up for a long time, so pull up a chair.
Gloss medium is most effective when applied over a painted base with some low relief detailing at the shoreline.
Lee