The water is E-Z Water, and we should have made the river before we put any plastic bridges or track in place, because we sustained a bit of heat gun damage during the E-Z Water process, which we later had to repair. Let that be a lesson to any who would use this stuff: keep anything made of materials that can be heat-damaged away from all E-Z Water areas until the water is done, and then protect the water with rags or foam rubber until the layout is done. The latter we did nicely, so the water came out without a scratch. It's very vulnerable to scratching. You can fix scratches easily with a heat gun, but it's very difficult to protect track and bridges and other things close to the water from a heat gun.
We'd wrapped up bridges with rags and wrapped aluminum foil around that, and put plywood pieces wrapped in foil over track and stuffed extra foil around edges, but the heat found cracks here and there and we had to use a propane torch to reheat and re-bend a few areas. It all ended up looking good and working good, but we learned a lesson (we'd never used E-Z Water before). Lesson two here is that one should always make bridges easy to remove.