When we removed the Charlton Branch section from Jason's layout we had to cut through the river. Although it went pretty well, the water did chip slightly and a couple of "chunks" of scenery separated from the subbase. I suppose I could have tried to blend it and add a new layer of Envirotex to blend everything together, but I wasn't sure that would even work and if it didn't work I was concerned it would mess up the remaining section of river and falls near Martin Machine that was in good shape.
A logical - and easier - solution is to not try to patch the river but instead add another fall and a mill pond at a higher elevation. This has the added advantage of allowing for another mill building/complex.
I did some rough measurements the other night and mocked things up. That seemed to go well enough that last nights "30 minutes a night" was spent cutting a mill pond surface (the leftover fascia piece - hence the color!) and the "land" - currently some 1" pink foam.