I don't have any photographs on the web at present, but this film clip shows an Accucraft C16 going under my waterfall on two different tracks, one in a tunnel and one behind the water cascade. I doubt that the latter is prototypical, but we wanted the visual effect.
Make sure to use waterproof materials, from the guy who knows. Here's mine below, I also covered it with some old rubber pond liner to protect the treated areas a bit more. Worked out fine though, now putting in one for two tracks instead of one. Oh yeah, make sure you make it big enough for the 1:20 stuff, someone will bring one over someday!
I made a curving tunnel that runs under a stream/waterfall. I used concrete slabs and sealed them before covering them with pond liner and then dirt. The rest is done the same way as if I had just done a waterfall without the tunnel.
I used a steel plate, with the edges broke up to create a drawer like box to go over the track, apply cement to the steel which makes a very nice looking river and water falls and prevents any leaks in the tunnel