Schorote, a contributor on the Facebook 6mm wargaming and terrain group (also has a website: BK (budget) Wargaming), has made bridges for his games. When I saw his pictures, it was one of those, "Doh! Why didn't I think of that?" moments. He kindly let me post them here:
According to Schorote, "the bridges are 6 cm long, 2 + 2 +2 cm, and 1,5 wide. the river top is 5mm below board surface while it is again 5mm deep."
A few years ago, I bought one of those profile or contour gauge. Now I have a use for it. My rivers are pretty uniform in width and the banks are not particularly variable. I think I can build some that will do the trick.