Hi Justin, I also fish the Deerfield, and I too encounter high numbers of rainbows. About 10 years ago, there seemed to be a mix bag of brook, rainbow and brown trout. Last year, there seemed to be an outstanding number of browns. Up until August, I was catching mostly browns. . . probably about 7 out of 10 fish. Then the rainbows started to appear. Could it be that the major flooding in May of 2006 caused the temperatures to create this phenomenon? Are browns more active (eating wise) in colder temps? This year, lots of rainbows (and lots of damaged fish). I find brook trout to be a rarity now. . .especially one over 12 or so inches.
As for the distribution, I've found rainbows in the rapids, riffles, and heads of pools. I've found browns typically in the slower (and shallower) waters, and really they seem to be anywhere.
Hope to see you on the river