Here's my $0.02:
Sure there are poisonous snakes in PA, but they are rare. In fact, the rattlesnake roundups I've been too have very few snakes.
I've been hunting, fishing and hiking northeastern and north central PA for over 40 years and I have seen 1 poisonous snake. That was on Penns Creek last year. I was so happy to see it I picked up a long stick and poked it for a few minutes. I got lots of photos and a short video of it.
I honestly think the snake phobia (unreasonable fear) folks exhibit is due in part to lack of knowledge and in part to primal fear.
Trust me when I say no snake wants to bite a human. They bite only in self defense. Why would they waste precious venom on a non-prey animal? Most rattlers will flee, given half a chance. Copperheads are known to be more aggressive.
A little knowledge of snake behavior goes a long way. Most pit vipers (rattlers & copperheads included) hunt at night. They will be more active in the early morning and late evening. In the morning, assuming the hunt was successful, they will sun themselves to increase their metabolism. Hard surfaces are efficient sunning platforms. Keep your hands off of rock ledges, especially those exposed to sunlight (good sunning areas).
When not hunting or sunning, they will hide. They like to hide under logs and rocks. Don't step over logs without poking around with a stick. You can even use a stick to sweep leaf litter and tall weeds if you are concerned about the presence of venomous snakes.
With a little common sense, knowledge and a walking stick the vast majority of snake bites can be avoided. Keep your eyes and ears open.
It may be wise to use the web to learn more about venomous snake behavior before hitting the stream. You may find the knowege more valuable than snake chaps.
The snakes are out there, but they should be objects of respect, not abject fear.