There is more good fishing around and on Pathfinder and Alcova Reservoirs than all of us together could shake a fly rod at and 99% of it is public land. The land that is not will be clearly marked. The folks at the North Platte Fly Shop on the way out of Casper to the lakes on 220 can tell you what flies are working and set you up with a guide if you think you really need one. I fished that area a LOT growing up and never had much issue with snakes or other critters.
The lake side of Alcova Dam was always good for me. I imagine wooly buggers would work a treat there if you let them sink a bit. I wish I could remember how to get to the one tank I used to like to fish between the two lakes. Late afternoon never failed to yield a bunch of fish. If you come across a rock tank along there, fish it. It's loaded but they only bit towards evening for me.
Another favorite spot was the confluence of Garden Creek and the North Platte just a stone's throw east of where the river comes right up to Fort Casper Rd on the west end of town. This is just west of the old Standard Refinery.
I know more.