I'm putting in a 2 mile water main out in the country in 3" pvc. Where the water flows, it flows great, but I've got a clog isolated in a 4000' section. Any bright ideas how to flush that out? I'm afraid a possum or something made its way into there, and I'm not sure how to even find it.

Buried or not?

Contact local fire department. Maybe they can blow it out with their pumper truck.