As water temps drop, so does the metabolism of warm water species of fish. And as temps drop and metabolism slows, so does the need for protien and thus less feeding. They may only eat once every week or two. So your chances of catching a bass in the winter drop drastically. Especially in rivers. It is not that they left. For the most part. I think fisherman migrate more than the fish do.
Where we these fish migrating to before the lakes were built???
And like Al, I love the challenge and the solitude. It is just a really great, great time to be out.