Well, you aren't going to end up with a large den of them because densites are not chosen by where it is easy to live in the summer but rather where they can survive the winter. If you were going to have denning gartersnakes on your property, they would already have been there for decades. As for protecting them from your dogs, you could train your dogs to leave them alone. Unfortunately, trying to teach a terrier not to chase animals is like trying to teach one to read Russian. I like terriers, but this is their one fatal flaw for me. Maybe you could treat the area where the snake hangs out with a chemical that is aversive to dogs but not snakes? If you could find a dead snake and cover it in habanero chili sauce and let your dog play with it, that would teach the lesson pretty well and not hurt the dog. The other solution is to provide the snakes safe passage and hiding areas that are difficult for the dog to access. I actually knew a person who installed a plastic pipe under their dog kennel to give snakes and toads a safe path to their pond.

Your description fits a "dog killed" snake. The way a dog attacks a snake: He quickly darts and withdraws as the snake tries to land a strike. The dog continues to test and wear down the snake with repreated fake attempts until he finds a small advantage and then he bites the snake in the middle impsrting a severe injury then quickly withdraws. On the next attack, the dog grabs the snake in the middle and fiercely shakes it snapping the ends of the snake like a shaken rug. A few of the grab and shake attacks spells the end of the snake, its spine broken in several places and the middle of the snake mangled.

If in doubt, call a pro - the zoo, university or local animal care, but there is a good article at www.gartersnake.info/articles/2008/garter-snakes-in-winter.php. Putting it outside in the winter, if he can't get himself below the frost line, could kill him. Here is a link that might be useful: Garter Snakes in Winter

Thank you! Hopefully the snake didn't have any deep puncture wounds from the cat's fangs and will make a full recovery. This is why I loathe free-ranging cats; they kill indiscriminately, for no reason other than the pleasure of killing, and they kill far more beneficial animals than pest rodent species. I keep my three cats indoors, so as to avoid being part of that huge problem, as well as to keep THEM from being attacked by larger predators.