Gary,
Thanks for the Cucuteni explanation. Soooo, here are my thoughts. In general, I think it's a really bad idea to capture wild garters, keep them in captivity for an extended period, and then return them to the wild. Some garters thrive in captivity, while others succomb to side-effects of stress from confinement.

A common scenario is that wild snakes thrive in the wild with low levels of parasites because free living allows them to escape reinfestation and cope with or eliminate a portion of their parasitic load. Captivity increases their stress levels and weakens their immune system. Moreover, snakes can't escape from parasites/pathogens that are confined with them in their enclosures, so their health declines over time. Altered diet, abnormal smells, different temperature/lighting regimes, etc. also contribute to their stress levels.

Again, some snakes thrive in captivity, but others don't. Releasing weakened or sick snakes back into their environment can be flirting with danger. It can be like releasing a heat seeking missile back into the wild because these snakes will encounter other wild snakes, and potentially hibernate/brumate in communal dens with them. If a weakened snake happens to enter a den with pneumonia, cryptosporidia, an abundance of communicable parasites, etc. it can spell the end for an entire colony in your area. It's a real danger that has been documented within lizard populations.

Your idea seems to soften the blow, though, since their confinement wouldn't be as dramatic. I'm not endorsing your idea, but I think it has merit. If you chose to overwinter them (particularily newborns) in a protected environment, their overwinter survival might increase. Then they could be released into your outdoor enclosure where the stress levels would hopefully be somewhat less. Before I'd release any snake back into the environment, though, I'd want to do as much as possible to ensure it isn't carrying a dangerous parasitic/pathogenic load, including providing appropriate medication. Anyway, a "large" outdoor enclosure with plenty of hiding places for individual snakes, shade, water, food, and so on would act as a proving ground for your idea instead of taking a chance with the health of wild snakes in your area. Of course, you'll want to check the laws in Washington about releasing captive animals and you'll essentially be responsible for the safety of all the snakes that yours might come into contact with (at least from an integrity point-of-view). This is why I wouldn't ever encourage housing and releasing wild-caught snakes. These are just my two cents. Spend them wisely .
Rick

P.S. If you choose to proceed, please become as educated as possible about keeping garter snake and maintaining their welfare by reading as much as possible. Look in the top left corner of the homepage (http://www.thamnophis.com/) for some suggested reading. Either way, it would be fun to have an outdoor enclosure as long as cats couldn't get in there.