Garters and Ribbons are both in the genus Thamnophis. So they are very closely related. Some species in that genus look similar, so are easily confused.
When you say north west coast, do you mean Pacific Northest (northern California, Oregon, Washington)?
According to my main book, Snakes of the United States and Canada, Northwestern Garter Snakes, Western Terrestrial Garter Snakes, and Common Garter Snakes are native to that area (maybe some more i missed). The pictures in the book show so many color variations of the Common Garter, some of which look very close to the snake I photographed today. So, you probably have been catching Garters that simply look similar to my Ribbon.
Ribbons are very common around here. I caught it in a text book area (in high woods next to a swamp). It's a small, very slender snake. That one would probably be an adult. The main visual difference between a Ribbon and a Garter are the clean labial scales around the mouth. Garters' mouth scales usually have dark marks.