In regard to the possibility of that medium sized mammal being a mountain
beaver, I would consider it very possible. Richard Rowlett had one in his
window well in Bellevue, and he kindly gave me the opportunity to view it
and take a few pictures before he planned to turn it loose. I gave the
critter some organic carrots in return.

This county link has photos:
http://dnr.metrokc.gov/wlr/waterres/lakes/critter.htm

Kevin Li
King County Environmental Lab
322 West Ewing St.
Seattle, WA 98119