Ring-billed Gulls (Larus delawarensis) are common winter visitors throughout western Oregon. They prefer fresh water environments, so you find them more often at inland sites and in estuaries than you do on the beach. They breed in eastern Oregon and along the Columbia River.
This is a first cycle Ring-billed. Unlike the larger species that take four years to reach maturity, Ring-billed Gulls are a three-year gull, so first cycle birds already show gray on the mantle. Note how far the wing-tips extend beyond the tail.