 Suisun Valley is a region in the California Coast Ranges that is bordered on the east side by the Blue Ridge of the Vaca Mountains, and on the west side by the Howell Mountains, which are also called the St. George Range.The valley is located almost entirely in Solano County with a northern terminus near the county line with Napa County, and a southern terminus at Suisun Bay in the estuary where the Sacramento River enters San Francisco Bay.Elevations on the valley floor range from above sea level, rising steeply to at Okell Hill on the north side, and to on the west side at Twin Sisters, a twin summit that is the high point of the Howell Mountains, as well as a prominent landmark in the valley.