Moist Pacific air reaches the Blue Hills through a gap in the coastal ranges called Chehalis Gap. The Union River, which rises in the hills, was impounded in a deep valley on the eastern flank of Gold Mountain by Casad Dam, completed in 1957. The dam creates the Union River Reservoir supplying over half of Bremerton's annual requirements. 95% of the watershed's land was purchased by Bremerton in the 1920s, and is entirely within the Blue Hills.