 Complete the following phrase:  In 1792, Lieutet William Broughton, a British explorer, led the first trip by non-natives as far up the Columbia River as the mouth of the Sandy River. He and his men camped on Sauvie Island, which lies between the Willamette and Columbia rivers directly opposite the mouth of the slough. Broughton encountered many Chinookans while exploring and mapping geographic features including Hayden Island and other islands in the Columbia just north of the slough. When 