Pass the cursor over the map to read details.
Detail from "A chart shewing part of the coast of N.W. America; with the tracks of His Majesty's sloop Discovery and armed tender Chatham," published in 1798. Lewis may have carried a tracing of Vancouver's map.1
On October 29, 1792, after pitching camp at today's Camas, Washington, opposite the mouth of Sandy River, Lieutenant Broughton and his crew rowed upriver to within an estimated two miles of "a sandy point terminating our view of the river," which he named for Captain Vancouver.2 Point Vancouver is believed to be at the bend of the river opposite the lower end of Rooster Rock State Park, where Lewis and Clark and the Corps of Discovery camped on November 2, 1805. Broughton estimated he was then about 100 miles up the Columbia from its mouth; Point Vancouver today is 128 miles above the center of the channel at the mouth.3 Had Broughton proceeded on another two miles, he could easily have seen Beacon Rock, just fourteen miles farther upstream. In fact, as Master's Mate John Sherriff reported, Indian informants told them that if they went up a little farther they would "meet with a fall, where were plenty of Salmon." However, the crew had only two days' provisions to see them back to their ship, so the next morning they headed back down the river.4
Either from the tracing of Vancouver's map, or from the map of the Northwest drawn especially for the Lewis and Clark Expedition by Nicholas King, which was based partly on Vancouver's chart, Clark created a map at Fort Mandan in the winter of 1804-1805 that reflected the captains' understanding of the geography that still lay ahead of them, including as much as was then known and conjectured about the Columbia River.
George Vancouver believed that the prominent peaks he and Broughton saw and named (Mt. Hood, Mt. St. Helens, and Mt. Rainier) were part of the Rocky Mountain range, and William Clark had no reason to think differently at the time he drew this map at Fort Mandan in the winter of 1804-1805. Point Vancouver would be about where the meridian crosses the Columbia at center. Lewis may have had limited advance knowledge of what lay west of Fort Mandan, but once they reached the Columbia, they knew more precisely where they were, and where they were going.
1. John Logan Allen, Lewis and Clark and the Image of the American Northwest (New York: Dover, 1975), pp. 96-97.
2. George Vancouver, A Voyage of Discovery to the North Pacific Ocean and Round the World, 1791-1795 (4 vols., ed. W. Kaye Lamb, London: The Hakluyt Society, 1984), 2:759-760. Lieutenant Broughton also named the "remarkable mountain" in view toward the southeast, for Lord Hood, Vice-Admiral of the British Navy.
3. River Mile Index, Main Stem, Columbia River. Hydrology and Hydraulics Committee, Pacific Northwest River Basins Commission, Revised July 1972.
4. Andrew David, ed., "John Sherriff on the Columbia, 1792: An Account of William Broughton's Exploration of the Columbia River," Pacific Northwest Quarterly, Vol. 82, No. 2 (April 1992), 58.