On 31 October 1805 Clark first saw this "remarkable high detached rock," the eroded core of an ancient volcano, which he estimated stood eight hundred feet above the riverbank and was four hundred yards in circumference. Here the river widened "and had everry appearance of being effected by the tide."
Camped on 2 November about five miles downstream from the Beacon Rock, Clark thought he noticed a tidal change, although today it is scarcely measurable. Lewis had a copy of a map showing the part of the Columbia River that had been explored by George Vancouver and his lieutenant, William Broughton, in 1792. It was perhaps that resource, as sketchy as it was, plus the knowledge they had just gone through the last rapid, that enabled the captains to conclude that they had reached tidewater.
When the explorers passed this landmark again on 6 April 1806, Lewis suggested that "it is only in the fall of the year when the river is low that the tides are persceptable as high as the beacon rock." From evidence near their campsite of the previous November, he figured, "the flood of this spring has been about 12 feet higher than it was at that time."
In 1915 river developments threatened to destroy the rock. Henry J. Biddle, a prominent Washingtonian and descendant of the original editor of Lewis and Clark's journals, bought it and eventually gave it to the state for a park. It was known as Castle Rock from 1811 until Biddle restored Clark's label in 1916.
From Discovering Lewis & Clark from the Air
Photography by Jim Wark
Text by Joseph Mussulman
Reproduced by permission of Mountain Press