The history of Seaside is wedded to the discovery and development of the Columbia River. The river was first sited in 1775 by the Spanish navigator Bruno de Hezeta (aka Heceta) who mistook the estuary for an inland bay, although its seething currents reminded him that "it may be the mouth of some great river or some passage to another sea." He did not attempt to enter.

On May 11, 1792, Captain Robert Gray of Boston, while seeking to expand his fur trade, navigated the bar and discovered the great river he named for his ship, the Columbia Rediviva. He first anchored near the site of today's Astoria, then sailed some 15 miles upstream, returning to his initial anchorage to spend five days among the hospitable Indians.

Word of Gray's monumental discovery moved rapidly with far- reaching effects. The British at once dispatched two ships to the river from the north Pacific--the Discovery, commanded by George Vancouver, and the Chatham, commanded by Captain Broughton. Vancouver was unable to find a channel at the entrance to accommodate his larger ship; however, the smaller Chatham explored some 100 miles upstream. By October of 1792, another British schooner was on the Columbia. Many followed.