About 30 kilometers north of Helena, the Missouri River cuts through a limestone canyon with walls rising over 370 meters straight from the water. Lewis and Clark named it the Gates of the Rocky Mountains in 1805, and the name has stuck because nothing better has suggested itself. The canyon is genuinely spectacular — the rock is white and pale gray, streaked with mineral deposits, and the walls press close enough on both sides that the river feels like a corridor.
Boat tours depart from a marina at the Gates of the Mountains Recreation Area and run through the canyon for about two hours, with time to observe wildlife — mountain goats on the cliffs, osprey, bald eagles — and hear commentary on the geology and the Lewis and Clark journey. The tours run from late May through mid-September. Reservations are advisable in July and August. The drive north from Helena on I-15 takes about 30 minutes, and the landscape along the way — the Missouri River valley opening out below the Continental Divide — is part of the experience. Allow a half-day.