Rainbow Falls is a treat, but not necessarily for the novelty of passing through U.S. Customs to access it.
Yes, you'll need to present your passport. But due to this fact, from the Goat Haunt checkpoint, other than a picnic table in the shade, the trails here are sparsely traveled. If you took the informative Canadian tour boat from Waterton Townsite, the majority of the passengers are on and off for a brief stay on U.S. soil in Glacier National Park before returning to the boat.
From the Ranger Station, follow the Waterton Lake Trail
west to the trail marked for Rainbow Falls. Through a mixture of forest and meadow you may pick ripe huckleberries in season. Follow the trail south along the Waterton River where moose can sometimes be seen feeding on twigs. Halfway along the trail is a scenic pond on the east side of the trail. Continue on and the trail ascends gradually as the river begins to cascade and cut through its rocky steps.
Scramble atop a long rocky outcrop for several good overlooks of a series of cascades over large blocks of fractured rock that comprise the falls. There will be plenty of time to look around before needing to leave to catch the next tour boat back to Waterton. You may return the way you came, but come back changed.
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Moose may be seen wading along the Waterton River. Wildflowers and huckleberries can be found in the meadows.