Every winter, 'snowmonsters' up to 5-6 meters tall appear on Japan's Tohoku mountains. Siberian winds carry moisture that becomes supercooled droplets, freezing instantly upon hitting trees and building up over weeks into towering ice formations called juhyo.
Until the 19th century, the Red River in the southern USA had a 260km-long natural log jam called the "Great Raft." Accumulated over centuries, grass and trees even grew on top of it, allowing people to walk across the river.