In Canada MOW crews regularly inspect for beavers, and blow up the beaver dams with dynamite, and they try to exterminate all the local beaver railfans, as they cause saturation of the ground, peat, and permafrost near RR tracks.
In Canada, Alaska, (and everywhere else for that matter) a small quiet trout stream quickly becomes a raging river torrent in rainy season.
In the flood of 1931 the Juniata River washed away a great portion of the PRR mainline, and the topography was totally changed forever, and the railbed needed to be totally relocated elsewhere.
Here a viaduct was needed to allow water to flow through: