Every few years, the Big Susitna causes damage to the tracks during breakup. What happens is that water will build up behind ice dams that develop in bends in the river. The water will then back up behind the dams, often covering the tracks. Because this is a slow process, not much damage usually occurs at this point. The real damage happens when the water builds up so much pressure that it flushes the ice dam suddenly down the river. Not only can the ice come up on the tracks with all of the hydraulic pressure behind it (as in this photo) but, the quickly rushing water that has backed up over the tracks will often take out all of the ballast and under cut the track as it tries to get back to the main part of the river. Here's another photo (Rotten Ice) that demonstrates that even Cats can have problems moving the ice away from the roadbed.