 You have large zones of the subsurface that never saw any water or oxygen because they're armored off by a frozen ground layer or under the water table. But when the ground thaws or the water table drops, new rock high in sulfates is exposed to the elements. That makes the water acidic and allows heavy metals to leach out and flow into the waterways. That can kill plants, insects, and fish. Of the water quality problems I deal with, this is the largest scale and the most daunting.