Over the past 15,000 years, the St. Croix River has carved out this valley, its power visible in the height difference between the bluffs and the river below. Nature is not the only force that has changed the river; during the logging era, people built rock and timber islands to break ice before it reached Nevers Dam, and today these islands create a refuge for aquatic species such as small-mouth bass and basking map turtles. Rivers constantly rise, fall, and shift, transforming the surrounding landscape through sudden changes like flash floods and through gradual processes in which fast-moving water erodes the shoreline while slower water drops sediments and creates sandbars. Many species are attuned to these changes: some shoreline trees disperse their seeds in the water, trees on sandbar islands provide important habitat for nesting herons, and the river's swift-moving current keeps some spots ice-free all winter, attracting hundreds of trumpeter swans.