Frozen River   Why do the river freeze? The deeper and wider the river, and the faster the water flow, the less likely it is to freeze. Moving water generates friction, which heats the water (even if only a bit). In the event that smaller rivers or streams freeze through from the surface to the river bed, fish will most likely already have escaped the impending ice trap by making a temperature-triggered journey from their summer habitats to their more suitable winter ones. In larger rivers, although they may appear completely frozen, a column of water usually remains liquid below the thick layer of ice. Fish have developed unique physical and behavioral adaptations that allow them to thrive there. Most fish, fresh and marine alike, are poikilothermic, meaning that the temperature inside their body is determined by the temperature of the water around them. So, much like the way humans are less active during cold winter weather, most aspects of fish activity changes in winter waters. Physical adaptations to winter conditions include a significantly slowed metabolism and slowed swimming abilities. Essentially, fish rely on the lowest energy input and output that they can withstand, hiding out under the ice until waters begin to warm again. Although this makes it difficult for fish to search for food and avoid predators, fish have developed several behavioral adaptations as a trade-off. As winter sets in, fish undergo changes in habitat preference, searching for areas of the river with larger boulders and rocks under which they can hide. They also shift to more overall activity at night; mostly as a means to avoid daytime predation, but also to escape the possibility of being trapped in their rocky hiding places by overnight formations of ice on the river bed. If these submerged ice clusters (called anchor ice,) form in the wrong place during colder nighttime temperatures, they can block the fish’s way out of their hiding place. (cincinnati.com, fishbio.com)