Anchor ice and why rivers freeze from the bottom
Standing water freezes top-down. Fast-moving water often does not, and the reason is turbulence.
In a turbulent stream, the whole water column is mixed continuously, so it cools uniformly rather than stratifying. It can drop very slightly below zero throughout — supercooled — without freezing, because ice needs a nucleation site. The streambed provides plenty: gravel, cobbles, aquatic plants. Ice forms there first, growing on the bottom as a spongy mass called anchor ice.
Anchor ice does peculiar things. It can build up enough to dam a channel and raise the water level upstream. When morning sun warms the bed slightly, or the stream's thermal balance shifts, large mats release at once and float up, carrying gravel and sometimes cobbles with them. That is a real mechanism of sediment transport in cold-region rivers, and it moves material that ordinary flow at the same discharge would leave in place.
For anything living on the bed, this is a difficult regime. Some invertebrates ride it out. Others are simply relocated downstream inside a raft of ice.