system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do canals not freeze under bridges when they freeze elsewhere?
gpt
Bridges are often pinch points where the water is compressed and the increased current makes it harder to freeze. They're expensive to build so they're often built across narrow points in the water. They often also have supporting posts in the middle.

Both the increased flow as the banks close in and the deflection in flow caused by the supporting posts cause increased turbulence and motion in the water, and calm water freezes much more easily than water in motion. So as a result the ice in that area takes longer to freeze and less time to melt because it's thinner.