Why do rivers and lakes not freeze over, even when it is below freezing?

Question

Is it because they have salt in them or because it is such a large amount of water? Because usually, it has to be very very cold before a river freezes over, right?

Answer 1

Drop the temperature far enough, for long enough time, then rivers and lakes could freeze.
However:
While water is moving, its temperatue can drop below zero and it doesn't freeze. Obviously, faster moving rivers are less likely to freeze.
For lakes, water's unique density behaviour protects them from freezing. Unlike almost all substances, the solid form of water (ice) is less dense than the liquid form. Ice cubes float. (For almost all other substances, the "cubes" would sink.)
So when lakes freeze, the ice stays at the top. This insulates the remaining water from the colder air above. If ice cubes sank, then lakes could freeze all the way to the bottom: the ice that formed would fall to the bottom, continually exposing the top water to the cold air.

Answer 2

Before a lake can freeze the water itself has to cool down to freezing point and this can take a long time. The more water there is and the deeper the water the longer it takes to cool down.

Moving water such as in rivers is quite 'energetic' and this energy creates heat. To freeze moving water it has to be very cold.

It's for these reasons that only the surface of a lake freezes and not the warmer water underneath and also why slow moving or static water freezes before faster moving water.

Answer 3

Can Rivers Freeze

Answer 4

All the lakes up here are frozen at this time of year. We ice fish by drilling a hole through about 2/3 feet of ice on the lake. It looks so cool to see the vehicles and fish house's on this large body of water in winter. However on occasion where the ice is to thin or they don't wait long enough, truck do fall through. Big fines are paid to retrieve them, lol. Around water that is constantly running such as a water fall or dam water will not freeze.
http://www.wgtn.net/Recreation/icefishing.htm
here is an idea of winter fishing :) hope you enjoy

Answer 5

lakes and ponds and some canals freeze quite quickly beacuse the water is not moving! rivers on the other hand have a current that keeps the water flowing- in this country is is never below freezing for long enough to even put a thin layer of ice on a river

Answer 6

They do freeze if it gets cold enough; but like you guessed the water's usually not pure, and there is a lot of it which slows the rate of cooling, and moving water freezes more slowly than still water; and since ice floats on top of water it acts as a protective layer and prevents the whole lake from freezing solid.

Answer 7

All bodies of water freeze over.

But the more the water moves (eg rivers & brooks where the water is rushing downhill), the longer it takes for clusters of ice to cover the surface (and below the surface, it tends to take even longer, as the earth under the lake is rarely as cold as the air above it).

Answer 8

The simple answer is that they do. But, being fairly large bodies of water and moving, means the ice that forms is broken up. This means it has to be that little bit colder.

Interestingly the Thames in London used to freeze over quite regularly. And then from 1831 it stopped because the old London Bridge was replaced and this speeded up the flow of the water.

Answer 9

Because the water is moving, I believe. This is the same sort of principle behind the suggestion to let water drip ever so slowly from faucets during a deep freeze to help prevent the pipes from freezing (and possibly bursting).

Answer 10

It's probably because of the turbulence in rivers. When ice does form near the surface, where it is coldest, the turbulent flow causes that ice to mix with warmer water at lower depths (assuming the cold weather isn't overwhelming). The heat of the Earth underneath ultimately counteracts the weather's freezing effect above.

Same goes for lakes and ponds: the surface of the water freezes first, cause that's where it's coldest, and ice won't mix with the lower, warmer water because ice is less dense than water, and there's no mixing action going on.