All these rivers are frozen for about eight to nine months in a year.

It's easier to move over snow when the rivers are frozen.

It was only November, but the rivers had been frozen for more than a month.

The river had been frozen, but of course the water flowed freely under the ice.

The river was frozen over in the winter of 1634-35.

The river is frozen over from the end of November until May.

The river was still frozen, even though the snow had blown in on warmer winds.

The river is frozen for more than half the year and floods each spring.

In winter when the river is frozen, snowmobiles run up and down it on the ice.

The river is generally frozen during the winter months.