It is called a glacier.
Glacier
A "slow moving river of ice" is a glacier. A slow moving chunk of ice floating down a river is a "floe".
glacier
You're describing a glacier.
a glacier == ==
glacier
A frozen river is called a frozen river. Rivers that normally flow as liquid water, are called frozen rivers when they become frozen.
ice
a glacier is ice. ice forms a landform called a mouth of a river
A slow moving body of ice is a glacier.
Boats that are used to clear mud and silt from the river bed are called dredgers- those that are used to clear ice from frozen rivers are ice-breakers.
River of Ice was created in 1940.
The water coming into the river is not from a smaller river, it is from melting ice.