The number of melting glaciers has been increasing every year due to global warming.
An ice river is some ice that moves (very slowly) like a river. That is a glacier. In fact, many inlets and sounds were created with glaciers, because glaciers moved through them like rivers... not very quickly, though.
The reading for 氷 is the one you didn't learn. Think of the glacier coming slowly to your door. It rings your doorbell and says heyo (ひょう).