Subglacial streams are streams which flow in a channel at the bottom of an ice-sheet or a glacier

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

, and are usually close to the glacier terminus

Glacier terminus

A glacier terminus, or snout, is the end of a glacier at any given point in time. Although glaciers seem motionless to the observer, in reality glaciers are in endless motion and the glacier terminus is always either advancing or retreating...

or snout. These are fed by streams on top of the glacier which descend through ducts in the ice. The pressures are very high in these streams due to the huge mass of ice above them, consequently, they have very high erosive power meaning they will increase the volume of water flowing through, maintaining the high pressure. Some subglacial streams have the strange property of 'flowing' uphill due to the colossal pressures that are present.