Warm based glacial ice is permeable to water in two ways. The first and basic way that warm ice is permeable to water is called primary permeability. This refers to the way that water is able to move through a 3-D network around the crystals of ice, also referred to as capillary action. The secondary permeability is a network of tunnels within the ice that exploit structural weaknesses within the glacier. Most of the water within the glacier moves about via secondary permeability.
Water enters the englacial tunnels via moulins.
Within the secondary permeability of a glacier two zones of water movement exist. One is movement through an unsaturated zone of the glacier. In this unsaturated zone the water is allowed to move freely as controlled by gravity, and flow is usually downward towards the base of the glacier. The second zone is one where the ice is saturated with water. Basically one can think of a glacier as having a water table.