It is important to realize that meltwater not only lubricates the underlying surface of the glacier, but if the subglacial water is under enough pressure it can offset the weight of the overlying ice and cause the glacier to slip, and maybe even cause the glacier to surge forward. In many cases the drainage systems may not be able to completely drain the meltwater in the glacial system. In this case the meltwater must be stored within the glacial system. The glacier can store the meltwater on top of, underneath, within, or dammed by the ice. Storage of Meltwater Water accumulates beneath glaciers where regions of low pressure are surrounded by regions of high pressure.
Water can also accumulate in a bedrock depression below the ice dome, such as a crater.
A water cupola forms if the glacier bed is flat and the surface ice has a central depression, thus forming a relatively low-pressure area in the center of the ice. This low-pressure region under the ice then fills with water. Englacial pockets of water form when a crevasse has been filled with water and the crevasse then closes, thus trapping pockets of water with in the ice. These pockets do not store large quantities of water. Supraglacial lakes form early in the melt season before the englacial system opens up.