First is little of no water, second is a system of discontinuous ponds. When the ice begins to encroach into the lake, stage three is reached. Stage four is a smaller ice dammed lake with floating ice masses. When the water reaches a maximum height for the dam, or the hydrostatic pressure, the water will break through the dam. Depending on the stability of the damming ice, the water will exit by way of a system of englacial passages, or through crevasses.
Water builds up by melting either by frictional heating of the ice, or yearly melting and runoff.
The lack of passages in the ice means the water find different means of emptying the lake. This process involves a build up of hydrostatic pressure. A small void between the ice and the bed is filled with water. If this process occurs rapidly, a glacial outburst flood is the result. The presence of ice dammed lakes can be an indication of glacial advance and represent recent retreat.