copy/paste from wikipedia " The mechanism by which the ice dam failed was as follows. Under pressure, the melting point of water decreases. As the depth of the water in Lake Missoula increased, the pressure at the bottom increased enough to lower the freezing point of water below the temperature of the ice forming the dam. This allowed liquid water to seep into minuscule cracks present in the ice dam. Over a period of time, the friction from water flowing through these cracks generated enough heat to melt the ice walls and enlarge the cracks. "
- http://www.glaciallakemissoula.org/story.html (nice picture)
and more recently in Iceland