The following is from Joe Upton's The Alaska Cruise Companion (an excellent book, by the way):
In April of 1986, Curt Gloyer, a pilot for Gulf Air Taxi, in Yakutat [a Tlingit village near the glacier], returning from dropping off a climbing party, noticed that Hubbard Glacier had surged all the way across the channel and essentially dammed Russell Fjord. He circled lower to make sure, amazed at the unusual site.
This was an event without precedent in local geologic history, and as soon as word got out, people and groups from all over the world converged on Yakutat to try and rescue the marine mammals trapped by the ice dam—primarily seals, sea lions, and porpoises. As the weather got warmer and the streams filled with snow melt, the water behind the glacier/dam began rising rapidly, eventually reaching almost 90 feet higher than the level of Disenchantment Bay on the other side.
Finally, in the middle of an October night, the water pressure became too great and burst through the glacier wall. By the time the first pilots got out the next morning, the big lake in Russell Fjord was pouring out like a huge waterfall.
For more details and lots of cool maps, visit Earthshots Satellite Images of Environmental Change Hubbard Glacier, Alaska.