Bretz was confident that a flood had occurred, but was unable to figure out where the water had come from. Originally, he proposed that the water was the result of increased runoff from melting glaciers. But even Bretz had a tough time imagining any significant volume of water melting rapidly enough to have such devastating impact. Not until 1930 did Bretz consider Glacial Lake Missoula as the possible source of water he was searching for. But the geologic evidence was elusive, and he did not fully embrace the idea until 1956. Unable to provide a clear, scientific argument for the source of flood water, Bretz went on to other activities. Bretz lived to the age of 98 and late in life had the satisfaction of seeing his theories validated. Perhaps it is poetic justice that in 1979, Bretz, at the age of 96, received the Penrose Medal, the Geological Society of Americas (GSA) highest award.