This was a giant oops.  A brand new dam was just filling with the spring mountain run-off for the first time in 1976.  It was an earthen dam built to span between basalt cliffs and hold back the Teton River.  The basin filled faster than expected. Water infiltrated the porous rock adjacent to the earthen structure and in just a matter of hours the reservoir emptied sending a 10 ft wall of water through the valley.