It was a July 16, 1979, release of radioactive mine tailings An earth/clay dike of a United Nuclear Corporation uranium mill settling/evaporating pond failed. The broken dam released 100 million U.S. gallons (380,000 m3) of radioactive liquids and 1,100 short tons (1,000 metric tonnes) of solid wastes, which settled out up to 70 miles (100 km) down the Puerco River and also near a Navajo farming community that uses surface waters. The pond was past its planned and licensed life and had been filled two feet (60 cm) deeper than design, despite evident cracking.

It dose not have an official INES rating, but was probably a level 4 incident.