In the summer of 1979, an earthen dam over the town of Church Rock, Utah, broke, flooding the arroyo below and then the bed of the Rio Puerco (an intermittent stream) on the southern border of the Navajo Nation. It was a small flood, but a dangerous one. It burned the feet of a boy who stepped into it, and caused sheep and crops along the banks to drop dead. That’s because the pond it came from had been used by a nearby uranium mine to store the tailings (residue) of its excavations — the water kept the radioactive dust from blowing away. The 93 million gallons of contaminated water that poured into the Rio Puerco remains the largest accidental release of radioactive material in U.S. history, bigger than the notorious Three Mile Island reactor meltdown that occurred 14 weeks later….