The Silt Remembers

USGS Lake Powell coring study reveals water quality calamities of the past

In June 1975, as winter clung stubbornly to the high San Juan Mountains of southwestern Colorado, a heavy rain fell on the still-icy tailings containment ponds at Standard Metal’s Mayflower Mill, just outside of Silverton. The combination of rain and ice caused one of the impoundments’ sand walls to give way, allowing some 75,000 tons of sludge within to break free and flow into the Animas River. 

The sludge—a combination of very fine and sandy mill tailings, loaded with acid-forming sulfites and a toxic soup of heavy metals—was carried by the swift current downstream to Durango, where the river ran the color of “aluminum paint,” as a Durango Herald reporter described it at the time. Of 31 rainbow trout placed in the water in cages, 27 were dead within 24 hours. Cyanide—often used to leach gold from ore—was detected at high levels in Farmington, where the Animas joins the San Juan River, which took on the same eerie hue. From there the material continued westward, ultimately settling into the San Juan River delta where it runs up against the slackwater of Lake Powell. 

Over time, the calamity faded from the collective memory. It was dredged back up when the Gold King Mine blew out in 2015, turning the Animas and San Juan Rivers TANG-orange and then electric yellow all the way into Utah. But the immediacy—and color—of the latter event quickly overshadowed the 1975 tailings spill. Lake Powell’s growing depository of silt, however, never forgets, and the memory of the 1975 event lurks some 45 feet deep within the sediment.