The silt remembers
USGS Lake Powell study reveals water calamities of the past

The silt remembers

As Lake Powell recedes, its silty, contaminated bottom could pose problems./ Courtesy of Land Desk

Jonathan Thompson / Land Desk - 12/02/2021

In June 1975, as winter clung stubbornly to the high San Juan Mountains, 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 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.

In 2018, U.S. Geological Survey scientists set out to unearth the sedimentary memories in Lake Powell, taking core samples from various locations around the reservoir but with a special focus on the San Juan River delta, since that’s where most of the Gold King residue would be. They recently released preliminary, raw data from the coring project, and Zak Podmore of the Salt Lake Tribune summarized their findings.

Yes, the Gold King spill was big and ugly – an estimated 540 tons of metals were carried down the river – and the record of the event shows up as a spike in zinc and lead a few meters under the silty surface. But the events of the 1970s, and the resulting spikes in zinc and lead, were significantly larger, according to the data. In fact, water quality in the 1970s was generally nasty, even when tailings piles weren’t bursting or alpine lakes weren’t flushing out mine workings. 

Why? Because water pollution laws weren’t yet in place. Yes, Congress passed the Clean Water Act in 1972. But the mining-specific provisions of the act were not finalized until later, so initially the law had little effect on the operating mines or mills in the San Juan Mountains such as Standard Metals’ Sunnyside Mine or Mayflower Mill. About 1,600 gallons per minute of acid mine drainage – or nearly one Gold King Mine spill every 36 hours – poured out of the Sunnyside’s American Tunnel. It carried with it 300 pounds of zinc each day, along with an assortment of other metals, all of which flowed directly into Cement Creek without any treatment, then into the Animas, the San Juan and eventually ending up in Lake Powell’s silt. Meanwhile, dozens of smaller, abandoned mines and tailings and waste rock piles around the area were adding their own toxic soup to the mix. 

Eventually the laws kicked in and the state started enforcing them more rigorously, forcing the Sunnyside Mine’s owners to treat their water discharges to a very high standard. And when the Sunnyside Mine shut down in 1991, it began a multi-million dollar cleanup effort, significantly improving water quality. That, too, is apparent in the sedimentary record, as is the 2005 shutdown of the water treatment plant which, by that time, was cleaning water from the Gold King Mine as well as the American Tunnel. 

These findings aren’t exactly revelatory. Yet I find it fascinating, nonetheless. It’s oddly reassuring to know that the silt remembers changes in water quality in much the same way trees remember the variations in climate over their lifetimes. It’s also a bit disturbing to know that as the lake’s level recedes, much of the silt and the stuff it contains – including radioactive waste left over from the Nuclear West – is remobilized, sullying the water in Lake Powell all over again, harming aquatic life and potentially contaminating drinking water for those downstream. 

Equally startling is the sheer volume of silt deposited in Lake Powell each year. In 1986, the Bureau of Reclamation conducted an extensive silt survey of the entire reservoir, finding 868,231 acre feet – equivalent to one cubic kilometer – of sediment had accumulated in the lake over the previous 23 years. If sediment has continued to pile up at that rate for the ensuing 35 years, now nearly 2.2 million acre feet of silt is sitting in Lake Powell, which is enough sand and mud to fill more than 1 million Olympic-size swimming pools.

The same 1986 study assured water officials that at that rate, it would take 700 years for sediment to completely fill the reservoir to its original capacity of 27 million acre feet. But at the current rate of climate change-induced lake level decline, the reservoir will be more sediment than water within a decade or less. The reservoir will be rendered useless and boaters will have to navigate it with silt-skipper crafts. The vast depository of sediment also poses challenges to proposals to “fill Lake Mead first,” by draining Lake Powell and dismantling Glen Canyon Dam. 

Jonathan Thompson is a fourth-generation Durangoan who runs the Land Desk, a thrice-weekly newsletter: visit He is also the author of River of Lost Souls, Behind the Slickrock Curtain and Sagebrush Empire.