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Post-fire runoff led to ‘dead zones’

University of New Mexico researcher Dave Van Horn discovered a puzzling "dead zone" as water from watersheds burned by the Las Conchas Fire passed his instruments beneath the Bernalillo bridge across the Rio Grande. (Dean Hanson/Albuquerque Journal)

University of New Mexico researcher Dave Van Horn discovered a puzzling “dead zone” as water from watersheds burned by the Las Conchas Fire passed his instruments beneath the Bernalillo bridge across the Rio Grande. (Dean Hanson/Albuquerque Journal)

Copyright © 2014 Albuquerque Journal

BERNALILLO – The Las Conchas fire was still burning in the Jemez Mountains when the Rio Grande turned black, smelling like a stale campfire, as flash floods washed through the mountains’ freshly burned watersheds.

But new research suggests something even more interesting was going on as the dark river made its way through central New Mexico. Dave Van Horn first spotted it July 22, 2011. Instruments the University of New Mexico scientist had installed in the bed of the Rio Grande beneath the main river crossing at Bernalillo measured a disturbing change in the water’s chemistry.

Dissolved oxygen, a key chemical component needed for life in water, had dropped precipitously. For fish and other organisms that live in rivers, that is bad news. “All higher organisms require oxygen,” said University of New Mexico professor Cliff Dahm.

University of New Mexico researchers Dave Van Horn, left, and Cliff Dahm are trying to solve the mystery of the impact of fire runoff on the chemistry of the Rio Grande. (Dean Hanson/Albuquerque Journal)

University of New Mexico researchers Dave Van Horn, left, and Cliff Dahm are trying to solve the mystery of the impact of fire runoff on the chemistry of the Rio Grande. (Dean Hanson/Albuquerque Journal)

The immediate cause was obvious – a thunderstorm washing off a watershed scarred by the fire. The ashen flow forced the Albuquerque Bernalillo County Water Utility Authority to stop using the river for its municipal drinking water supply.

But the data Van Horn, Dahm and their colleagues collected over the rest of the summer suggests something more extreme than what had been previously understood was going on.

On July 30, 2011, the problem was worse. Van Horn’s Bernalillo bridge instruments detected zero dissolved oxygen in the river, something the scientists say is nearly unprecedented. The slug of dead water showed up a few hours later in Albuquerque and, by the following day, the problem was detected at instruments at the north end of Isleta Pueblo.

The basics of how dissolved oxygen levels drop in a body of water are well understood by scientists. Bacteria in water are always consuming some of the oxygen. They need it to live. If you suddenly provide a lot of extra food for the bacteria, they begin consuming more oxygen, until there is not enough left and then the cycle shuts down. This is how water becomes stagnant. Too much organic material dumped into a river can lead to “dead zones” as bacteria consume all the oxygen. If you are a fish or other aquatic organism, this is a problem. You either find refuge, perhaps in a safe backwater, or you die, Dahm explained.

In this case, fish, such as the endangered Rio Grande silvery minnow, seem to have found refuges and survived, according to Van Horn and Dahm.

But a major puzzle remains: Why was the impact of Las Conchas runoff on the Rio Grande so severe? Somehow, the flushes of ashen water off the burn scars had briefly turned the entire Rio Grande stagnant. Dahm, an expert in the chemistry of rivers, had only seen an oxygen drop that extreme in a river once before – on rivers flowing off Mount Saint Helens in the weeks after the Washington volcano’s catastrophic May 18, 1980, eruption.

It happened four more times in August – thunderstorms in the mountains, a flow of ash down the river and dissolved oxygen dropping to near zero. In the two years since, Van Horn, Dahm and their colleagues have been trying to unravel the mystery and to understand what that might tell us about coping with increasingly severe wildfires in the watersheds above the Rio Grande.

Runoff from the normally dry Peralta Canyon wash west of Cochiti Pueblo seems to be at the root of a mystery on the Rio Grande, as runoff from areas burned by the Las Conchas Fire caused big changes in the river. (Dean Hanson/Albuquerque Journal)

Runoff from the normally dry Peralta Canyon wash west of Cochiti Pueblo seems to be at the root of a mystery on the Rio Grande, as runoff from areas burned by the Las Conchas Fire caused big changes in the river. (Dean Hanson/Albuquerque Journal)

Working with the National Weather Service, Van Horn and Dahm now made their way back upstream and used the Weather Service’s radar data to try to pin down exactly which parts of the watershed upstream contributed to the problems downstream. The research comes as water agencies and government officials are looking seriously at what sort of steps might be taken to improve forest health and reduce the risk of wildfire in New Mexico’s mountains.

Standing in a dry arroyo recently at the bottom of Peralta Canyon, Van Horn and Dahm pointed to a burned area in the high country upstream, where the Las Conchas fire had denuded forest slopes, allowing water to flow off more quickly when the post-fire thunderstorms hit.

“It’s a combination of charcoal and ash, and woody debris and sediment,” Dahm said as he looked at a channel bearing the clear signs of the recent high flood flows. “This is an obvious example of where that occurred.”

Picking through sediments along the edge of the channel, Van Horn found a layer of black, sooty-looking soil apparently left behind by the receding water.

The research is a reminder, Dahm said, that understanding the Rio Grande and managing its fragile ecosystem requires attention to more than just the water moving down the river’s main channel. The mountain watersheds and dry arroyos that feed it during storms are also a critical piece of the puzzle, he said.

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