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Posted 8/1/2011

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By Ariane Pinson
District Technical Writer / Editor


You just don’t expect fish to drown, and it is almost counter intuitive that dead fish down in the valley could somehow be the result of a fire high up in the mountains.  

Yet, in this hot, dry post-La Niña summer, fish in the Rio Grande have become the latest casualty of the Southwest’s multi-year drought.

This year, more than a decade of drought has been intensified by a warm winter, except for a few days in February, and very little rain or snow since the last monsoon season. Tinder-dry-pine- forests, low humidity and high early-summer winds combined to create a ferocious fire season. More land in Arizona, New Mexico and Texas burned this year than in any other. And, both Arizona and New Mexico experienced their largest fires ever, including the Wallow Fire in Arizona and the Las Conchas Fire in the Jemez Mountains of New Mexico.

But, just when we thought the worst was over, now that the fires are contained, come reports of rafts of dead fish.        

In mid-July, there was a significant fishkill in the Gila Box along the Gila River in Arizona, in a drainage badly affected by the Wallow Fire. And, in late July, carp, catfish and white suckers washed up dead along the banks of the Rio Grande. Corps biologist Sarah Beck explains that fishkills occur on a stream when you have a very localized segment of the stream in which conditions become rapidly intolerable for the fish.  

“A river flushes itself, diluting bad material and washing dead fish downstream,” she said.

In the case of the Rio Grande, she said, reports are that the fishkill occurred primarily in the vicinity of Peña Blanca, N.M., where Peralta Canyon enters the Rio Grande. A storm high up in the mountains over the Las Conchas burn area dumped rain on the ash-covered landscape, producing surface runoff rich in ash and charcoal. This runoff entered Peralta Creek and moved swiftly down-canyon to its confluence with the Rio Grande, where the ashy, charcoal-rich water produced a visible plume in the river. The volume of water entering the Rio Grande rapidly raised the main river 8 inches. 

Although the plume was quickly diluted as it moved downstream, where it first entered the main stem it would have been sufficiently concentrated to do great harm to fish. This ashy water would damage the gills of the fish in a way that would limit or prevent the gills from taking up oxygen from the water. At the same time, oxygen levels in the river fell rapidly because of the addition of organic matter along with the ash. In short, the fish rapidly began to have difficulty breathing, and some couldn’t survive.

Were Rio Grande silvery minnows affected by this plume from Peralta Canyon? It’s hard to know.  Beck points out that only large fish were found dead along the river, but dead minnows would be harder to see compared to larger fish, and more likely to be scavenged, so the impact, if any, of this event on the minnow is unknowable.

Will we be seeing more fishkills in the future? In all likelihood, yes. There has been very little rain over the burn scar, and each drainage will need three or four large rain events to completely flush the ash and nutrients from the burned areas. 

albuquerque district Corps of Engineers fish fishkills gila river La Nina Las Conchas Fire new mexico rio grande silvery minnow USACE Wallow fire wildfire