Step pool

High-intensity fires can dramatically change step pools like this one, according to a study.

This is scary.

Step pools may soon become an endangered species.

Blank stares. What?

Well, think back to the last time you strolled along a mountain stream. Where did you stop to flick your fly rod, deploy a bobber or set up a couple of canvas chairs to enjoy the wine and cheese in the picnic basket?

Odds are, we’re talking about a step pool — where the stream brims over a log or vein of granite or a couple of boulders rolled together. The stream makes a little waterfall passing over this barrier, which digs a little hole on the downstream side. The resulting pool provides a critical habitat, not just for trout, but for everything that wriggles, creeps and crawls along the stream bottom.

Well, turns out in the era of megafires and monsoon floods, you can add step pools to the endangered species list in the mountainous drainages of Rim Country and the White Mountains.

This conclusion comes from a study published in the peer-reviewed journal of the Geological Society of America. The researchers did an intensive study of the impact of the 2012 Waldo Canyon Fire in Colorado on the drainages of the Pike National Forest.

The study has implications for the White Mountains and Rim Country, which not only have more streams and lakes than almost anywhere in the Southwest — but also contain the watershed that sustains Phoenix — the fifth largest city in the nation.

The researches discovered that high-intensity crown fires like the Rodeo-Chediski, the Wallow and other recent megafires in Arizona have a very different impact on streams than the low-intensity, mostly ground fires to which the forest is adapted. These high intensity fires not only kill most of the trees in their path, the intense heat they generate makes the soil “hydrophobic,” which means it can absorb nearly as much rain.

The step pools play an unexpectedly central role in stream health. They slow the flow of the stream and provide habitat for many aquatic organisms, effectively buffering the effects of floods, drought and fire. Individual pools display amazing stability, remaining essentially in place for decades at a time.

But what happens after a high-intensity wildfire of the kind that’s increasingly afflicting Arizona’s forests?

Bad stuff, concluded the researchers, based on an in-depth, interdisciplinary study over the course of three years in seven step-pool channels burned by fires of different severity. The team collected samples, measured stream flow and used LiDAR laser scanning to precisely map changes in stream channels, above and below the step pools.

The researchers found that a severe burn followed by heavy summer rains blasted away step pools that had remained in place for decades — perhaps centuries. In low-severity fires, the step pools survived unscathed even if big storms dumped water on the watershed right after the fire stopped burning.

On the other hand, high-intensity fires like the Wallow Fire caused dramatic changes in the step pool system, especially if followed by heavy rains. That’s bad news for Arizona, since the May-June fire season comes right before the heavy rains of the monsoon.

The researchers concluded “even the smallest, rainfall of garden variety destabilized step pool habitat present and the types of benthic (water-based) organisms able to colonize after the fire.”

Numerous climate projects suggest Arizona will experience a dramatic rise in both mega-fires and subsequent flooding.

In 2017, the 7,000-acre Highline Fire burned on the face of the Mogollon Rim. A subsequent monsoon downpour generated a flash flood on Ellison Creek that killed 10 people, trapped by a 30-foot-tall debris flow rushing off the denuded slopes.

Many areas ravaged by the Wallow and Rodeo-Chediski fires have also struggled to recover. In many places, the ponderosa pine forests have not returned and stream function and topography has changed dramatically.

The researchers concluded land managers don’t have to worry about how streams will respond to low-intensity fires, but may need to consider major interventions to keep a wildfire from destroying stream functioning for decades after the ashes cool.

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