By Ashley Rust, PhD
Question: Colorado, along with many states in the Rocky Mountain West, experienced a record-breaking fire season in 2020 where over 650,000 acres burned in our state. How will the trout populations in nearby streams be impacted? What should anglers expect to see in the rivers this year?
Dr. Rust Replies: Yes, 2020 was an unprecedented year, Colorado experienced three of the largest fires in the state’s history; the Cameron Peak fire, the East Troublesome fire and the Pine Gulch fire, each burning well over 100,000 acres a piece. And this is after the Grizzly Creek fire had engulfed forest areas around I-70 and caused highway closures.
With climate change, our fire season each year is two months longer on average, starting a month earlier in the spring and lasting a month longer in the fall. Combine a longer, drier season with current forest management practices and more people living in the wildland-urban interface and we are observing fires that are larger, more severe, and more costly than ever before. We are reckoning with our attitude of fire suppression, which has been a warfare on fires, allowing forests to age and fuels to accumulate. However, fires are a part of the forest landscape.
Fires are a natural disturbance, and the inhabitants of the West, including our beloved trout, have evolved with fire. Native plants, insects and fish have all adapted to return after fire. If you take the “long-view” fires generally help revitalize ecosystems, acting as a natural re-set in climax communities.
I have spent much of the last decade studying how wildfires disrupt water quality, impact water supplies and affect aquatic life. I have been on the ground sampling aquatic insect and fish populations immediately after and many years following fires in Colorado. And I have utilized public data to evaluate the most common water quality responses in streams disturbed by hundreds of wildfires. My main observation is that Mother Nature is incredibly resilient and ecosystems recover healthier after fire when enough time has passed. I hope to convince you to be patient, remain hopeful, and observe some remarkable landscape scale changes and recovery.
Wildfires are evaluated by their burn severity, a spectrum where foresters consider how much of the vegetation was combusted. Low severity fires leave much of the vegetation intact, are more like crown fires, and do not disrupt the hydrology, water quality, insect or fish populations in the streams within and below the burn scar. Moderate and high severity fires, where vegetation is completely combusted and the ground is scorched, result in higher streamflows and compromised water quality for 1-5 years after the fire. The greatest impacts on streams have been observed after rainstorms. In moderate to high severity burn scars, the forest floor becomes hydrophobic because the organic material has been cooked, reducing infiltration capacity of the soil, preventing rainwater from percolating into the ground and causing rain to accumulate as surface runoff, delivering more water to the streams after rain. The higher flows can scour stream bottoms, flushing fine sediment and material from the system.
During Colorado’s monsoon season late in the summer, short intense rain-events also physically dislodge soil from burned landscapes increasing erosion and delivering soil to streams. The eroded soil carries nutrients, like nitrates and phosphates, and absorbed heavy metals from ash and minerals to the stream. But it is the dirt alone that causes the most commonly-observed disruption: higher suspended solids in streams within and below wildfire areas. Algal growth has been observed to increase in some areas after fire because the streams are receiving more nutrients from the burned landscape and the canopy over the stream is open allowing more sunlight. These are temporary effects that are more common in landscapes where the fire burned at a moderate to high severity; large but low severity fires do little to change the stream. So, within and below the high severity burn areas, anglers can expect to see muddier water after each rain event, shifting deposits of ash and fine material and maybe more algae growing in streams. Generally, after 1-5 years these impacts dissipate as the landscape recovers and vegetation returns, stabilizing soils.
Aquatic insect populations experience the disruption from fire in and below burned areas. Surprisingly, the density and total number of insects inhabiting streams generally remains the same. But there is a species shift in diversity, from a wide variety of orders and sensitive species, to a few hearty pollution tolerant species. The stoneflies, most caddisflies, and most mayflies are temporarily absent from the community after fire and there are more chironomids (midges). The high flows that follow rainstorms in high severity burn areas scour the streambed, removing many species and the fine sediment embedded between rocks. This acts as a re-set to the ecosystem, and the early pioneer, pollution tolerant insect species come back first and the diversity of sensitive species tend to return in 2-5 years. Anglers may notice changes in their favorite local hatch, but impacts are extremely localized to within and directly below high severity burn areas, so hatches may be patchy throughout a burned landscape.
Stream fish, including trout, can be killed directly from the heat of the fire and from high suspended solids (turbid water) that follow monsoon storms. The suspended solids in the water can clog the fish gills and suffocate them. Patchy fish kills can occur 1-3 years after a fire if fish are trapped in streams with poor water quality. However, even when fish kills occur, and I have observed several after fires, the fish return and populations rebound quickly. The key to fish survival in streams is connectivity. If a fish can escape a poor water quality event, such as muddy water after an intense rainstorm, by swimming upstream, downstream, or into another tributary, they can survive. And even when fish kills do occur, if the stream has high connectivity, other fish will be quick to re-populate the open habitat. I have personally observed several trout populations rebound with higher numbers of adults and young of year 2-3 years after a fire caused a fish kill or burned the area upstream. While the purple fireweed is still carpeting the burned landscape and the aspens begin to re-establish, the fish and the population of insects they rely on for food are back stronger than before. For them, the fire was not a catastrophe but a moment that rejuvenated their ecosystem.
We have to get used to a future with more fires. It is hard to watch, but the change that follows as the landscape recovers from black and scorched to a fresh bright green, scattered with wildflowers and new young tree growth is awe-inspiring.
Ashley Rust is a researcher and professor at the Colorado School of Mines.
