Wildfire Effects On Aquatic Ecosystems
The 1988 Fires burned approximately half of Yellowstone National Park and provided a significant natural laboratory to review the effects of wildfire on aquatic ecosystems. Photo George Wuerthner
Most people assume that wildfire harms aquatic ecosystems and fisheries. But such assumptions are being challenged by new research.
This narrative misleadingly portrays mixed-intensity forest fires as being bad for fish. It does so by selectively focusing on some short-term impacts while ignoring both the habitat benefits from intense fires and the harms from logging done under fire-related justifications.There is no doubt that under some conditions, especially immediately after a high-severity blaze, aquatic ecosystems can suffer temporary degradation.
Increasingly, we are beginning to understand that wildfires are a crucial positive influence on aquatic ecosystems.
- Most fish populations recover rapidly after a major fire.
- Wildfire often improves conditions for native fish.
These findings make sense since native fish have evolved wih mixed and high severity fires for thousands of years. Mixed to high severity blazes are not some kind of aberration, but have periodically charred most western plant communities.
Some of the evidence for this co-evolution of wildfire and native fish is the existance of high quality aquatic ecosystems now found in places where major wildlife occurred such as Yellowstone National Park, 1910 Big Burn as well as the North Fork of the Flathead by Glacier Park, the Bob Marshall Wilderness and other reference watersheds around the West.
WILDFIRE INFLUENCES ON FISH
Fire influences on fish can generally be broken down into three categories: short-term, delayed response, and long-term effects. The short-term impacts would usually be considered neutral or negative, while the long-term effects, on the whole, would be regarded as a positive influence.
Mixed to high severity blazes create improved aquatic habitat through the input of down wood. To be clear, high severity blazes are the major source of snags and down wood in aquatic ecosystems of the West. The more intense the fire, the more wood that enters waterways. This wood provides structural diversity, slows the current reducing erosion, provides haibtat for aquatic insects, and hiding cover for fish.
Another benefit of large fires is smoke. Smoke from wildfire can cool waterways, improving the survival of salmon, trout and other aquatic life that require colder water.
Yellowstone cuthroat trout. Photo George Wuerthner
When a watershed burns, the loss of plant cover and subsequent changes in sediment flow, changes in water temperature, changes in debris flow, and the release of nutrients into the system often alter streams.
Nevertheless, since drought is almost a prerequisite for large blazes, the actual negative impacts on fish are low water flows as a consequence of drought. Low precipitation results in greater dewatering of tributary streams for irrigation, causing a decline in spawning success and recruitment. Reduction in the amount of water leads to higher temperatures and greater concentrations of pollutants, negatively affecting fish and aquatic systems.
Climate change is a more significant threat to western aquatic ecosystems since it affects the entire region, not just one or two headwater drainages.
Cache Creek was one of the most heavily burned drainages of the 1988 Yellowstone Fires, however, fish returned to the creek within one year. Photo George Wuerthner
In most of the West, small headwater streams are typically quite cold due to high elevation and snowmelt as a water source. The water temperature in such streams remains well within the tolerance of trout and other aquatic insects, even if streamside vegetation is removed. Rising temperatures may be a positive benefit (if you think more fish is “good”), increasing biological activity, growth rates, and food supplies. But again, like any generalization, there are exceptions.
Middle Fork Boise River, Boise NF, Idaho after a wildfire. Photo George Wuerthner
For instance, research on burned vs. unburned drainages in the Boise River, Idaho, documented that “fish in streams most dramatically impacted by wildfire grew faster, but matured earlier in life with some evidence for shorter overall life span resulting from early reproduction.” The cause of this finding was theorized due to warmer temperatures and, in some cases, fewer fish resulting in reduced competition for food.
North Fork of the John Day River, Oregon. Photo George Wuerthner
Fish recovery after a wildfire is relatively rapid. A study of the John Day River in Oregon after major high-severity wildfires concluded that “within four years distribution of juvenile steelhead (anadromous rainbow trout Oncorhynchus mykiss) and resident rainbow trout was similar to that before the fire.”
East Fork Bitteroot River, Montana. Photo George Wuerthner
A surprising finding after a major wildfire burned the East Fork of the Bitterroot River in Montana is that native cutthroat trout increased while non-native trout declined. At least in some cases, a wildfire may promote native fish recovery.
High-severity fire increased the biomass of some aquatic insects in the River of No Return Wilderness. This “fire pulse” of increased productivity led to an increase in insect-eating birds and bats.
However, some lower-elevation waters may rise above lethal temperatures for fish if enough streamside vegetation is killed or destroyed by fire.
WILDFIRE, STREAM CHANNEL ALTERNATIONS, AND SEDIMENTATION
Fire-induced vegetation loss can also affect stream flow and timing. For example, Snowmelt may come earlier and proceed more rapidly in burned watersheds. Plus, the loss of trees and shrubs can reduce the amount of moisture transpired by plants, increasing soil moisture and leading to higher stream flows. These higher flows, especially in steeper first-order headwater streams, can mobilize sediments and debris, increasing incision and downcutting, thus affecting channel morphology.
Sediment flows record climatic changes in fire frequency and size with warmer periods like the Medieval Warm Spell when many large fires are recorded in sediment profiles; cooler periods had far fewer large blazes.
Abundance of down wood and logs in Cache Creek, Yellowstone NP, Wyoming. Photo George Wuerthner
Nevertheless, how higher flow affects individual streams has much to do with the stream size, steepness, and bedrock characteristics. For example, the upper headwaters of Cache Creek in Yellowstone National Park is a steep, short tributary of the Lamar River and suffered significant sediment flows after the 1988 Yellowstone Fires.
More than 80% of the Cache Creek drainage burned in the 1988 Yellowstone Park blazes. The watershed is composed of loosely consolidated volcanic debris. After the fire, subsequent heavy summer thunderstorms contributed to major changes in stream channel combined with significant sediment flow that initially led to a decline in aquatic insects and fish.
The much greater floods, like those in Yellowstone in the summer of 2022, result in more erosion than those resulting from even high severity wildfire. Seen here is the Gardiner River where flooding destroyed the main entrance road into the park. Photo George Wuerthner
This minor flow increase resulting from wildfire compares to the natural variation is much smaller than that results from a major flood that may change flows by as much as 161% over the long-term average. In other words, when you reach the level of a major river, the effects of a fire are minor compared to other natural events like floods or droughts.
One researcher involved in extensive studies of the aftermath of the 1988 Yellowstone fires concluded: “Current evidence suggests, however, that even in the case of extensive high-severity fires, local extirpation of fishes is patchy, and recolonization is rapid. Lasting detrimental effects on fish populations have been limited to areas where native populations have declined and become increasingly isolated because of anthropogenic activities.”
Wood and logs clogs a tributary of Cache Creek after the 1988 fires, Yellowstone NP, Wyoming. Photo George Wuerthner
Wayne Minshall, now deceased, formerly at the Stream Ecology Center at Idaho State University in Pocatello, studied the effects of Yellowstone’s fires on stream systems. They found that burned watersheds in Cache Creek and other small tributaries of the Lamar River (where more than 50% of the area had been scotched) had more sheet erosion, gully formation, and soil mass movement compared to unburned control streams.
Though these channel alternations may initially be seen as unfavorable, they are, for the most part, temporary. The regrowth of vegetation stimulated by the increase in sunlight, water, nutrients, and fertilization from the fire’s ashes rapidly reduces erosion and sediment flow. Within a few years, the stream systems begin to stabilize.
Indeed, a recent study of wildfire effects on trout in the Rio Grande found that initially, there was some fish kill due to post-fire sediment loads, but stream conditions had stabilized within three years. Fish populations had returned to pre-fire conditions.
In a comparison of sediment flow in the Lamar River prior to the 1988 fires with post-fire conditions, Roy Ewing found that sediment transport initially increased but diminished by 1992 to less than pre-fire levels. Much of the decrease in sediment transport was due to storage behind fallen logs and other debris that had begun to trap gravel.
After the initial rush of fine sediments is reduced, stream flows stabilize the newly deposited gravel. They even may create an important new source for spawning habitat.
To build redds to protect and incubate eggs, spawning salmon look for well-oxygenated gravel beds in river shallows. The size of the gravel is critical. Spring Chinook tend to favor gravel ranging from 25-100 millimeters in diameter (think bullseye marbles to softballs), while Oregon Coast coho prefer finer gravels (10-50 mm).
Because rivers carry gravel downstream over time, sprawling headwater creeks and rivers require regular gravel influxes. Wildfires are one of the episodic processes that provide this input.
WILDFIRE IS AN IMPORTANT SOURCE OF WOODY DEBRIS
Wildfires are often the major source of episodic input of logs and woody debris into drainages critical to functioning aquatic ecosystems. Photo George Wuerthner
Another generally positive benefit of fires is a large amount of woody debris—logs, branches, and other burnt materials that are carried or fall into rivers. These logs and other materials reduce water velocity contributing to greater channel stability over time, somewhat countering the effects of higher flows.
Researchers in Montana found that sizeable woody debris was critical to forming pools for bull trout. Still, in areas with logging, there was often a need for more logs in streams, affecting the habitat for bull trout. A study in Alaska found a similar increase in pool formation due to woody debris input.
Wildfire is a major source of fallen wood into aquatic ecosystems. Photo George Wuerthner
The additional wood and logs also create cover and food resources for aquatic insects and fish. A comparison between clearcut forests and burned forests in Wyoming showed more than twice as much woody debris in streams in drainages that had burned compared to those in logged areas.
Furthermore, since even on the most intensely burned sites, there is an abundance of snags and logs that remain on site for decades, the fires continued to contribute fish habitat to streams for years after the burn. In a sense, episodic high-severity fires are a long-term source of woody debris to streams that may provide logs and wood for the next hundred years.
Wayne Minshall found that an average of 28 additional pieces of large woody debris per 50-m reach was recorded in third-order burned sites relative to only eight pieces gained in third-order reference streams in 1989, the first year following the 1988 Yellowstone wildfires.
WILDFIRE COMPARED TO LOGGING EFFECTS
The persistence of siltation from logging roads is fundamentally different from the intermittent pulses associated with forest fires. Aquatic ecosystems are adapted to and even benefit from occasional sediment pulse. But chronic sedimentation from logging road is new and creates significant problems.
Logging roads are a chronic and long term source of sedimentation into streams. Clearwater NF, Idaho. Photo George Wuerthner
In many instances, post-fire logging can increase sediment, and logging roads are a source of chronic sedimentation. Fish can adapt to short-term sedimentation, such as after a wildfire, but continuous sedimentation from logging roads often harms aquatic ecosystems.
Salvage or post fire logging both removes the physical wood from the drainage, but can increase long term sedimentation from logging roads. Photo George Wuerthner
One difference between fires and logging activity, particularly “salvage logging,” is the repeated remobilization of sediments every time machinery and road construction occurs in a watershed. ( It should be noted that the FS “restoration” projects may occur over a 30 year period recurring logging and livestock grazing). While a blaze may release an initial flush of sediments, within a few years, sediment flow tends to decline to pre-fire levels or even lower as the post-fire slopes revegetate and fallen woody debris begins to trap sediments both on the slopes and in the streams.
Logging road bulldozed to facilitate post fire logging. Boise NF, Idaho. George Wuerthner
Logging, however, may repeatedly disturb slopes, releasing sediments for years or decades depending on how long logging continues in the drainage. As a result, logging roads may contribute to 90% of the sedimentation. In addition, since most logging roads are not fully restored, including the restoration of slope lens and revegetation, they are a long-term sedimentation source.
Clear cutting and logging removes critical sources of logs for aquatic ecosystems. Photo George Wuerthner
Another difference between fires and human activities is the structural component. While logging removes wood from the site and streams, fires add wood to the sites and streams. Over the long term, the input of fallen snags creates more fish habitat..
Cache Creek post 1988 Yellowstone Fires. Photo George Wuerthner
In the majority of Yellowstone streams monitored by the USFWS, macroinvertebrates increased between 1988 and 1991, which may be attributed to higher post-fire primary productivity.
Prior to 1988, Montana State University entomologist George Roemhild, had sampled aquatic insects throughout the park. He resampled many of those sites in 1991 and 1992, some three and four years post-fire, and found no large changes in the number or diversity of stonelies, mayflies, or caddis flies before and after the fires in the park as a whole.
So what was the effect on fish? As with Yellowstone’s 1988, fish in small headwater drainages like Cache Creek suffered some mortality from the blazes. Again, temperatures were not to blame; smoke increased ammonia levels in tiny streams to lethal conditions. But within a year, fish had recolonized all these streams.
Despite the severity of blazes that charred many of Yellowstone’s major watersheds, researchers could find no evidence of fire-related effects on fish populations in any of the park’s major rivers, including the Gibbon, Madison, Firehole, Yellowstone, Lamar, and Gardner. Furthermore, post-fire data shows that trout growth rates in these rivers were among the highest recorded.
Clear Creek, a tributary to Yellowstone Lake is also a major Yellowstone cutthroat trout spawning site. Photo George Wuerthner
Researchers also conducted an inventory of cutthroat trout spawning runs in Yellowstone Lake. Before the fires, some 58 tributaries of Yellowstone Lake had cutthroat trout spawning runs, and in 2000, at least 60 streams were documented to have trout spawning activity. Again, this suggests no direct long-term negative impacts on fisheries.
Indeed, evidence suggests that overall the effects of wildfire were positive, including the deposition of more woody debris that has increased habitat structure and higher fish growth rates due to the influx of nutrients.
Kelly Creek within the Great Burn proposed wilderness Idaho is today one of the premier fisheries in the West despite the fact the entire drainage burned during the 1910 Big Burn. Photo George Wuerthner
What can we say about the long terms impacts of fires on the West’s fisheries? Well, all you need to do is look back in time. Many of the West’s last strongholds for native fish and high-quality fish habitat are areas that burned extensively in the past. For example, the drainages of the Selway River, North Fork of the Clearwater, St. Joe River, Kelly Creek, and the Lochua River in Idaho were extensively burned in the 1910 blazes that charred more than 3.5 million acres of the Northern Rockies. Today they are among the most famous trout streams in northern Idaho and known as refugia for native species like Westslope cutthroat that are endangered elsewhere.
The MIddle Fork of the Flathead River, Montana. Photo George Wuerthner
A similar conclusion could be made about the North and Middle Fork of the Flathead Rivers in Montana. Both drainages have burned extensively in the past, and today are among the last refuge and stronghold for bull trout and west slope cutthroat trout.
Research on fire ring history documents even larger fires in Yellowstone in the centuries past. Despite these large blazes, Yellowstone remains a premier fishery.
One of the key differences between the impacts associated with fires and those from other human activities like logging and livestock grazing is the temporal component. While a thunderstorm may send massive amounts of sediments from fire-denuded slopes into a stream, such events only occur for a short time after the blazes. Very shortly after a blaze, new plant growth stimulated by the fire-released nutrients and greater sunlight begins to take hold of slopes. This, combined with the down woody debris that acts as mini check dams, work to reduce sediment flow. As a result, fish populations can deal with a short-term impact on habitat quality and quickly recover from population declines.
Cattle grazing is one of the major impacts on aquatic ecosystems around the West. Livestock compact soils, destroy streamside vegetation, and often break down banks. Since this occurs year after year, it’s overall effects on aquatic ecosystems is far greater than a wildfire. Photo George Wuerthner
The same thing can be said about fires and livestock impacts. Year after year, cattle trample streambanks, destroying bank structures and removing vegetation; At the same time, fires may temporarily upset the stream channel stability, but over the long term, it has a chance to stabilize and eventually improve as riparian vegetation regrows and channel structure is stabilized by the addition of wood, and more streamside vegetation.
All the research suggests that the adverse effects of fires are localized and short-term, while the positive results appear to be long-term and more widely distributed in a watershed. Any regional effect of fires is dwarfed by the negative impacts of dewatering combined with drought. If there are any lessons we take away from the recent large wildfire, influences are well within the natural range of variation for aquatic ecosystems. Fish are well adapted for coping with these occasional blazes.
George Wuerthner is an ecologist and former hunting guide with a degree in wildlife biology
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