The Effects of Wildfires 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. There is no doubt that under some conditions, especially immediately after a high-severity blaze, aquatic ecosystems can suffer temporary degradation. But increasingly, we are beginning to understand that wildfires are a crucial positive influence on aquatic ecosystems.


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. Rieman et al. report no known examples of stream fish extirpation due to wildfire.

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.

In general, as the size of the watershed increases, the more of the area that typically is unburned. Thus a larger drainage provides a greater buffering effect upon the watershed. Therefore, first-order headwater streams tend to suffer the greatest alterations and changes due to fires compared to larger, 3rd, and 4th-order segments downstream.

When you reach a river the size of Yellowstone or Sacramento, alterations due explicitly to fires are minimal. In other words, the short-term adverse effects of fire and ecological impacts on aquatic ecosystems in most rivers are insignificant.

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 flows lead to greater dewatering of tributary streams for irrigation, causing a decline in spawning success and recruitment. Low flows also reduce the amount of water in streams, leading to higher temperatures and greater concentrations of pollutants, negatively affecting fish and aquatic systems.

For instance, the loss of vegetation after a blaze has several effects. Removing screening streamside vegetation, particularly on smaller streams, can lead to higher water temperatures. If temperatures rise too high, they may be lethal to some fish like bull trout.

Climate change is a more significant threat to western aquatic ecosystems since it affects the entire region, not just one or two headwater drainages.

However, 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.

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.” This finding was theorized due to warmer temperatures and, in some cases, fewer fish resulting in reduced competition for food.

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.”

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.


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.

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.

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.

Yet researchers found that the further downstream one moved from the headwaters, the less fire affected aquatic ecosystems and flow characteristics.

For instance, measurements taken on the Yellowstone River outside of the park showed that the overall effects of the fires were relatively minor, with runoff increasing only 4-5% because of fires.

This minor flow increase compares to the natural variation 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.”

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 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.

It’s important to note that 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 often harms aquatic ecosystems.


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.

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.

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 priod 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, 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.

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.

Fish and aquatic insects can cope with a few years of limited reproduction due to high sediment flow. Still, they can’t deal with extended periods of repeated flushes of fresh new sediments. This is one of the significant differences between logging and its effects on fish habitat compared to fire-induced habitat changes.

Another short-term effect of fire is a greater loss of organic materials—at least in first-order high-gradient streams. Much of the organic matter that supports macroinvertebrates is leaf fall, grass, and other organic matter that falls into streams. The higher velocity of stream flow resulting post-fire can transport more organic materials downstream, reducing the organic material that aquatic insects depend upon for food.

Plus, the organic material that is retained in the streams shifts to high amounts of charcoal that is inedible for most species. Although certain species, including some mayflies, can feed on charcoal materials and may increase, most stream macroinvertebrates find such sources inedible.

These effects, however, are often very short-term. Post-fire revegetation of streamside vegetation is often rapid and even enhanced by removing streamside conifers and other evergreen vegetation.

Studies of Cache Creek in Yellowstone showed dramatic changes in post-fire stream invertebrates. The loss of streamside vegetation contributed to a decrease in organic matter, such as leaves, leading to a decline in aquatic insects that shred debris.

Countering these effects was increased algae production due to greater sunlight penetration that favored aquatic insects like mayflies and riffle beetles that feed on algae. Still, the biggest long-term effect of the fires on Cache Creek aquatic insect populations was not due to changes in food resources but rather a consequence of the alterations in stream channel morphology that occurred post-fire.

The findings in Cache Creek differed significantly from results measured in other parts of Yellowstone. In the majority of 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; ammonia from smoke increased levels in tiny streams to lethal conditions. But within a year, fish had recolonized all these streams.

Dead trout outside Yellowstone were documented in several wilderness streams in nearby Forest Service wilderness areas two years later. These fish died from high sediment loads from several severe but very localized thunderstorms. These storms in August of 1990 created a “flash flood” that washed in massive quantities of gravel and dirt into several streams, including Crandall Creek and Jones Creek—both in the North Absaroka Wilderness east of the park.

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.

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.

The general overall conclusion from Yellowstone research is that small, high-gradient streams like Cache Creek, where more than 50% of the drainage was burned, suffered major changes in stream channel morphology that included downcutting, channel scouring, and loss of pool habitat. Responding to this, the macroinvertebrate populations shifted towards more generalist species. And fish populations declined but did not disappear from these drainages.

On the other hand, larger downstream segments of these watersheds have suffered no negative impacts from fires. Indeed, some evidence suggests that overall the effects 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.

Even though nearly half of Yellowstone’s acreage was within the perimeter of a major burn, no noticeable short-term or long-term adverse fire effects on fish or aquatic invertebrates were observed in any of the larger rivers or to fish populations.

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.

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.

And 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 simulated by the fire-released nutrients and greater sunlight begins to take hold of slopes, combined with the down woody debris, that acts as mini check dams, both 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.

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 wildlife influences are well within the natural range of variation for aquatic ecosystems. Fish are well adapted for coping with these occasional blazes.

George Wuerthner has published 36 books including Wildfire: A Century of Failed Forest Policy