Long-Term Wildfire Exposure of North American Lakes: Watershed Burning and Smoke Plumes

North America's lakes are more than just scenic backdrops; they are vital engines of the planet, providing drinking water, supporting diverse life, and sustaining local economies. For decades, scientists have watched how wildfires reshape the land, but the story of how these fires affect the water has remained incomplete. We know that when a fire burns the land surrounding a lake, it can wash ash and soil into the water, changing its chemistry. However, there is another, less visible way fires reach the water: through the air. Smoke carries tiny particles, nutrients, and chemicals hundreds of miles, drifting over landscapes that never saw a flame. As wildfires grow larger and more frequent, a critical question has emerged: are lakes being changed primarily by the fires burning their immediate neighborhoods, or by the vast clouds of smoke that drift across the continent?

A new study spanning sixteen years offers a definitive answer to this question. Researchers mapped the history of wildfires and smoke across nearly one million lakes in Canada and the United States, from 2008 to 2024. They found that while the total area of land burned by fire has grown, the number of lakes whose surrounding land actually burns has remained relatively steady and unpredictable. In contrast, the number of lakes covered by smoke has exploded. What was once a rare event for most lakes has become a near-universal experience. The study reveals that smoke is now the dominant way wildfires influence inland waters, affecting far more lakes for longer periods than the direct burning of the land ever could.

The researchers began by looking at the sheer scale of fire on the continent. Since 2008, the total area burned each year has increased, driven largely by a surge in fires within high-latitude forests, particularly those north of 50 degrees. These northern forests are burning more intensely and frequently, a shift that has changed the nature of the smoke itself. Forest fires produce significantly more smoke per acre than fires in grasslands or shrublands because the trees hold more fuel and burn for longer. This means that as fires move northward into these dense woodlands, they are generating massive plumes of smoke that travel great distances.

When the team examined how this fire activity touched the lakes, a clear divergence appeared. They tracked how many lakes had fire burning directly within their drainage basins—the land area that drains water into a specific lake. Despite the rise in total fire activity, the number of lakes experiencing this direct burning has not followed a steady upward trend. In any given year, roughly 25,000 lakes have fire in their basins, but this number fluctuates wildly. Some years see as few as 7,000 lakes affected, while others see nearly 63,000. The key finding is that even when the total fire season is record-breaking, it does not guarantee that more lakes will have fire burning right next to them. Fires are simply not hitting the specific spots where lakes are located in a consistent pattern.

The story changes completely when looking at smoke. The researchers counted how many days each year a lake was covered by a smoke plume. In 2008, only about 1.6 percent of lakes, roughly 16,000, experienced more than 20 days of smoke. By 2023, that number had skyrocketed to 88 percent, or about 866,000 lakes. The smoke season itself has stretched. Between 2008 and 2017, the period when smoke regularly blanketed thousands of lakes lasted about 165 days. From 2018 to 2024, that window expanded to 208 days, starting earlier in the spring and ending later in the fall. During this extended season, the average number of lakes under smoke each day increased by nearly 80 percent.

This shift has profound implications for how we understand the impact of fire on water. The study shows that the threat is no longer just about the fire burning the ground around a lake. It is about the atmosphere itself becoming a delivery system for fire's effects. Smoke carries ash, nutrients, and contaminants across thousands of miles, depositing them into lakes that are far removed from the flames. This means a lake in a region with no active fires can still be chemically altered by a fire burning hundreds of miles away. The researchers noted that lakes exposed to this smoke tend to be at higher elevations, sitting in or downwind of the mountainous, forested regions where these massive fires are most common.

The intensity of this exposure is also changing. While severe burning of a lake's basin—where more than half the land is scorched—remains a rare event, extreme smoke exposure is becoming common. In 2023, some lakes experienced smoke for over 150 days. The study suggests that this growing dominance of smoke is linked to the specific type of fire driving the trend. The fires moving into the northern forests produce up to 20 times more emissions per unit of area than fires in drier, southern regions. Because these northern fires burn with lower efficiency, often smoldering rather than flaring, they release more particulate matter and black carbon into the air, creating the thick, persistent plumes that now cover the continent's lakes.

The researchers were careful to note the limits of their data. Their analysis tracked the presence of smoke plumes high in the atmosphere, detected by satellites, rather than measuring the exact concentration of smoke at the water's surface. While a plume overhead does not always mean heavy smoke is settling on the lake, it does indicate that the lake is under the influence of the fire's emissions. Even without direct deposition, the presence of smoke overhead can alter how much sunlight reaches the water and change surface temperatures, both of which are critical for the life within the lake.

This work reshapes the understanding of wildfire impacts on freshwater ecosystems. For years, the focus has been on the immediate aftermath of a fire burning a watershed. The new data shows that the real story is much broader and more persistent. The influence of wildfire on North American lakes is no longer a local event confined to the burn scar; it is a continental phenomenon driven by the atmosphere. As fires continue to intensify and shift northward, the smoke they produce will likely continue to expand its reach, affecting the water quality, biology, and chemistry of lakes far beyond the fire lines. This suggests that protecting these water resources will require looking beyond the edges of the burned land and understanding the vast, invisible pathways that connect the fire, the sky, and the water.