Climate change helped transport the water from there to here, but the extent of the damage done is mainly the responsibility of BC’s out-of-control logging industry. 

A short-wave infrared image of this summer’s 20,000-hectare July Mountain Fire (reddish brown area). The Coldwater River snakes along the fire’s lower edge on the left and then punches through the centre of the burn as it heads toward Merritt.

LET’S SURVEY SOME OF THE DAMAGE and the circumstances that led to the Lower Mainland being cut off from the rest of Canada and the flooding of Merritt and Princeton in mid November.

The Tank Hill Underpass, just east of Lytton, was built in 1957 to allow the newly widened Trans Canada Highway to pass underneath the CPR Railway. In all the years since, there’s no record of the culvert below the highway not having enough capacity to safely transport water under the structure. In February of 1963, for example, the structure survived a 24-hour rainfall of 69.9 millimetres (2.8 inches) that washed out both the highway and railway at a point closer to Lytton. In the “Great Coastal Storm” of 2007 that hit BC, Washington and Oregon in early December, Lytton recorded a 24-hour rainfall of 106.9 millimetres and the underpass was unscathed. But on November 14, 2021, the structure washed out after the fall of 61.9 millimetres of rain in the previous 24-hour period. What had changed?

The washed-out Tank Hill Underpass (Photo: BC Ministry of Transportation)

The 807-hectare watershed above the underpass was severely burned during last summer’s 84,000-hectare Lytton Creek Fire. The ministry of forests’ historical record of forest fires shows that watershed had no previous record of fire. This summer’s fire, rather than the quantity of rain that fell, may have determined the underpass’s fate.

The hydrological impact of forest fires is well understood. A 2011 study, conducted by US Forest Service scientists, noted: “Basins with high-burn severity, especially those with steep, previously forested terrain, have flashier hydrographs and can produce peak-flows orders of magnitude greater than pre-fire conditions.” (See report attached at end of story.)

Note the scientists’ use of the expression “orders of magnitude greater.” As you know, one order of magnitude means 10 times greater. Two orders of magnitude means 100 times greater. And so on.

Why would a forest fire have such a large impact on the hydrological function of a forest? Here’s the short answer from those scientists: “This is due to fundamental changes in the hydrology of burnt watersheds, especially in the short term (1-3 years). Consumption of the canopy and forest-floor organic horizon that formerly intercepted precipitation, moderated infiltration, and protected mineral soil, results in decreased evapotranspiration and infiltration, and increased runoff. Further, newly exposed soil surfaces are subject to rain-drop erosion, which may be exacerbated by fire induced soil-water repellency. Though the hydrologic impacts of high-severity wildfire have been well documented in the scientific literature, the socio-political ramifications of a latent, continuous, and highly unpredictable disturbance regime (i.e. post-fire flooding and sedimentation) has not been addressed.”

At about the same time as the Tank Hill Underpass was washing out, more serious trouble was brewing 50 kilometres to the east. The Coldwater River began to surge over its banks where it joins the Nicola River at Merritt. The west end of the town was flooded above the level that hydrologists had determined would likely be the worst case scenario—the 200-year flood plain—for future floods from melting snowpack. No one foresaw Merritt being flooded by a mid-fall rainstorm. As many as 7000 residents were forced to evacuate. Yet Merritt itself recorded only 31.4 millimetres of rain (1.2 inches) in the critical 48-hour period on November 14 and 15. By way of comparison, a total of 28 millimetres fell in a 48-hour period on November 24 and 25, 1962 and no flooding was reported. Similarly, 36 millimetres fell on Merritt in a 48-hour period during 2007’s Great Coastal Storm, yet no flooding occurred.

The Merritt flood was obviously the result of rainfall, but where did that rain fall? The town is near the junction of the Coldwater and Nicola rivers and just downstream from where Clapperton Creek—which drains the Nicola Plateau—joins the Nicola.

Flooding in Merritt, looking southeast across the west end of town. The Coldwater River cuts across the centre of the photo. The smaller Nicola River is in the foreground. One of the town’s mills is visible in the background.

Photographs of the Merritt flood posted online show that the Coldwater River was swollen and moving much faster than the meandering Nicola River. While not much rain fell directly on Merritt, precipitation was even lower towards Kamloops. The wave of water, then, likely originated south of Merritt near the headwaters of the Coldwater. That area, too, experienced a large forest fire last summer. The 20,000-hectare July Mountain Fire was contained almost entirely within the Coldwater River’s watershed.

The same physical factors associated with severely burned forest that may have caused the washout of the Tank Hill Underpass were also in play in the July Mountain Fire area, but in this case the burned area was 25 times larger. As well, there was a much larger area of recent clearcuts and young plantations in the watershed that had severely diminished the watershed’s natural ability to slow the rate at which water could move over the land before it reached the creeks and rivers that led to Merritt. UBC forest scientists XuJian Joe Yu and Younes Alila have found that removing forest in BC has a much greater impact on flooding than previously believed. They found, for example, that even small rates of logging can double the frequency of flooding and large rates of forest removal can result in up to fourfold increases in the frequency of large floods.

Extensive clearcut logging has been allowed throughout the watershed since the early 1980s, but the rate of logging accelerated dramatically after 2009 when the ministry of forests introduced a salvage logging program. The program’s objective was to remove lodgepole pine killed by the Mountain Pine Beetle, but the logging companies were permitted to remove all trees. In the Lillooet and Merritt Timber Supply Areas, only 20 percent of the logging between 2010 and 2019 was related to salvaging dead pine. To make the salvage logging more commercially attractive, companies were permitted to take stands of any species of healthy trees as well. The result was widespread devastation of healthy primary forests and loss of hydrological function in the Coldwater watershed. The time-lapse video below, which runs from 1984 to 2020, shows a portion of the Coldwater River’s watershed that was burned by the July Mountain Fire. Watch for the sudden acceleration in the cut that occurs in 2010.

Time-lapse video of logging in the area of the Coldwater watershed that was subsequently burned by the July Mountain Fire (Google Earth Time-lapse generator)

Seventy-five kilometres southeast of Merritt, Princeton also flooded. It lies at the confluence of the Similkameen and Tulameen rivers. The town saw 66 millimetres of rain—just over 4 inches—over the 3-day period between November 13 and November 15. The last big flood there occurred in 1972, but that event was dramatically different from the November 14 flood. 1972’s soaking was the result of warm temperatures quickly melting a huge winter snowpack in late May. No record of a November flood ever occurring in Princeton could be found by this reporter. But, like the Coldwater, the Similkameen watershed experienced a large forest fire this past summer, entirely within its watershed—the 15,000-hectare Garrison Lake Fire—and extensive clearcutting has been allowed throughout the watersheds of both the Similkameen and the Tulameen. The time-lapse video below records the logging from 1984 to 2000 in the area of the Similkameen’s watershed that was burned by the Garrison Lake Fire.

Time-lapse video of logging in the area of the Similkameen River watershed, 1984-2020, that was subsequently burned by the Garrison Lake Fire in 2021 (Google Earth Time-lapse generator)

If forest fires are an important factor in flooding and water damage to infrastructure—and the scientists tell us that they are—then BC is likely in for a hell of a ride in the coming years. The current forest policy of liquidating as much primary forest as is necessary to compete in the export market for wood products—80 to 90 percent of logging in BC is for exports—is creating roughly 250,000 hectares of new clearcuts each year. Clearcuts and plantations have a higher fire hazard than primary forest, and as the fraction of BC that’s covered by clearcuts and plantations grows, forest fires are becoming larger. More and more of BC will be in that state that the forest scientists described as having “flashier hydrographs” and “can produce peak-flows orders of magnitude greater than pre-fire conditions.” More and more of BC will be unable to control movement of water across the landscape, whether it has burned or not. In short, the government’s current obsession with “export competitiveness” is leading directly to hell.

You can see where all this is heading, can’t you? The cost of the flooding in Merritt and Princeton alone will likely be in the hundreds of millions. The cost of repairing the highway infrastructure and making it more flood and landslide resistant could be of a similar magnitude or greater.

The wise thing to do next—now that we can see how climate change and the current forest management regime in BC are going to synergistically combine to produce physical chaos and social misery—would be to reduce the amount of logging in BC. Let the Chinese and American buyers of BC forests figure out some other way to grow. Instead, the logging companies will keep denuding the land as quickly as the export market will allow. The political class will decide that highways and bridges and flood-prone communities will now need to be reengineered and rebuilt to withstand higher levels of water and sliding hillsides, at whatever great cost. The financial and emotional costs of flooded-out lives will just have to be paid. But who will pay?

Not the logging companies who caused it. Not the American or Chinese consumers of our forest products. Not the government officials who allowed it to happen.

The cost of mitigating against climate change will become just another part of the immense public subsidization of the logging industry in BC. The blissfully unaware public will pay whatever is needed without even knowing they are paying for it.

This story was edited on November 25, 2021 to reflect updated data for the record of precipitation that fell on Lytton and Merritt, including the historical data. The information that the ministry of forests’ record of historical forest fires showed no previous fire had occurred in the Tank Hill watershed was also added at that time.

Please also see the note below in the comments section about rainfall amounts in the “Great Coastal Storm” of 2007.

Hydrological Impacts of forest fires.pdf

David Broadland does not consent to the destruction of life on Earth. Read more of David’ s stories about BC’ s logging industry at evergreenalliance.ca.