Extreme hydrometeorological events induce abrupt and widespread changes in Pacific Northwest freshwater temperatures

The Pacific Northwest experienced four disruptive, if not devastating, extreme hydrometeorological events in 2021. Here, we explore the impacts of these extreme events on water temperatures for rivers and lakes across this vast region of North America including British Columbia's Nechako Watershed.
Published in Earth & Environment
Extreme hydrometeorological events induce abrupt and widespread changes in Pacific Northwest freshwater temperatures
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Introduction

In 2021, the Pacific Northwest of North America experienced several disruptive, if not devastating, extreme hydrometeorological events. That year alone, four extreme hydrometeorological events caused wide-ranging impacts in the region including increased morbidity and mortality during an unprecedented early summer heat dome, catastrophic flooding following a sequence of autumn atmospheric rivers, and spikes in hydroelectricity consumption due to two Arctic air outbreaks. Aside from these socio-economic impacts, the four extreme hydrometeorological events of 2021 resulted in abrupt and widespread changes in water temperatures in the countless creeks, rivers and lakes across the Pacific Northwest of North America. Our study (Déry et al. 2024) applies an extensive observational database of hourly water temperatures for 554 sites across this vast region to understand how waterways responded to spikes in air temperature and intense precipitation events. This blog entry reports on some of the field work activities that contributed to this extensive observational database of water temperatures for northern BC and that motivated the authors to undertake the present study. We focus here on two periods during the 2021 field season: 1) the heat dome of early summer 2021, and 2) the sequence of atmospheric rivers of autumn 2021. 

The early summer 2021 heat dome

On an early morning of late June 2021, a University of Northern British Columbia (UNBC) field crew prepared for another long day crisscrossing parts of northern British Columbia (BC) to collect water temperature data. The day’s  forecast called for extremely hot conditions with a forecasted record high temperature of 38°C, when the average high for that time of the year was 20.5°C. Despite being only a few days after the summer solstice, a heat dome smothered the area, requiring the field crew to take extra precautions with skin protection from the sun’s intense rays and from heat exhaustion, in addition to the usual pepper spray to deter bears and personal protection equipment. The destination for field work encompassed the traditional and unceded territory of  Nak’azdli Whut’en First Nation near Fort St. James, BC where, in collaboration with the Nation, data were being collected from stream temperature loggers on the Tsilcoh and Necoslie rivers.  

The spring freshet had long passed in these two waterways and flows were already quite low in response to the ongoing heatwave (Fig. 1). With the low flows unable to buffer the extreme heat, stream temperatures soared in both waterways, even approaching a daily mean of 28°C in the murky Necoslie River. Seven days into the heat dome, water temperatures for the Necoslie and Tsilcoh rivers rose by 6.2°C and 5.2°C, respectively, relative to the prior week’s conditions. These low-lying streams were especially sensitive to rapidly increasing air temperatures, warming by 0.76°C (Necoslie River) and 0.63°C (Tsilcoh River) for each 1°C increase in air temperatures during the heat dome. This high thermal sensitivity to the heat dome characterized many low-lying streams across the Pacific Northwest of North America, particularly those on the receding limb of the spring freshet. This was in stark contrast to headwater streams such as Rhine Creek (see Fig. 2) for which pulses of snow and glacier melt suppressed the rapid increase in air temperatures.

Figure 1: Daily runoff for the Tsilcoh River (black filled curve) and mean daily water temperature for the Tsilcoh and Necoslie rivers in 2021. The grey bar marks the period of the heat dome or Event 2 in our study.

The autumn 2021 atmospheric rivers

Several months following the early summer heat dome, a series of intense atmospheric rivers made landfall along western North America’s Pacific Coast in autumn of 2021. This included a pair of atmospheric rivers that brought copious amounts of precipitation with up to 330 mm reported at Hope, BC. This deluge caused catastrophic flooding in southwestern BC and northwestern Washington state, resulting in five fatalities, the loss of countless farm animals and devastating structural damages. In the fall of 2021, a team from UNBC led a two-month field campaign titled the Tahtsa Ranges Atmospheric River Experiment or TRARE to study this phenomenon in the upper Nechako Watershed on Cheslatta T’en traditional and unceded territory. A key aspect of this field campaign was to assess how landfalling atmospheric rivers influence stream temperatures. Thus, several water temperature loggers were deployed in alpine creeks draining to the Nechako Reservoir, a massive body of water managed for hydroelectricity production.

Preceding the mid-November 2021 flooding event, several atmospheric rivers made landfall in proximity to the upper Nechako Watershed in September and October 2021. Of note is the 20-22 September 2021 event that resulted in over 50 mm of precipitation at Huckleberry Mine, the base of operations for the TRARE field campaign. Waterways in the area including the Kemano River and Rhine Creek experienced a spike in runoff and a surge in air temperatures with the infusion of warm sub-tropical air into the area (Fig. 2). Despite the rise in air temperatures, however, water temperatures saw minimal changes given the cloudy conditions, copious precipitation, alpine snowmelt and the resulting pulse of water flowing through the montane waterways. These montane waterways were therefore more resilient to ambient warming relative to their low-land counterparts.

Figure 2: Daily runoff for the Kemano River (black filled curve) and mean daily water temperatures for the Kemano River and Rhine Creek in 2021. An atmospheric river made landfall along the north-central coast of BC on 20-22 September 2021 causing water levels to jump rapidly with river runoff peaking at 70 mm day-1.

Southwestern BC and northwestern Washington experienced similar conditions in mid-November 2021. While the area directly affected by flooding saw a surge in air temperatures as southwesterly winds advected relatively warm air, the inclement weather and rapid alpine snowmelt suppressed the warming of stream temperatures. Indeed, waterways in the flooded area saw minimal changes in stream temperatures given these ambient conditions. However, regions further to the south, namely in Oregon, observed a marked increase in freshwater temperatures as they remained in the warm and clear air sector of the two storms.

Conclusion

Our work demonstrated that the four extreme hydrometeorological events of 2021 including the early summer heat dome and devastating pair of autumn atmospheric rivers induced drastic changes in water temperatures across the Pacific Northwest of North America. As climate change amplifies, these types of extreme events may become more frequent, persistent and intense with potential deleterious impacts to water quality, aquatic species and their habitats. A prime example is the recent drought in BC that spurred a record-breaking wildfire season in the summer of 2023. There is therefore an ever-increasing need to not only expand stream temperature monitoring across the Pacific Northwest of North America but also extend this study to encompass additional extreme hydrometeorological events including floods and droughts to mitigate their impacts on aquatic environments and ecosystems. The UNBC research team is therefore dedicating considerable effort to enhance monitoring of freshwater temperatures, and other variables, as extreme hydrometeorological events continue to afflict northern BC.

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