Climate change is causing more weather extremes that directly influence water. Northern Canada is getting more than its fair share of impacts. This post summarizes noteworthy hydrological events in Yukon during the 2020-21 hydrological year (October 2020 to September 2021).
Southern Lakes:
The combination of record snowpack, delayed summer conditions and an intense warm spell at the end of June caused the lakes to peak at a record high elevation since hydrometric records started (1950). This was the event of the year in Yukon, and it was associated with a return period of about 200 years (including the event itself in the record), which means that this event would occur, on average, once every 200 years.
Interestingly, back in the early 2000s, with a shorter hydrometric record, an event of that intensity would have been associated with a return period of 5000 years, a nearly impossible event. The peak date, July 10 (at Marsh Lake), was also the earliest on record by two full weeks. The pictures above (from the Department of Community Services, Government of Yukon) and below (shared by Dr. John Pomeroy, Professor from the University of Saskatchewan) present temporary protection measures, respectively in front of, and behind Army Beach, Marsh Lake.
Beyond the important social and economic cost of the flood, it is important to outline that this event could have been even worse:
- The warm spell (June 27 to July 6) was relatively short and maximum air temperatures only flirted with historical records at most locations (as opposed to what happened in British Columbian were new maximum air temperature records were set several days in a row). The abrupt end to the heat contributed to breaking the momentum of the snowmelt runoff wave.
- There was still plenty of snow left at higher elevation after the warm spell and this snow melted gradually during the following weeks.
- Áa Tlein (Atlin Lake) peaked on August 17 (also setting a record), more than an entire month after Tagish Lake (the other large lake driving that system). A more synchronized hydrological response between Tagish Lake and Áa Tlein could have generated higher water levels in Yukon.
- Satellite images revealed that most glaciers remained snow covered down to a relatively low elevation until the end of summer. The snow layer protects glaciers from sun radiation, and this suggests that glacier melt contribution was below recent years’ average in 2021.
It is also of interest to mention that the Näkhų Chù (Takhini River) peaked early in July, establishing a new record for that period. It is possible that its inflow into the Tàgé Cho Gé (Yukon River in Northern Tutchone) caused slightly higher water levels up to Kwanlin (Whitehorse).
Kwanlin (Whitehorse and surroundings):
Snowmelt generated property flooding at many locations in April and May. There were anecdotal reports of basement damage in several subdivisions, especially where the ground is largely impervious. This event may affect the insurance coverage of several Yukoners in the future, even those living very far from, and much higher than, any river or lake.
Citizens of Whitehorse are now discovering a slightly changed sediment deposition pattern in the Yukon River channel across Riverdale as the late-fall water level drops. Very high flows are known to mobilize a significant amount of sediment and therefore to change the alignment of river channels.
Deisleen Kwáan” (Teslin) watershed:
Melting of the significant snowpack combined with a few rain events generated very high water levels in Teslin Lake and the Teslin River. The maximum water surface elevation (reached on June 24) in the community of Teslin corresponded to a return period of about 50 years (including the event itself in the record). The snowpack depletion at higher elevation was timely, given the warm period that immediately followed.
Carmacks area:
After an intense river ice formation event in the Tàgé Cho Gé (Yukon River) in November 2020, spring breakup was relatively gentle, a scenario normally imposed by Tàa’an Män (Lake Laberge in Southern Tutchone). However, significant snowmelt in the Deisleen Kwáan” (Teslin River), Little Salmon River, and Gyò Cho Chú (Big Salmon Rivers) caused the discharge to rise significantly by the end of June. The maximum water level at Carmacks corresponded to an approximate 50-year event (the most significant event in the partial record). High flows from Tàa’an Män contributed to maintaining a higher-than-usual water level in the Yukon River during the following months.
On the Tsâwnjik Chu (Nordenskiold River), the intense river ice formation event of November 2020 (causing significant backwater at several locations along the lower part of the river) was partially responsible for the dynamic breakup event of April 2021. Fortunately, cold nights imposed a fairly gradual snowmelt rate, and spring freshet water levels, although high, remained lower than those associated with most ice-related events.
Ts’ekínyäk Chú (Pelly River) and Nä`chòo ndek (Stewart River) watersheds
As observed along other rivers of Yukon, river ice formation at some locations along the Ts’ekínyäk Chú and Nä`chòo ndek generated higher-than-usual water levels through dynamic ice congestion. Spring breakup produced some ice jamming activity, and this was followed in late May and early June by a progressive snowmelt. Although the volume of water was significant, maximum water levels remained close to normal. A rain runoff event in August produced high flows in both watersheds, but conditions soon returned to normal as winter is approaching.
A road washout was reported on the Robert Campbell Highway near Faro as a result of rapid snowmelt. Prior to that, ice formation was causing concerns at some stream crossings in central Yukon, and it is still unclear what generated this significant ice production (a potential combination of high late-fall flows, an early winter, and a snowy winter).
ʼAt’aayaat Chù’ (White River):
This river system produced relatively high flows during the entire summer of 2021 (with a dryer mid-summer period), but of most interest is a very sharp runoff event (a 600 m3/s rise in 6 hours) on August 7. The wave was detected as far downstream as the Tágà Shäw (Yukon River in Southern Tutchone) at Dawson on August 9. A similar wave had occurred in August 2020 and it is of interest to better understand the exact origin (potentially a local rainstorm) and the impact of these events.
Dawson area
The very dynamic ice formation in the Tágà Shäw (Yukon River) in November 2020 (this caused the water level to rise by more than 2 metres at Dawson) set the table for an interesting spring. Fortunately, the colder weather that occurred on April 27-28 in central Yukon caused an interruption in the snowmelt process, and this probably left enough time for the ice cover to lose some strength before it became mobile on May 6.
Interestingly, because of the initial ice jam location just in front of Dawson, water levels were relatively higher upstream of town (with several 20-year-old trees broken, see picture below by YukonU) than downstream of town (slightly lower than a 10-year event). In the following days, ice jams occurred downstream of Dawson and into Alaska. It had been anticipated that summer water levels in the Yukon River would remain below those associated with spring breakup, given the lag in hydrological responses from the different river systems (even with the higher-than-average snowpack).
On the Tr’ondëk (Klondike River), an intense freeze-up event during the first days of November 2020 also meant that spring breakup could be problematic. Ice jams of moderate intensity formed in several locations in early May 2021 (maximum water levels corresponding to an annual probability of 10%, or a 10-year event) and they released between May 5 and May 7. This outcome was welcomed in a context where slightly higher air temperatures could have caused severe ice jam floods (more intense snowmelt runoff pushing harder on fragile ice cover sections).
Water levels in both Hunker and Bonanza Creeks were a concern during that period. While the former was affected by a significant snowmelt flow, the channel of the latter was blocked by the ice left on the banks of the Klondike River, which caused minor flooding at the Dawson City RV Park and Campground (see picture below by YukonU).
Aalseix̱’ (Alsek River in Tlingit) or Àłsêxh (Alsek River in Southern Tutchone):
Flows in the Aalseix̱’ system, which includes glaciers and high mountains covered with snow, were higher than normal during most of the summer season, with a close-to-record peak in August 2021. This was only the second time that the flow was evaluated above 2000 m3/s since 1974 (when the Water Survey of Canada started to operate the station). Indeed, the river used to peak, on average, at 1000 m3/s, but there is now a new reality imposed by the diversion of the Tänshı̨̄ (Kaskawulsh Glacier in the Southern Tutchone) runoff directly towards the Gulf of Alaska (summer 2016).
Liard River
Water levels were relatively high in June as the significant snowpack melted. Hydrological conditions went back to normal in July.
Ch’oodeenjìk (Porcupine River) Watershed:
Winter 2020-2021 was relatively long, and it was followed by a gradual melting of a thinner-than-usual snowpack. As a result of the relatively low spring flows, most ice jams in the river system were of low intensity. Summer conditions were dramatically dry (see picture below shared by Environment and Climate Change Canada), as opposed to what happened in the south, with close to record low flows several weeks in a row. August runoff events brought hydrological conditions back to normal.
Deh Cho (MacKenzie River), Northwest Territories
The town of Fort Simpson was affected by unprecedent ice jam flooding in May 2021 (an event with an approximate return period of 100 years). This was partially caused by record high water levels in 2020 in the river system, another probable signature of a changing climate.
2021 river ice formation perspective:
At locations where river water levels are close to normal, freeze-up intensity should correspond to average conditions. However, along rivers where water levels are currently well above the historical mean for this time of year, intense river ice formation could occur, especially if the early winter forecasted by Environment and Climate Change Canada materializes. High freeze-up water levels rarely result in significant flooding, but they increase the likelyhood of high river ice breakup water levels during the following spring.