High water levels in Yukon's Southern Lakes

Given the current situation in Yukon’s Southern Lakes, this post is meant to provide information to better understand their hydrology.

Outlet of Marsh Lake on July 5, 2021

Q: What controls water levels in Southern Lakes?

A: The Lakes are part of the headwaters of the Yukon River. Like any lake, they respond to what flows in (river hydrology) and what flows out (river hydraulics). The water that flows into Southern Lakes mainly comes from snowmelt. Rain and glacier melt contribute smaller portions of the inflow, depending on weather conditions.

Usually, the first Lakes to respond to snowmelt in the spring are Bennett Lake and Marsh Lake. Tagish Lake, which has the largest sub-watershed and contains the largest mountains then becomes the dominant contributor to the inflow. At the end of June or early in July , Tagish Lake controls the water level in Bennett Lake (the water in Bennett Lake is backed up by the high level in Tagish Lake) and in Marsh Lake (the downstream lake that receives all the water). In turn, Atlin Lake contributes a large portion of the inflow into Tagish Lake. In 2007, during the last great flood, Atlin Lake was sending more flow into Tagish Lake then it does as of July 5, 2021.

The flow that gets out of Southern Lakes is mainly controlled by 3 features: Lewes dam (a minor constriction to flow), Miles Canyon (a major natural flow constriction) and Schwatka Lake (that imposes a water level into the system). The higher the water level in Marsh Lake, the greater the evacuation capacity. At the end of summer (on average late August), inflows (glacier melt and rainfall runoff) become smaller than outflows and the lake levels naturally fall. 

Q: What caused the current high water levels?

A: It is a rare combination of a record snow fall in southwest Yukon, a late spring snowmelt with some rain, and high air temperatures that caused the snow to melt relatively suddenly. It was known that there would be a lot of meltwater in Southern Lakes this summer, with a chance that it could melt gradually, even with a portion of the snowpack persisting until next winter. This is not what is happening.

Q: What was predictable and what wasn't?

A: The Southern Lakes form one of the most predictable watersheds in Yukon, on a seasonal basis as well as in the short term. Nonetheless, a water level forecast can never be more accurate than the weather forecast used for the hydrological simulation, regardless of the model. There was a number of warning lights on the dashboard:

  • Summer and fall 2020 were wet - water levels in rivers were high when winter started and the water content in the ground was above average.
  • Winter 2020-21 broke several snowpack records and snow accumulated until the end of April. At that point, it was known that there was potential for a high-water level year, but the exact scenario can never be foreseen several months in advance.
  • May was relatively rainy in many locations, especially south of Whitehorse (one can ask backcountry skiers how many sunny days they had at the White Pass this spring).
  • Flows in some tributaries of Southern Lakes were breaking records early in June despite the relatively normal weather, because of the sudden snowmelt at mid and high altitude. The high-water event was taking shape at that time.
  • Water levels in Southern Lakes were at their highest level for a specific day starting on June 20 with no room for a widespread rain event or a heat wave.
  • Environment and Climate Change Canada released a heat wave warning for northern British Columbia and Southern Yukon on June 22, which is when the probability of flooding increased considerably.

After June 26, the snowpack in the Southern Lakes watershed was exposed to air temperatures above 25oC, which does not usually happen, at least not several days in a row. Each snow patch on each mountain of the enormous watershed released its own little stream of water while melting at a fast rate. This quickly reached streams and river entering each lake. It is therefore not surprising that the water level in Bennett, Altin, Tagish and Marsh Lake was rising by more than 10 cm per day for several days in a row.

Q: Can the water levels rise much further?

A: The short answer is not really unless intense rain occurs. The warm weather has been almost continuous in recent days yet flows in all monitored small tributaries of the Southern Lakes are now declining. This indicates that the bulk of the snowmelt is behind us and suggests that glaciers do not have the same runoff potential as the record snowpack. First, a real decline in the rate of rise of Atlin Lake would be welcomed (it does not seem to slow down at the same rate as other lakes). Then, the whole lake system needs a chance to recover (to digest this significant snow-driven runoff event). This means that water levels could continue to rise slightly for several days.

The possible scenarios are:

  • Warm and dry conditions: A limited rise (in the order of 30 cm) followed by a very gradual decline in water levels (those glaciers will continue to produce runoff).
  • Very wet conditions: A rise in water levels (a rise beyond 30 cm, depending on the rain intensity and duration)
  • Normally warm and wet conditions: A limited rise (in the order of 15 cm) followed by a decline in water levels (this is the preferred scenario).

People have mentioned that we are still weeks away from the normal peak water level date for Southern Lakes. Indeed, the water level has never peaked before July 20th since 1970. However, during an extreme summer like 2021, historical trends should not be considered as the most likely scenario. Moreover, it is important to keep in mind that, at such a high water level, the evacuation capacity of the system has never been so high.

Yukoners that are affected by high water levels and those who are supporting flood protection efforts should not be too worried about heat, light rain or even isolated intense rain, because the ground is now very dry at many locations and can absorb water. Wind waves are becoming the most important threat in the short term as we hope for normal weather conditions for the second half of summer.

Q: Can climate change affect the frequency and severity of flooding events in Southern Lakes?

A: Yes. Climate change means more precipitation and more heat, which is what caused the current conditions in Southern Lakes. The previous water level record in 2007 was caused by a sequence of  significant snowpack melt followed by  mid-summer rains. This flood was estimated to have an annual probability of 0.5% (or a return period of 200 years). With another flood  15 years later, it seems that this type of event will become more frequent (occurring maybe every 20 to 50 years). Beyond the unusual combination of hydrological conditions that have caused the current situation and the 2007 flood, there is potential for this system to produce even higher water levels. For example, the heat wave could have come one month sooner, when the snowpack was melting gradually.

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Questions about adaptation could be discussed later as people are currently bravely fighting the current flood in the heat and bugs, in solidarity to Yukoners in need.