Some people in Yukon may wonder, looking out the window, how river systems will handle such a significant amount of snow in the months to come.
What is a flood?
A flood is when water starts flowing out of a channel, on the floodplain. Most river channels are naturally formed by erosion and sediment transport. Their size (width and depth) is shaped to carry the maximum annual flow every second year, on average, without causing flooding. Floods can happen when there is excess water in the watershed due to snowmelt or rain, or when the channel is blocked by ice jams, wood jams, or landslides.
Flooding processes in large rivers* of Yukon
In Yukon, the highest water levels in large rivers (those without glaciers in the headwaters) are most often the result of spring snowmelt, what is referred to as the “freshet period”. Typically, water will be highest in early May and early July – corresponding with the timing of low elevation snow melt and high elevation snow melt, respectively. This is the case of the Liard River, Pelly River, Stewart River, Peel River, Porcupine River and Lower Yukon River. Glacier melt normally causes maximum water levels later in summer, between late July to early September, in the Upper Yukon River (Southern Lakes), Alsek River and White River.
Two common processes can generate flooding in the spring: 1. Ice jams taking place during the river ice breakup period, and 2. high flows occurring after breakup, when the bulk of the snowmelt happens.
- The Pelly River, Lower Yukon River, Porcupine River, Liard River (BC and NWT side), and Peel River (NWT side) are known for their intense river ice breakup events that may lead to severe ice jams. This process is less frequent in the Liard River (Yukon side) and in the Stewart River, and it is virtually absent of the Upper Yukon River (upstream of Carmacks). Large ice jams can form when the snowpack in the watershed melts faster than the ice cover degrades in the river. This leads to a chaotic breakup of the ice.
- Higher-than-average flows occur in the spring when runoff (water reaching the streams and flowing into larger rivers) is either caused by intense snowmelt (high air temperatures with sunny conditions), by rainfalls (particularly if rain falls on snow), or by a sequence of both weather conditions. Any river in Yukon can be affected by high water levels during the freshet period.
The following figure presents a rule of thumb indicating how the probability (qualitative) of ice jam and open water floods is linked with weather conditions.
In general, flooding will happen if:
- Maximum air temperatures are above 15oC for several days in a row (or if there a single, very warm day) with no freezing at night.
- More than 15 mm of rain falls on a large portion of the watershed in less than 24 hours.
Spring 2021 outlook
The snowpack contains more water than average in most regions of Yukon as of March 1st, 2021. Does this mean that the chances of flooding are higher? The simple answer is yes. More snow will require additional time to melt, and the probability of seeing an intense rain event or a sequence of warm days during the melt period is therefore higher. Moreover, there is currently more water in the ground than in the Spring of 2020, a result of the very wet 2020 Summer and Fall. This higher saturation level gives less room for ground absorption of excessive snowmelt and rain.
However, ideal weather conditions, such as those that prevailed in most watersheds of southern and central Yukon during the Spring of 2020, could still happen this year. To avoid severe flooding, we hope for:
- Sunny and cold conditions in March and April to promote early snowmelt on south facing mountain sides
- Limited additional snow and limited rain
- No consistent warm conditions over entire watersheds until most of the snowpack is depleted
- A gradual migration of spring conditions from South to North
To those who believe that the warmer-than-average Winter 2020-2021 may contribute to a lower probability of large ice jams this Spring, unfortunately, there is always enough ice in the rivers of Yukon (mentioned above) to generate an ice jam flood if breakup occurs suddenly.
Flood forecasting is a complex task that relies on several sources of information and models. Environment Yukon takes the responsibility of seasonal flood forecasts and shares this information, for each major watershed of Yukon, through communications such as the Snow Bulletin.
*Floods in small watersheds (Klondike River, Nordenskiold River, or smaller rivers) or in urban environments are a different story.