Most rivers in Canada carry some flow year-round, even during the heart of winter, even under a thick ice cover. You may not hear it, you may not see it, but the channel is alive. I personally like to say that the water does not like to freeze, it wants to keep flowing.
Water, in its liquid state, is critical to life in its many forms. It is also useful to produce low carbon emitting energy when the demand is peaking. Multiple right holders and stakeholders have an interest in knowing the amount of water that flows in a river at any time of year. But how does that work?
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During the open water season, roughly from mid-May to mid-October in most watersheds across Yukon, estimating the flow, or discharge, relies on the development of a site-specific equation that relates the water level (in meters, above a known vertical benchmark) to the flow (in cubic meters per second). The relationship of water level to flow is referred to as a rating curve. Once a rating curve is established, water level measurements can easily (in most cases) be converted to accurate discharge estimates.
In winter however, the presence of stationary ice in the river channel makes discharge estimation much more complicated. Different ice-types can give a unique personality to river channels when it comes to the relationship between water level and flow. The YukonU Research Center is currently exploring the personality of different rivers in Yukon (Takhini, Nordenskiold, and Dezadeash rivers) in partnership with the Government of Canada to better understand the interaction between different forms of ice (border ice, anchor ice, smooth or rough surface ice, ice jams, etc.) and the water level. Here is how we can consider winter in the North from a hydrometric point of view:
Late Fall to early winter:
When stationary ice starts forming in a channel, this slows the water velocity and blocks part of the channel, which leads to a rise in water level. This is why the water surface can rise significantly during the river ice formation period. However, the formation of ice at upstream locations and in small tributaries also translate into flow instabilities that are transferred downstream, which leads to apparently chaotic water level time series. As soon as stationary ice forms, the open-water relationship between water level and flow is no longer valid and flow estimation enters a period of great uncertainty.
Mid-winter:
Water levels are usually more stable once the ice cover is formed everywhere in the river system, but some watercourses, especially small rivers and streams, keep on “responding” to air temperature variations through detectable water level changes. During this period, it is possible to drill holes in the ice cover to measure the water velocity at different locations across the channel in order to determine the under-ice discharge. This helps to tell the winter discharge story. About half of the rivers in Yukon are relatively simple to “read” from an hydrological perspective during mid-winter.
Spring:
As the sun radiation increases and air temperatures rise, the ice starts to melt and to move. This generates apparently chaotic water levels, sometimes accompanied by sudden rises (ice jams) and drops (ice cover or ice jam mobilization), therefore significantly complicating flow estimation. The figure below illustrates how the movement (by about 150 m) of a single ice slab in a large river can generate water level (flow in this case) variations during a few hours.
The impact of all ice movements upstream of, and at a water level measurement station generates several superimposed waves and water level fluctuations. The following figure provides an example about the interpretation of ice processes using water level variations during the river ice breakup period on the Klondike River in 2018.
Dynamic river ice breakup events are particularly hard on instruments that are measuring water levels year-round. Once all stationary ice has left the channel (there could still be ice running downstream), estimating the discharge can rely on the open-water rating curve again.
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Understanding the personality and behavior of rivers during winter is only one of multiple tools that are being developed to improve the accuracy of winter discharge estimates.
Why is this important? If the winter discharge was never estimated, it would be hard to know how much hydropower can be produced ahead of time, it would be impossible to forecast river ice breakup and ice jam floods, it would be demanding to determine the impact of river conditions on some wildlife species, and it would be difficult to quantify the impact of climate change on the hydrological regime of northern rivers during half of the year.