Outdoor lovers have noticed that the ice cover in Chasàn Chùa (McIntyre Creek) close to Yukon University Ayamdigut Campus can change very quickly during the first half of winter and can also present significantly different aspects from one observation point to the next during any given day. This ice cover often seems chaotic compared to the relatively flat frozen surface that is found in larger rivers. Even though temperatures are low, moving water resists freezing and any river or stream wants to keep flowing; it does not want to freeze solid.
How does an ice cover form in small steep creeks?
Like most mountain streams, the formation of ice in Chasàn Chùa is largely three-dimensional. It can be described in 5 stages.
Stage 1: Water temperature cooling
In the fall, the water temperature can be warmer than the air temperature because it still carries heat absorbed by ponds as well as by the channel banks and bed during the summer period. Also, the groundwater temperature in most watersheds is well-above freezing and it supplies lateral heat to the channel during cold months (note that some streams are losing water to the surrounding ground, and it is known that this is the case for some segments of Chasàn Chùa).
Stage 2: Ice shells
During the first cold nights of late fall, the first ice to form in steep creeks will be transparent ice shells, or spray ice. This ice forms on cold surfaces next to the stream, but it is only sporadically in contact with the water (it is not floating and has no effect on water levels). At locations characterized by significant groundwater heat supply and a fast flow, this will be the only ice cover to form during the entire winter season, even during the coldest nights (for example at the pedestrian bridge close to the blue [Kopper King] pumping station).
Stage 3: Anchor ice
When undisturbed, freshwater freezes at 0oC, but fast flowing water struggles with the transition from liquid to solid. However, Nature can create some magic: frazil. These tiny crystals can form in the flow and eventually stick to submerged surfaces to form anchor ice (a white blanket on channel bed). Then, they can grow into the flow and occupy a significant portion of the flowing cross-section, forcing the water level to rise. Border ice can also extend from the banks and thicken downward into the water, but also upward by the freezing of successive overflow layers. At that point, the groundwater contribution, and its associated heat, is blocked, which means that the creek loses its heat faster and produces more ice.
Stage 4: Massive overflow
During the first very cold periods of winter, the creek keeps on losing its heat to the atmosphere and anchor ice production cannot be stopped. This generates overflow and exposes liquid water to more heat loss. Eventually, the channel can entirely fill with ice and water, (more than 1.5 m of water as opposed to 0.3 m without ice) and overbank flooding and icing can result (this is often observed at the bridge below Ayamdigut Campus). Where massive ice production is observed, water from the creek channel actually pushes groundwater into the banks (it temporarily becomes a losing stream). In turn, the decreasing flow velocity in the channel allows for a thin surface ice layer to form.
This is a challenging period for aquatic life and fish will seek thermal refuge, mainly dominant groundwater sources. People can be surprised by slush and water under the snow layer when stepping on the floodplain and the channel boundary can be difficult to identify.
Stage 5: Free-spanning ice cover
Eventually, air temperatures warm up and heat losses are reduced. Once some submerged ice starts to melt, a snowball effect begins between the increasing channel insulation and a reduction in water levels. Within a few hours (often less than one day), the flow can melt its way back under the massive ice accumulation, a process reinforced by heat coming from upstream.
Once this hydrological state is achieved, and once the groundwater heat flux is reestablished, the water in the channel can warm up by a few tenths of a degree, even with all that ice and snow in the channel. This heat is carried downstream to melt more ice and extend the area of free-spanning ice cover. People can even hear the water flowing freely under the snow and the partially collapsed ice cover or they can see it through small open areas in the ice cover. Some cantilevered ice features are fragile (the ice cover may only be a few centimeter-thick and not well supported) and it is not recommended to step on this ice cover. Interestingly, many segments of the creek become thermal refuges for aquatic life in the middle of winter and the under-ice temperature may remain relatively warm until the snowmelt period.
This free-spanning ice cover and snow are very efficient insulating layers. As observed early in January 2022, when night temperatures dropped below -40°C, ice production did not resume, and the water level remained unchanged. This is not always the case, and during some winters (or at other locations and in other creeks), stages 3 and 4 may be reinitiated during mid-winter cold spells.
Why is it important to study these processes?
In terms of cumulative length, small steep creeks are probably dominant water courses in Canada compared with larger rivers and low-gradient channels. These are also the creeks that feed larger rivers downstream. Their winter regime is still poorly understood and therefore, they deserve scientific attention from and hydrological and ecological point of view.
In winter, the altered biological activity in a creek, the reduced contact area between water and the atmosphere, and the formation of ice at the watershed scale that causes downstream flow fluctuations can significantly affect contaminant concentration levels. The under ice water quality is a topic of interest for the mining industry as well as for regulators.
Moreover, among the numerous channels that the cold regions transportation infrastructure crosses, small steep creeks are the most common. In Yukon, the Department of Highways and Public Works devotes significant resources each winter and spring to avoid ice-induced overflow and to replace damaged or aged culverts. Adaptation and mitigation strategies to reduce the impact of ice processes (including ice-induced overflow) are currently not optimal from a heat and hydrological angle, and this represents a cost to society that justifies research investments.
Finally, steep streams are also used for electricity production in Yukon (Chasàn Chùa is actually one of them) and other cold regions. The downstream impact of an altered flow and thermal regime (more electricity is needed in winter) on ice processes, water levels, and habitats is still poorly understood and rarely accurately anticipated. Additional research on this topic is needed.
What are the next research activities for YukonU’s work along Chasàn Chùa ?
YukonU Research Centre began studying winter hydrological processes in Chasàn Chùa during the fall of 2020. Continuous water level, water temperature, and water conductivity data sets have been collected at five locations, photos of the creek have been taken on a regular basis, ice coverage data has been compiled, and flow measurement trainings have been organized. Students (supported by Scholarly Activity Grants) and researchers have visited the creek below campus tens of times over the last 15 months. Maude Bergeron-Lambert and Anna Smith, two devoted YukonU students, have provided continuous support to the project (collected data is presented here).
As these efforts continue, new research activities will now focus on the interaction between the creek channel and the water table, including during the winter period. Boreholes will be drilled, and additional instruments will be tested and installed. Partnerships will also be developed with other scientists and organizations.