(this post is written with the collaboration of estimated colleagues Avery Zammit, Stephanie Saal, and Tyler de Jong)
Some residents of Whitehorse have had the privilege of witnessing the high water and ice elevation at the Chasàn Chùa (McIntyre Creek) pedestrian bridge below Ayamdigut Campus in the last seven days.
This freeze-up process is mainly caused by a sudden drop in air temperatures below -20°C and it is exacerbated by a high discharge (the creek is used for power generation). In steep streams (e.g., made of rapids), ice initially forms at the bed, forcing the water to flow on top of it. The formation of this “anchor ice” (cloud-like, porous masses of ice under water) occurs faster at specific locations (e.g., sweepers and cascades), and this translates into the development of a series of ice dams (that block the entire channel cross-section) and pools (that can be surprisingly deep).
The ice formation process is explained in a former blog post.
The purpose of this short blog post is to emphasize that the creek, especially the reach between the pumping station and Mountain View Drive, is very dangerous during the summer because of the fast current and the numerous sweepers. During winter, this reach becomes extremely dangerous:
- The water depth can increase from the normal 0.5 m to more than 1.5 m in a few days (it was very high between Nov 24 and Dec 1 at the bridge below campus).
- Emerging anchor ice is generally full of voids and is structurally weak.
- Snow near the creek may be soaked by overflow.
- Once the ice cover appears white or when it is covered by dry snow, this means that the water has found its way through and under the ice. It also implies that fast-flowing conditions are restored, which can be noticed by the sound of water flowing and splashing under the ice.
- The partial ice cover left after freeze-up is generally thin and in a free-spanning state. It may bend or break under its own weight because the water no longer supports it by buoyancy. Cracks may be hidden by snow, making them invisible. One would be lucky if it supported a dog.
- Crossing such a creek with a snowmobile is the worst idea possible.
- The sweepers are still in place under the ice, and drowning is a real possibility if one falls through the ice.
- The water under the cover is at 0°C, and the creek bed is slippery. If someone were to break through the ice, standing up would be a challenge, getting out of the channel would be another challenge, and walking back to safety with soaked clothes would be a third challenge. Some readers may be familiar with Jack London’s To Build a Fire.
A sign on the trail states that the bridge may become icy (likely because this overflow process has happened in the past). People using those trails should note that the bridge is the safest alternative for crossing the creek in the area. When there is overflow near or on the bridge, turning around represents the safest option.
During the heart of winter, the creek channel may become entirely hidden by snow (someone unfamiliar with the site may wander away from the trail and reach McIntyre Creek without knowing). The free-spanning ice cover can collapse easily, even if there are footsteps in the snow, even later in winter, including when it is cold or when temperatures warm up.
Bottom line, be safe out there.