Whitehorse Climate Change Adaptation Plan - Yukon College

to shifting seasonal conditions because of its directional flow (flows south to north) and its
many tributaries. Seasonally, river levels rise in the fall and spring, driven by increased levels
of precipitation, and ebb at the end of the summer. Cold weather can cause ice-jamming and
flooding as the river does not freeze completely during winter months.
Causes of Flooding
There are numerous natural and anthropogenic reasons for flooding. In the spring, the river ice
breaks and moves with the current, potentially creating conditions for an ice-jam and resulting
in flooding. Ice jamming can also be problematic in winter when triggered by widely fluctuating
seasonal temperatures (i.e., from very cold to very warm). Flooding can also result from the
thawing of permafrost, which adds more water to the river drainage system. Permafrost can be
melted by forest fires or by a heavy snowfall. Forest fires can melt about 3.6 m of permafrost.
Heavy snowfall can trap heat from the ground, acting as an insulator to melt permafrost,
especially if the snowfall occurs early in the season.
Localized flooding can happen any time through the summer due to heavy precipitation events,
such as thunderstorms. According to climatologists, thunderstorms are most likely to occur in
the Whitehorse area during June and July. Thunderstorms are caused by the sun heating the
earth’s surface and causing a large amount of warm air to rise. When the warm air is cooled by
mixing with unstable atmospheric conditions at very high altitudes, a relatively sudden burst
of precipitation is produced. This burst of precipitation can exceed the natural or engineered
drainage capacity of a region, resulting in localized flooding. Thunderstorm activity can continue
until the end of July and into August. After August, limited daylight decreases the surface area
heated by the sun reducing the likelihood of a storm occurring.
Development along the Yukon River floodplain and through the river corridor has exacerbated
the threat of floods. Since the installation of the new hydraulic turbines at the power dams,
workers have observed flooding along the riverfront as the freezing of the river is less extensive
during winter3 .
Impacts of Flooding
Flooding directly impacts residents by damaging infrastructure and property and potentially
threatening the lives of residents. Damage from flooding is evident throughout the history of
Whitehorse. In December 1962, about two feet of water and slush covered the streets and
evacuation was necessary. High precipitation over a short period broke the record precipitation
for June in 1985, resulting in localized flooding. A mudslide along Robert Service Way was
created when 50.5 mm of rain fell on the area, blocking half of the road. In February 1968,
there was an important flood on Sixth and Seventh avenues, particularly in the low-lying
west end of the city. Flooding was caused by unseasonably warm weather which melted the
snow. The resulting melt-water exceeded the coping capacity of the local sewage system. To
complicate matters, the city crew did not want to increase the pressure in the sewers in case
the water backed up into other basements. Another early spring thaw occurred on March 4th ,
1968. The problems started when basements on Jarvis, Fifth and Sixth avenues, and beyond
were flooded by melting surface water. Heavy flooding along Second Avenue in Whitehorse in
1985 submerged a vehicle in about a metre of water. That same year, heavy precipitation in the
summer negatively affected the tourism season and the local economy.
The Marwell subdivision in Whitehorse has historically experienced greater issues with flooding
than anywhere else. Flood levels in Marwell have measured up to about one metre of water. A
flood in January 1982 reached more than 100 m into Marwell, threatening houses along Silver
3. Phone conversation with City of Whitehorse’s employees, October 2009.
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