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1. Fall and early winter Coastal watersheds in BC are vulnerable to flooding during this period. The most common mechanism to trigger flooding is a major rain event accompanied by warm temperatures and occurring on a relatively shallow snowpack. This most commonly happens between the middle of October and the end of December although floods in January are not unknown. As the season progresses, the deepening snowpack has more absorptive quantities (i.e. it is able to store more rainfall like a sponge) and the probability of getting warm, wet storms (commonly referred to as Hawaiian or Pineapple Express events) decreases. The River Forecast Centre monitors coastal weather forecasts, snow conditions and stream levels throughout the fall, starting in mid-October. Almost all coastal rivers have their floods of record during this season and rivers which have historically caused damage include the Bulkley River, Bella Coola River, Squamish River and the tributaries to the Fraser River in the lower Fraser valley.
Interior snow fed rivers and streams generally have their highest flood levels as the result of snowmelt in the spring. Rain occurring after a warm spell that has caused rivers to swell can exacerbate the flood levels. The timing of the peak flows depends on the weather sequences and, to some extent, the average elevation of the basin. When the snowline is relatively low at the beginning of the melt season, an extended warm spell will have a greater effect than the same warm spell later in the year with a much higher snowline. The River Forecast Centre monitors rivers throughout the province during the freshet, with particular emphasis on the Fraser River basin for which hydrologic models are used to make short term flow forecasts. Other natural rivers such as the Similkameen, the Elk and the Skeena are also monitored. Because of the large number of dams on the main stem of the Columbia River, there is little danger of major flooding under normal circumstances. |
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