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B.C. rivers could have ‘extremely low’ flows this summer

The Nechako River

Seasonal forecasts from Environment and Climate Change Canada favour an increased likelihood of above normal summer temperatures (June-July-August) across British Columbia.

In the short-term, B.C. forecast to see high pressure building into the second week of June, with temperatures expecting to rebound to seasonally hot conditions.

Flood risk from snowmelt is largely over for the season, as most rivers have experienced their peak flows and are now on or near the falling limb of the seasonal hydrograph. Flood risk from extreme rainfall events remains a possibility throughout the BC Interior.

Eastern and north-eastern B.C. is particularly susceptible to extreme precipitation associated with cold upper low systems that are common in June and into July; these systems tend to drive extreme rainfall-driven flooding irrespective of snow cover.

With diminished snowpacks and early melt this year, risks have shifted towards the increased likelihood of low flow conditions this summer in all areas of the province.

There are a few trends in current conditions that are particularly concerning. The first is extremely low seasonal flows in coastal lowland streams, particularly on Vancouver Island. Many gauged rivers are flowing in the second to fifth percentile range, including some that are approaching or exceeding historic minimum flows for early-June (for example the San Juan River and Carnation Creek). If dry conditions persist, extremely low flows will emerge as the summer progresses.

The second trend is the rapid transition that is occurring in rivers which are now on the receding limb of the snowmelt hydrograph. With lower starting snowpacks and dry spring weather, overall freshet volumes are well below normal this year.

With the influence from snowmelt waning, rivers are vulnerable to extremely low flows this summer, particularly if dry weather persists through the summer. While antecedent conditions are one important factor for summer low flows, summer weather is also of critical importance.

Long-range precipitation is difficult to forecast accurately over long lead times, therefore creating uncertainty over how the 2019 summer season will play out. While continued dry weather would drive extremely low flows this season, there is a similar chance that wet weather could dominate the summer season and at least partially ease the risk of low flows.

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