Spring Freshet Review of 2021
Bulletins posted on 2021-05-20 by Ottawa River Regulating Committee
Freshet occurred early this year. At the end of February, the snow water content was close to average in most of the basin; however, weather conditions in March were favourable to cause a significant portion of the snow to directly evaporate rather than run off into the river system. In late March, warmer weather combined with rainfall on the southern and central parts of the watershed caused the remaining snow cover to melt resulting in a rise in flows and levels on the main stem of the Ottawa River. With warm temperatures, a reduced snow cover and below-normal precipitation in early April, flows and levels on the main stem of the Ottawa River quickly peaked. Only two weeks after the freshet had started, flows and levels were back to pre-freshet conditions on the Ottawa River.
With earlier than normal snowmelt over all of the basin and reduced risk of flooding due to little precipitation in April, most runoff from northern areas was stored in the principal reservoirs, resulting in no second peak on the main stem of the Ottawa River. A second peak often occurs when snowmelt runoff water from the northern portion of the basin flows down the main stem of the Ottawa River. Reservoirs were filled several weeks ahead of normal given the early spring and reduced risk of flooding. Despite receiving little precipitation this spring, most reservoirs are close to their target summer recreational levels.
Levels and flows in most locations on the main stem of the Ottawa River between Mattawa and Montreal have continued to be below normal since the 2nd week of April when most spring runoff from the central and southern parts of the basin had left the river. While levels and flows are below normal for this time of the year in most locations, these conditions are not unprecedented. In mid-May, levels and flows were similar to those observed in 2012 and were higher than those observed in 2010.
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