State of Lake Winnipeg: 1999 to 2007 – Highlights

Climate Conditions

Given that Lake Winnipeg extends about 436 kilometres from its southern point near Netley-Libau marsh to the far north, the lake experiences different climate conditions across the south and north basins. The climate of the Lake Winnipeg watershed is classified as cold continental, with arid conditions throughout, particularly over the warm season through the prairies and plains to the west and south of the lake.

Air temperatures over Lake Winnipeg vary over a north-south gradient. For the period of 1999 to 2007, mean air temperature was 0.8°C at The Pas in the north, nearly two degrees lower than that at Gimli (2.5°C), located near the south end of Lake Winnipeg. Over the nine-year period, the warmest year was 2006 (The Pas 2.0°C, Gimli 3.7°C) and the coolest 2004 (The Pas –0.8°C, Gimli 0.9°C). The occurrence of air temperatures above or below what might normally be expected has implications for Lake Winnipeg. For example, in years where temperatures are higher than normal, water losses from the lake and the length of the open water season may be expected to increase, and rates of biological productivity (including the formation of algal blooms) may rise.

Annual mean precipitation in the Lake Winnipeg watershed is for the most part between 400 and 600 mm, but as low as 200 mm in some areas. Precipitation in the immediate vicinity of Lake Winnipeg was at or below the normal of 488 mm in most years from 1999 to 2007. The driest year was in 2006 (275 mm) and the wettest in 2004 (576 mm), the latter of which followed a strong and persistent drought that affected much of the Lake Winnipeg watershed in 2002 and 2003. Total precipitation occurs primarily in the form of rainfall between May and September. Snowfall, which represents between approximately 15 and 20% of the total precipitation in the south basin and up to twice that percentage in the north basin, contributes to sediment- and nutrient-laden flows during the spring melt in rivers upstream of Lake Winnipeg.