Prairie streams often exist for only a few days or weeks during the spring snowmelt, and perhaps briefly after heavy rainstorms. Prairie dugout Glaciers left hundreds of thousands of small lakes and sloughs dotting the flat plains and rolling terrain of this region. Some lake basins in southern Alberta and Saskatchewan have internal drainage with no surface outflow, even in wet years. Because there is no outflow, surface runoff is balanced by evaporation, so these water bodies expand in wet years and shrink or disappear entirely in dry years. Dependable surface water supplies come mainly from large natural and constructed water bodies.
Dugouts are also a common means of storing local runoff.
They are usually designed to provide sufficient water to last 2 years, after accounting for evaporation losses. If there are several consecutive dry years, they cannot maintain the supply. Central Canada Ontario and Quebec are generally well supplied with water provided by about 1000 millimetres of precipitation each year. The relatively high potential evapotranspiration in the warmest parts of southern Ontario often leaves soils without adequate moisture during the growing season. Quebec, on the other hand, has slightly lower potential evapotranspiration and more precipitation, so is more like Atlantic Canada with respect to soil moisture and surface runoff.