Why does flooding happen in Calgary?

Understanding river flow rates

The Bow and Elbow Rivers can look very different throughout the year. 

River levels generally rise every spring. Normal spring time flows can range between 70-400 m3/s on the Bow River and 15-80 m3/s on the Elbow River. Then beginning in mid-July we see them slowly go back down towards their winter levels. Each year flows will be different, based on the amount of snowmelt and rain. See Historical Data on River Flow Rates​.

Most of the time, a river is large enough to contain its flow within its banks. But if flows are high enough, the river can’t hold all the water, and it flows over normally dry land, through the floodplain. These tables show some possible impacts when levels start to rise. Watch this video to understand flow rates​.

Bow River

Bow River flows Impacts in Calgary
Greater than 280 m3/s
 Unsafe boating conditions; watch for boating advisory.
Greater than 290 m3/s
 Some pathways may be impacted.
Greater than 500 m3/s
 Potential basement flooding due to higher groundwater. Flooding in some streets and parks.
Greater than 850 m3/s
 ​Overland flooding in some communities. Evacuation may begin. View our flood maps to see which areas would be affected.

Elbow River

Elbow River flows Impacts in Calgary
Greater than 11 m3/s
 Some pathways may be impacted.
Greater than 50 m3/s
 Unsafe boating conditions; watch for boating advisory.
Greater than 120 m3/s
 Potential basement flooding due to higher groundwater. Flooding in some streets and parks.
Greater than 150 m3/s
 ​Overland flooding in some communities. Evacuation may begin. View our flood maps​ to see which areas would be affected.​

Flood Readiness Infographic

Click image for full size version

Calgary has two types of flooding

Flooding type Highest risk season Cause Impacts
River Flooding Mid-May to mid-July Heavy rainfall in the mountains and foothills combined with snow melt which drains into our rivers.

Flooding happens quickly and with little warning because of the short, steep distance the rivers travel from the mountains to Calgary.

Rivers and creeks can overflow their banks when full of rain water and snowmelt. This is called “overland flooding”. High river levels may also cause groundwater to rise and storm or sanitary sewer system backups that can flood basements.

In winter, flooding from the river or high groundwater may be caused by ice jams. Properties in our river valleys, including downtown and Beltline can be impacted.

To see if your home, work or community is at risk, check our Flood Map.
Local stormwater flooding Summer Thunderstorms over Calgary

Rain water may flow through streets, or pool in low spots until it can drain into the stormwater system. Heavy rain may cause drainage issues, storm system backups or sanitary sewer backups that can flood basements.

If the rain is so intense that the stormwater system is overwhelmed, water may flow onto private properties. This is sometimes called “overland flooding”.

In winter or spring, snowmelt can cause local flooding if storm drains (catch basins) are blocked with snow and ice.
Flood Readiness Infographic

Click image for full size version

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