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Understanding river flow rates

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

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 high groundwater 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.

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.

Explore Calgary’s River Flood story map to find out which areas have flood risk.

Calgary has unique river systems that can create flooding

The Bow River has a large watershed drainage area of 8000 km2. The Bow River begins at Bow Glacier north of Banff. When it rains, this watershed can produce a lot of runoff, which drains into the Bow River and flows to Calgary.

The Elbow River has a smaller watershed drainage area of 1200 km2. It begins at Elbow Lake in Highwood Pass west of Bragg Creek.

Explore our Healthy Rivers story map to learn about Calgary’s watersheds and healthy rivers.

For both our rivers, it’s a short, steep trip from the mountains to where they meet in the heart of Calgary.

Weather conditions in Calgary can change rapidly. Our proximity to the Rocky Mountains can make accurate weather forecasting a challenge. Weather forecasters often have difficulty predicting how weather systems will behave once they reach the mountains.

The City of Calgary continually monitors weather and river conditions so that we have as much time as possible to responsd to a potential flood.

Learning about river flows is the first step to understanding rivers in and around Calgary, and improving flood resiliency in our communities.

Bow and Elbow river flows

The Bow and Elbow Rivers can look very different throughout the year. Sometimes there is less flow, sometimes there is more. The levels generally rise every spring. Then they slowly go back down to their normal winter levels. Each year flows will be different, based on the amount of snowmelt and rain.

Watch this video for more information about river flows.

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.

Flow in rivers is typically measured in cubic metres per second (m3/s). One cubic metre of water would fill a box one metre high, one metre wide, and one metre deep. That’s equal to 1,000 litres, and would weigh one metric tonne. A flow of one cubic metre per second means that if you stand on the bank of the river, one cubic metre of water is going by you every second.

The illustration and tables below show normal river flows and the impact of high flows.

Bow River flow- upstream of where the Bow River meets the Elbow River

Bow River flow rate Probability of flow occurring in any given year (without flood mitigation) ​Impact
40 (m3/s)
Normal winter flow
​More than 50% The City monitors for ice jams.
70-400 (m3/s)
Normal early summer flow
More than 50% Some riverside pathways impacted.
280 (m3/s)
Riverbanks overflow
More than 50% ​High groundwater and potential basement flooding. Flooding in some streets and parks
500 (m3/s)
Riverbanks overflow
29% ​High groundwater and potential basement flooding. Flooding in some streets and parks
850 (m3/s)
Overland flooding - impacts to buildings begin
​12% Overbank flooding may reach some homes.
Evacuation may begin.
Flood emergency response required.

Elbow River flow downstream of Glenmore Reservoir

Elbow River flow rate ​Chance of occurring in any given year ​Effect
2 - 35 flow rate (m3/s)
Normal winter flow
More than 50% ​The City monitors for ice jams
15 - 65 (m3/s)
Normal early summer flow
More than 50% ​Unsafe Boating Conditions
50 (m3/s)
Riverbanks overflow
​More than 50% Potentially unsafe boating conditions; watch for boating advisory.
120 (m3/s)
Riverbanks overflow
​26% High groundwater and potential basement flooding (could occur at lower river flow rates). Flooding in some streets and parks
150 (m3/s)
Overland flooding - impacts to buildings begin
​20% Overland flooding may reach some homes. Evacuation may begin. Flood emergency response required.

We can’t stop our rivers from flooding, but we are working to reduce the impact of floods. Government and citizens have a role to play in preparing for flooding.