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Understanding flooding

Learn more about how flooding happens in Calgary, the common types of flooding we experience, what are the causes and triggers of flooding, the communities most impacted, and how we monitor flood and water-level conditions.

Types of flooding
Learn more about the types of flooding that can happen in your community.

River flow rates
Learn more about what triggers impact river flows for the Elbow River and Bow River, as well as, the common warnings and alerts related to Calgary river conditions.

Flood monitoring
See how our team of river engineers keeps a watchful eye on Calgary's watersheds.

Flood risk
Determine if you live in a community that may be impacted by flooding and learn more about how you may be impacted.

Flood readiness
Find resources to help you prepare for potential flooding.

Understanding our river system and its impact on flooding

Calgary sits at the meeting point of two large rivers, the Bow River and the Elbow River, as well as several smaller creeks.

  • The Bow River has a large watershed drainage area and drops from the mountains to Calgary at a steep slope.
  • The Elbow River also drops in elevation at a steep slope and has a short trip from the mountains to where it meets the Bow River in the heart of our city.

The risk of having two short and steep river systems is that when they experience fast and high flow rates, they can reach Calgary quickly, with little-to-no warning.

The period between May 15 and July 15 is when we are most likely to experience river flooding. Historically, this is when we receive our largest rainfalls. However, flooding can occur any time of the year due to sewer backups, high ground water tables or ice jams.

Flood-related words and definitions

When you are monitoring river conditions or weather, you may hear words or alerts related to flooding. It's vital to understand what they mean so you can be prepared and take action.

Flood: A flood occurs when water flows beyond its normal confines over what would normally be dry land (i.e., a river overflowing its banks onto dry land).

100-year flood: A 100-year flood event describes a flood that has a one per cent chance of being equalled or exceeded in magnitude in any given year. It is not a flood that occurs once every 100 years. Often 100-year flooding is written as 1:100.

Floodway: The floodway includes the channel of a river and adjacent land areas. Floodways carry the bulk of floodwater downstream and are usually the area where flow velocity is the highest and flow depth is significant.

Flood fringe: The flood fringe refers to the land area outside the floodway but is subject to inundation by flooding.

Floodplain: A floodplain is the land area adjacent to the river that is subject to flooding. The floodplain is made up of two sections: the floodway and the flood fringe.

Watershed: The area of land where all of the water that is under it or drains off of it goes into the same place.

Flow rate: The flow rate tells us how much water is flowing in a certain amount of time. It's measured in cubic metres per second. A cubic metre is 1,000 litres of water.