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Overland flooding

As snow melts in the Rocky Mountains and combines with heavy rainfall, people in low-lying areas (or flood plains) may be impacted by flooding. These areas include communities near the Bow and Elbow rivers.

What causes river flooding?

The main factor for flooding is rainfall. Although the amount of snow and the rate of snowmelt are contributing factors to river flooding, the highest risk is from heavy rainfall in the watershed.

The rain flows off the mountains into the river rapidly and the runoff then travels a relatively short distance through steep river channels to arrive in Calgary. By this time, the runoff is flowing rapidly, which may lead to river flooding with little or no warning. The period from May 15 to July 15 is when the risk of river flooding increases in our city because these are the rainiest months.

Understanding provincial warnings

If necessary, the Government of Alberta may issue a warning or advisory based on current river flow.

View current advisories and warnings.

  • Spring runoff advisory: means that stream levels are rising or expected to rise due to snowmelt and no major flooding is expected. Minor flooding in low-lying areas is possible. Anyone situated close to the streams affected (property owners and the general public) is advised to be cautious of rising levels.
  • High stream flow advisory: means that stream levels are rising or expected to rise rapidly and no major flooding is expected. Minor flooding in low-lying areas is possible. Anyone situated close to the streams affected (campers, fishermen, boaters and the general public) is advised to be cautious of the rising levels.
  • Flood watch: means that stream levels are rising and will approach or may exceed bank full. Flooding of areas adjacent to these streams may occur. Anyone situated close to the streams is advised to take appropriate precautionary measures.
  • Flood warning: means that rising river levels will result in flooding of areas adjacent to the streams affected. Anyone situated close to the river should take appropriate measures to avoid flood damage.

What is overland flooding?

Overland flooding happens when water overflows the banks from streams, rivers and lakes, or through storm water backup. Depending on how severe the flooding is, the water can slowly creep up backyards or it can rapidly spread throughout the streets. In winter, frozen catch basins, storm drains or piled snow, combined with quick thawing and/or rain can cause overland flooding, which can enter basements or garages.