Slowly moving rotational slumps and mudflows are fairly common on the steep slopes along the valleys of the Bow and Elbow rivers.

Rotational slumps are landslides that occur when a slumping block slides on a curved failure surface, causing the upper surface to tilt back.

Mudflows occur when unconsolidated materials become saturated with water during snowmelt or rainstorms and flow downslope.

Rotational slumps are landslides that occur when a slumping block slides on a curved failure surface

At times, combinations of both types of landslide occur together, for example at the Wildwood slide site in north Calgary.

The Wildwood slide site in north Calgary (Photo: P. Wozniak)

The stage was set for failing slopes when large quantities of glacial and lake sediments were deposited during the Ice Age in the broad pre-glacial Bow and Elbow river valleys. The rivers cut down through these sediments to create the steep slopes that we see along the rivers today. These steep slopes are unstable and may fail when the ground becomes saturated with water.

The bluffs along the Bow River in northwest Calgary are an example of a steep slope cut by the Bow River in unstable glacial sediments. Lake sediments were deposited in Glacial Lake Calgary (Photo: B. Groulx)

Landslides are natural phenomena, but some slope failures of the last several decades were caused in part by human activity. Removal of material from the base of slopes, placing earthfill at the top of slopes, and excessive lawn and garden irrigation on top of bluffs contribute to slope failures.

To minimize the occurrence of landslides in Calgary, the city has implemented land-use and engineering guidelines. Since 1978, an evaluation of slope stability by a qualified engineer must precede development at any site where final design slopes exceed 15% (about 9°).