Concretions on the Athabasca River

September 03, 2010


Photographer: Darcy R Zelman 
Summary Authors: Darcy R Zelman; Jim Foster

The photo above features huge concretions that form the Grand Rapids on the Athabasca River, 160 miles (257 km) downstream from the town of Athabasca, Alberta. These 6-10 feet (2-3 m) wide concretions congregate in the river bed when they're dislodged by the fast flowing river water from the nearly vertical sandstone wall along the east side of the valley. As they tumble into the river, they create natural dams and rapids. The rapids run for approximately one mile (1.6 km) and fall about 35 feet (11 m) during the first half mile (0.8 km). Photo taken on August 29, 2009.

Photo Details: Camera: Canon PowerShot S5 IS; Focal Length: 6.0mm; Aperture: f/4.0; Exposure Time: 0.0013 s (1/800); ISO equiv: 80.