Monday, June 14, 2010

Debris Flow on a Sunny Day in Oliver, British Columbia

Photo - Richard Algurie special to Vancouver Sun

A debris flood came down a small creek near Oliver, British Columbia approximately 12 miles north of Washington State's border with Canada in the Okanagan Valley. Early reports indicate the debris flow may have been associated with a dam over topping or break at a small lake in the upper watershed of the creek.

The apex of the fan can readily be seen in the photo above where the creek exits the steep terrain onto the alluvial fan. As debris filled the the creek channel it spread out over the surface of the fan. Terraces along the sides of the Okanagan Valley can be seen on the hillside in the photo. The terraces are common features along the valley side and developed as the lobe of ice in the valley retreated to the north approximately 15,000 years ago.    

Photo - Spencer Whitney of Rustico Farms & Cellars. This photo shows that the muddy almost lava like nature of the mud flow on the lower part of the deposit.

Alluvial fans are geologically hazardous areas with risks consisting of debris flow/flood hazard, flood hazard, and erosion hazard. I took the picture below in the same valley on the Washington State side of the border. This fan is much steeper. A similar fan 600 feet to the north had a house on the central part of the fan. The take away message is that the small creeks associated with these alluvial fans are capable of behaving in a very dangerous manner and home and building site selection should take that into account.

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