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A Journey to a New Land

Woolly Mammoth
Multimedia Library

Dr. John Clague

Department of Earth Sciences, Simon Fraser University

Fraser River Floodplain

Below the McCallum terrace you have an almost perfectly flat surface that we call a floodplain. This is the surface that regularly gets inundated during floods by the Fraser River and tributary rivers like the Harrison for example, or farther down the valley, the Chilliwack River. Over time these floods will deposit their load of silt and sand on that surface and build it up. The material settles out as it would through water to form a perfectly flat surface.

So we are looking at a surface or a flood plain that has built up over millennia by countless floods that have left their load of silt and sand and built it up to this more or less flat plain. This plain is banked up against the surrounding bedrock hills, the much older hills, and against this McCallum terrace, which is also older.

The floodplain is a fairly young geologic feature – it has formed just in the last 10,000 years. So it is younger than the ice age McCallum terrace and much younger than the surrounding mountains. It is a very young feature in our landscape, and it’s still forming. Or it would form if the river could escape its banks and continue to deposit silt and clays. But now the river is dyked to prevent these inhabited, developed surfaces from being flooded, for very good reason.