Physical Geography

In the Gravel Reach, the Fraser runs through a large U-shaped post-glacial trough, shaped by the ice and sediments of a series of glaciations.

Morphology

The Gravel Reach is a wandering braided-river channel formed on a confined cobble-gravel alluvial fan that stretches between Laidlaw and Sumas Mountain. The river's gradient declines rapidly downstream of the Canyon, and the resulting slower flow causes the river to deposit the coarse sediments it carries.

Patterns of deposition and erosion shape the river channels and a network of perennial and seasonal back-channels. Islands have formed where gravel bars grew high enough to trap finer sediments in which plants could take root.

Hydrology

Melting snow in the upper reaches of the Fraser basin supplies the river with most of its water. A spring flood (freshet) is part of the annual hydrological cycle.

• Flow rate (in cubic metres/second or cms) through the Gravel Reach can range from a winter low of less than 850 cms to well over 7000 cms at peak freshet (measured at Hope).


• Mean annual discharge is 3540 cms.


• Water elevation normally increases 3.5 to 4 metres at freshet, and routinely inundates a significant portion of the undiked areas of the floodplain.