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.
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.
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.