Searching for the Lower Fraser’s lost streams

In support of our efforts to conserve and restore wild salmon habitat, we’ve worked with Dr. Tara Martin’s Conservation Decisions Lab at the University of British Columbia to quantify the lost streams of the Lower Fraser River. This research has been motivated by a desire to understand what fraction of the area’s historical salmon habitat remains accessible today.

What the maps don’t show

The initial attempt to map lost streams followed Fisheries and Oceans Canada’s approach of digitizing historical maps. However, historical mapping focused primarily on locating valuable timber or farmlands, not mapping streams. While some larger rivers exist on these maps, smaller streams that may have been important for salmon were largely overlooked. In order to sharpen the historical picture, it needed to be viewed in a different way.

Digital elevation models can be used to infer where you might expect streams given the topography of the landscape. In BC, the Freshwater Atlas has systematic mapping of existing streams at a 1:20,000 scale. By comparing a stream network created from a digital elevation model with those in the Freshwater Atlas, streams that may have existed historically (but have since been replaced with pipes, drainage ditches or canals) can be identified.

Restoration targets

By combining the extent of lost streams with information on barriers to fish passage (like dams, culverts, flood gates), the amount of historical salmon habitat that remains accessible to salmon today can be determined. The inclusion of lost streams is important to inform our understanding of remaining habitat, as it guards against shifting baselines and can inform restoration goals. By considering the costs and benefits of restoring access for salmon beyond existing human barriers, these historically productive ecosystems could be restored in a way that is economically efficient and ecologically valuable.

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