This weekend’s predicted storms could be bad news for future salmon runs.

Coho and chinook salmon, which are now entering streams and rivers throughout the region, will have a hard few days because of increased polluted stormwater runoff throughout the region. Impervious surfaces such as roads and parking lots keep water from infiltrating the ground, pushing it instead into streambeds. Along the way, the runoff picks up oil, brake dust and other toxins that can harm salmon. High energy floods can also wipe out salmon nests, killing juvenile fish even before they hatch.

The recently released State of Our Watersheds Report by the treaty tribes in western Washington documents how we’ve yet to reverse the trend of increasing pavement.

First, what are the impacts of impervious surfaces on salmon? According to the report:

As impervious surface increases in a watershed, stream temperatures and sediment transport are likely to increase and instream biodiversity decrease by reducing the number of insect and fish species. (Impervious surface) contributes to pollutants in stormwater runoff, which can contaminate local aquatic systems. Contaminated runoff poses significant threats to freshwater, estuarine, and marine species, including the Pacific Northwest’s salmon and steelhead runs. The addition of impervious surface reduces water infiltration and increases runoff, causing higher peak flows during wet times and lower dry weather flows due to lack of groundwater recharge.

So how bad is the problem here? Bad and getting worse:

Excluding federal lands, impervious surface area increased to about 7% in 2011, an increase of 2.6% since 2006. By 2026, the forecast population for Puget Sound will increase by over 750,000 and an increase in impervious surface to over 1,574 square miles.