What happens when river flows get lower in the summer and higher in the winter?
Extremes at both ends are bad news for salmon. Lower flows in the summer lead to high temperatures that threaten salmon survival. Higher flows in the winter disrupt their egg nests and harm juvenile salmon.
This is what salmon face in the Hoh River, where the Hoh Tribe reports decreasing summer flows and increasing flows during the rainy winters. This finding is reported in the State of Our Watersheds Report, recently released by the Northwest Indian Fisheries Commission.
Over half a century of data from the Hoh River gauge at Highway 101 shows that the amount of the river’s streamflow is changing. Peak flow values show increased winter streamflow while summer mean low flow values show a decreasing trend at precisely the time when streamflow is needed most and when water temperatures are at their highest.
Decreasing flows in the summer means that salmon are getting less water when they it most, when temperatures increase and the extra water could lower the temperature. Salmon need both clean and cool water to survive.
Higher flows in the winter mean that juvenile and adult salmon face more intense floods that could wipe out redds, according to the Columbia River Intertribal Fish Commission:
The increased winter flows scour the riverbeds, disturb nests, and cause physical damage to both salmon eggs and juveniles, while the lower summer flows increase water temperatures further and reduces the overall habitat available to salmon.