Reservoirs can save the Potlatch River basin from getting too hot and dry for steelhead

The Potlatch River in northern Idaho receives most of its annual precipitation from December to June. Thus, there is a natural pattern of high flows in the late winter/early spring followed by decreasing flows through the summer.

However, thousands of acres of timber and meadow areas have been converted into cropland, which has altered flow patterns that resulted in higher and more concentrated peak springtime flows and lower summer base flows compared to historical flow patterns. Presently, base flows in the Potlatch River are severely limited with many tributaries becoming de-watered or intermittent during the summer months.

Flow conditions in Potlatch River — *Flow conditions in the Potlatch River in April (left) and August (right) 2023*

Adequate summer stream flows are critical to juvenile steelhead survival in the Potlatch River. When flows diminish during the summer, juvenile steelhead and other fish get trapped in isolated pools until fall rains re-charge the water in the streams.

In some years, the lack of flow, low oxygen, warm temperatures, and lack of cover in these pools affect the ability for fish to survive the summer. Fish and Game studies have shown a strong relationship between juvenile steelhead survival and summer flows in the Potlatch, with higher survival during years of higher flows.

Dead fish in the Potlatch — *A drying pool with dead juvenile fish (including steelhead) during the summer in the Potlatch River basin.*

Survival and flow in Potlatch — *Relationship between juvenile steelhead survival and summer streamflow in the Potlatch River.*

To improve summer flow conditions in the Potlatch River basin, stream habitat restoration efforts are underway in numerous locations. One of the most promising efforts involves releasing water from headwater reservoirs to supplement summer stream flows. Fish and Game conducted a pilot study using Spring Valley Reservoir at the headwaters of Little Bear Creek as a way to keep a constant flow of water in the stream during the summer.

Results showed that releasing even a small and constant amount of water from the reservoir maintained perennial flow in more than 10 miles of stream habitat in Spring Valley and Little Bear creeks. These efforts improved juvenile steelhead growth and survival during this time.

Fish and Game is working to secure funding to increase the size of Spring Valley Reservoir to hold enough water to supplement downstream flows for wild steelhead every year while still maintaining a popular fishing opportunity for the local communities. This project would likely also improve the recreational fishery in the reservoir. Ultimately, improving summer flow conditions in the Potlatch River will benefit the survival of juvenile steelhead during the stressful summer months and ultimately support more fish!