Sediment is a concern in this watershed because the rivers pass through a network of forest roads and culverts that can fail and cause landslides. The glacier-fed rivers already have a naturally high amount of sediment, but silt from glacier melt is suspected to increase unnaturally because of human-caused climate change.
The tribe wants to know how the quantity and timing of sediment affects salmon spawning. The Lower Sauk River was identified by the Skagit Chinook Recovery Plan as having a poor rate of egg survival because of high amounts of sediment. In the Suiattle River, chinook salmon spawn in the mouths of tributary creeks, where the water is clearer.
The tribe and USGS are sampling the river, measuring temperature, flow, suspended sediment and turbidity at several locations, taking physical water samples as well as using an automatic sampler and continuous turbidity sensors. The Puget Sound Partnership provided funding for the majority of the project.
Physical suspended sediment samples provide the best data by measuring sediment throughout the water column. Automatic sampling is a cost-effective way to keep track of what’s happening in the river between the time-consuming and labor-intensive physical measurements.
“A lot of modeling has been done, but this is the first time sampling has been done this thoroughly,” said Scott Morris, watershed manager for the Sauk-Suiattle Tribe. “The sediment data we collect will provide critical information for future watershed models that incorporate climate change scenarios with sediment delivery to the Skagit River delta and Puget Sound.”
The mineral content of the sediment will be analyzed so researchers will know whether it’s natural accumulation from glacier melt, or if a forest road has failed.
“If there’s a lot of road sediment, we’ll know we have work to do,” Morris said. “We’ll have the numbers, not just assumptions. We can work it into the recovery plan.”
For more information, contact: Scott Morris, watershed manager, Sauk-Suiattle Tribe at 360-436-0347 or [email protected]; Kari Neumeyer, information officer, NWIFC at 360-424-8226 or [email protected].