Sediment samples fill in the gaps for fishery managers

Five years of sampling have provided resources managers with concrete information about how much sediment is traveling downstream in the Sauk River watershed, and when.

“Before doing this study, we had to rely on best guesses for the quantity and timing of suspended sediment,” said Scott Morris, water quality coordinator for the Sauk-Suiattle Tribe. “Now, we have actual numbers and a range of suspended sediment associated with different flows to better inform our understanding of these natural processes.”

The Sauk-Suiattle Tribe collected suspended sediment and water temperature data from stream gauges along the Sauk, a Skagit River tributary, in partnership with the U.S. Geological Survey.

“Suspended sediment is not inherently good or bad for fish,” Morris said. “It all depends on quantity, timing, duration, grain size, location in the watershed, as well as a host of factors such as availability of side channels or other refuge habitat where fish can hide from excessive sediment during storms.”

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 and the lower Sauk River, spawning beds are presumed to be impaired as they get embedded by too many fine grains,” Morris said. “In the Skagit River delta, on the other hand, suspended sediment could be helpful to offset losses of estuarine habitat because of sea-level rise associated with global warming.”

The Skagit also is impaired by three dams on the upper river that prevent sediment from traveling downstream.

Sediment can help create new channels, allowing the river to meander and increasing habitat complexity. But too much sediment can smother salmon egg nests, or redds. The granules fill in the spaces around the gravel, blocking the flow of oxygen and preventing the hatched fry from emerging.

“It all depends on where the Sauk’s sediment ends up being distributed,” Morris said. “Restoration projects aim to direct flows back to their historic tidelands, which could distribute some of the river’s sediment back to areas that have lost their natural connection.”

The glacier-fed Suiattle and Sauk rivers already have a naturally high amount of sediment before they pass through forest roads and culverts that can fail and cause landslides, increasing turbidity. The amount of silt from glacier melt also is on the rise because of climate change.

The rivers are important spawning and rearing grounds for several species of salmon, including chinook, steelhead and bull trout, which are all listed under the federal Endangered Species Act.

Results of the study are available here.

Photo: Sauk-Suiattle watershed manager Scott Morris, left, records data as technician Lucas Wilson, center, and water quality field coordinator Andrew McDonnell sample water from the Sauk River near Darrington.