Sediment Study in the North Fork Stillaguamish River,

2006 - ongoing.

Above: Building artificial redds

The Stillaguamish Tribe has created artificial Chinook salmon redds, which are salmon nests made of gravel, in the North Fork (NF) Stillaguamish River. The purpose of the study is to shed some light on the impact of fine sediment intrusion on incubating Chinook salmon eggs.

The Tribe created 18 artificial redds in 6 reaches of the NF. To collect sediment, a total of 108 in situ samplers in the form of 2-liter plastic buckets were filled with clean gravel and incubated in the river between September and December 2006, typical egg incubation time for Chinook. Each redd contained six bucket samplers placed within the dimensions of an "egg pocket,” and samplers were buried under clean gravels to a bottom depth of 30 cm to mimic a Chinook redd. The buckets were removed periodically during the testing period and the contents will be “particle-sized” with a sophisticated laboratory shaker (see photo) to determine the percent of fine sediment (< 6.3 mm) intrusion. The Tribe will then relate the percent of fine sediment intrusion with published estimates of the survival of juvenile Chinook to the emergence stage of their life cycle. Fine sediment < 6.3 mm and < 0.850 mm has been shown to either entomb or suffocate incubating Chinook.

Of the redds that were recovered, the sites near Segelson Creek (North Fork river mile 29) faired the best in that sediments < 6.3 mm and < 0.850 mm occupied approximately 8 and 4 % of the total mass of sediments in the buckets respectively. According to other studies done this would translate into 46 - 66% survival of incubating Chinook to their emergence. Sites just above the confluence with Boulder River (river mile 25.5) and below the Steelhead Haven Landslide (river mile 20) didn't fair as well in that survival was estimated to be between 15 - 30%.

Above: Sieves for sorting sediment collected in artificial redds.

This year Stillaguamish Natural Resources Staff built redds in similar locations as last year, but also added redds above and below the confluence with Deer Creek. Deer Creek has a long history of transporting tons of fine sediment into the North Fork Stillaguamish. Redds were put in from September 7th - October 16th 2007. The first set of buckets were retrieved on November 1st, 2nd and 6th 2007. We will attempt to collect buckets again in December 2007 and January 2008 to document the changes in fine sediment concentration following sediment delivery flows.

Above: Sediment sorting machine.

Observations from our first collection indicate that a high flows in October began scouring out one redd and buried most of the other redds with anywhere between 3 - 25 cm of sand impacted gravels and mixed fines.

This project will provide much needed insight on the survival to emergence (STE) of naturally spawning North Fork Chinook salmon, which is listed as a data gap in the Recovery Plan. This study is funded by Environmental Protection Agency, and coincides with a similar study on the South Fork Stillaguamish, being conducted by Snohomish County Surface Water Management and funded by the Salmon Recovery Funding Board