The average and maximum number of pieces of wood per 100 meters in agricultural stream channels is 70 percent less than what is found in forested and rural residential lands. The loss of pool area is associated with the removal and reduction of large wood debris, increases in sediment supply, and increased peak flows. Channel slope also influences the stability of the wood once it has entered the stream. Generally speaking, the spacing between pools in the Stillaguamish decreases with an increase in wood pieces and a decrease in channel slope. Sedimentation problems have been a concern to fish biologists in the Stillaguamish since at least the late 1950s.
Landslides associated with human land uses are the primary source of sediment.
A total of 1080 landslides have been inventoried for the period from the early 1940s to the early 1990s. A total of 851 landslides delivered sediment to stream channels, and of these, at least 40 percent delivered sediment directly to fish-bearing waters. Increases in peak streamflows exacerbate sediment problems. Streamflow measurements from the North Fork show a systematic increase in peakflows. Because this trend is not found in the South Fork streamflow data, it suggests a relationship between land use activities more prevalent in the North Fork.