There are several habitat limiting factors negatively affecting salmon and their ecosystems in the Stillaguamish watershed. The major factors are discussed below. In the floodplains of the Stillaguamish, the mainstem Stillaguamish has lost more than 31 percent of its side channel habitat, primarily from the construction of dikes and revetments. The side channels of the North and South Forks have been decreased by about one-third of historic levels. The losses are mainly due to filling, and can be attributed to the combined effects of revetments, agriculture, and railroad and road construction.
Side channels provide critical rearing and refuge habitats for coho and other salmon.
The riparian forests of floodplains and upland areas are also a limiting factor. Eleven of the 27 sub-basins identified in the Stillaguamish watershed have more than 70 percent degraded riparian forests. Eight of these sub-basins have suffered more than 90 percent riparian degradation. Riparian zones associated with agriculture and rural residential land use are the most severely degraded. The loss of riparian forests has resulted in a dramatic decrease in large woody debris and associated pool habitat, both of which are key to productive salmon habitat.