From WNPS:
Pacific Willow is a large, deciduous, spreading multi-stemmed tree or shrub which grows in riparian areas. This plant produces drooping catkins. Males catkins are green to yellow, females catkins are green or pale yellow. This plant produces capsules which split open to release the seeds. Each seed is surrounded by a tuft of hairs and grows on stalks which are diminutive (0.8 to 4 mm) in length. Once fruits develop, seeds are dispersed in the wind. Pacific willow responds well to herbivory and fire, especially when beavers chew them down. After their main trunk is cut down, the tree will send up many trunks and become a multi-stemmed shrub. Mature bark is grayish-brown and rugged, but new growth is often bright yellow, sometimes green.
 
Pacific willow is a cornerstone plant in PNW wetlands and provides many benefits to the environment in which they grow. The leaves are highly desired as browse by deer, American beavers, elk, and snowshoe hares. It provides habitat (either as dense shrub or as tall trees) for birds and small mammals. Beavers build dens and dams from the branches. Growing along streams, Pacific willows makes shade over water, cooling the water temperature and improving salmon habitat. Once the branches fall into the water, the woody debris provides habitat cover and creates deep pools for fish and other river animals.
 
This plant is a poster child for restoration projects in the PNW. Twigs can be cut and inserted into moist ground as live stakes. The twigs will regenerate into new established plants. These live stakes are very cost effective and grow rapidly.  Pacific willow quickly creates a canopy and competes well with invasive plants like reed canarygrass. The bark and roots of willows assist other plants in establishing in new sites by acting as a rooting hormone.
 
Primary photo credit: Matt Lavin on Flickr (CC BY-SA 2.0 Deed)
Secondary photo credit: Matt Lavin on Flickr (CC BY-SA 2.0 Deed)