Juvenile salmon actively seek safety from predators in downed / submerged Alder Trees. The distance between the twigs appears to enhance their safe harbor. Some of the the twigs quickly become colonized by black fly larvae, Hydropsyche larvae and other food sources for the small salmon.
In-stream Large woody debris (LWD) is important for a variety of reasons. In this case, it provides these fish with a place to "hide" from a predator. Or, it might just make it more difficult for the American Mink to catch the fish among the branches.