These are catkins on a speckled alder tree. The tiny flowers on the male catkins are shedding pollen that pollinates the female flowers. The male catkins are 5 to 10 centimenters long.
While gathering firewood today around our camp on Ima Lake in the BWCA, I worked my way through a dense thicket of speckled alder. The pollen burst away from the catkins in cloudy puffs.