I'm with you on the comparative photos. What I see is are normally developing thickets enlarging as one would expect over a twenty years period. It definitely matters what time of the year the photos were taken and whether it was a dry or wet period. Mature willow and alder thickets such as those shown are really hard for humans to navigate around in but plenty of birds and small animals have no trouble. They and moose further shape the thickets by eating the bark and tender shoots of alders and willows (diamond willow) but this only makes willows and alders grow more than before being munched on. Each thicket is unique and is a microcosm of it's own which we know little about because we rarely hang out in them.