Good info on the beavers, guys.
I see Beaver in the larger rivers and then in sloughs, but do not come across as many in the smaller streams as I did in my youth.

It is odd how wildlife changes in different time periods.
In coastal WA, we had a ton of skunks as a kid, very few Raccoons, an no Opossums..
now it is possums and Raccoons-Skunks are rare. Even Porcupine seem to have come from nowhere to be seen often as roadkill.

Nutria are more common than Muskrat.

Osprey nested in trees and it is now 95+% artificial platform, and Bald Eagles and Peregrines are commonplace-nice post DDT recovery.

Canada Geese were not in anywhere near the numbers they are now, but in my area, ducks are far less common. No Wood Ducks in a couple of decades.

The fish-as we old(er) timers know, are a total different ballgame as well. On the lower Chehalis (pre-1980's), there were carcasses of huge kings in the Fall, and upper Quinault was full of Bald Eagles feeding on carcasses in feb/march. Not much anymore.

The lack of carcasses feeding the entire stream ecosystem is something that was finally fully realized-bringing small "repair" by bringing dead hatchery fish and dumping them in some small streams is a start, but that has to be something that is impossible to fix for the big picture where there are little fish population numbers so no carcasses to be harvested. How can streams be starved of food and be expected to revive yearly?

So back to the great comments by RIVRGUY and CARCASSMAN, YES! Alot more than one or two things to blame here.