History of Beavers in the Pacific Northwest
Beavers historically occupied much more area than they do currently. Their ponds create habitat for a wide array of wildlife and plant species. For this reason, some people call beavers a "keystone" species because of how they affect so many other populations. Without beavers to create ponds and wetlands, a great deal of aquatic habitat is lost for many other species as well. Prior to European settlement, beavers could choose any area they wished to live in, but the amount of lands available for beaver occupation is much reduced now compared to pre-European settlement. Our landscape is quite different than it was historically...there is a lot less beaver habitat. There is also a lot less good spawning habitat for fish, and there are a lot more people with varied economic and other interests in the lands around them.
The Benefits of Beavers by the National Parks Conservation Association is a great article for information about historical beaver populations and their ecology.