The North American beaver, Castor canadensis, has recovered from historic overtrapping, recolonizing much of its former range as its population expanded. Previous studies using historical aerial photos document recent increases in number of beaver ponds, but the long-term sustainability of beaver populations and their ponds over centuries of landscape alteration is unknown. This paper analyzes the fate of beaver ponds mapped in 1868 near Ishpeming, Michigan, USA. Of the 64 beaver dam and pond sites mapped in the 1860s, 72 % were still discernible in 2014. Land use changes that altered the terrain (mining, residential development) or stream paths (channelization) were the main sources of beaver pond loss. This remarkable consistency in beaver pond placement over the last 150 years is evidence of the beaver’s resilience.
There are beaver dams near our family cottage in the UP of Michigan. We've been fishing them for about 50 years. The beavers don't maintain the dams continuously, but cycle their maintenance. My father told me that they do this to allow the vegetation to grow back. Of course, they have also been trapped, which I am sure has an impact.
That said, when the beavers abandon a dam for several years, it does not disappear. Water flows through at some points, and it slowly sinks, but I think it would take 30 years or more for it to disappear entirely. They improve them really quickly. There is one large dam that spans maybe 200 ft that they abandoned sometime when I was a preteen, but then rebuilt in one season about a decade later.