Beaver Dams can last a Long Time.

[BillT]

Based on old maps from around Ishpeming, MI, near where my Dad grew up.

Beaver dams can last hundreds of years.

http://news.sciencemag.org/plants-animals/2015/12/150-year-old-map-reveals-beaver-dams-can-last-centuries?utm_campaign=email-news-latest&et_rid=33537079&et_cid=138606

[Mugwump]

We have a few in the area here too. The wealthy Norris family in St Charles have such a dam on their property. In the 70's the spent over $25K to have the dam evaluated because it appeared to have a leak of good size. While the engineers were evaluating the project and drawing up a plan, over about 6 months time. The local and seldom seen beavers repaired the dam.  ....work cancelled because the engineers declared that they could do no better.

[BillT]

There's a reason that engineering schools have beavers as their mascots.