Beaver dams can last centuries, 1868 map shows

Posted by Jeff on Dec 13, 2015 @ 4:14 am in Conservation | 0 comments | Last modified: December 12, 2015

Beavers aren’t just busy — they’re swamped. But while building and maintaining a marsh can take time, it’s apparently worth the investment. The rodents’ ecosystem-shaping homes have long been known for their durability, and a recent study offers unique evidence that individual beaver dams can persist for centuries.

That evidence comes via an 1868 map commissioned by Lewis H. Morgan, a prominent American anthropologist who also worked as a railroad director. While overseeing a rail project through Michigan’s Upper Peninsula in the 1860s, Morgan came across something that amazed him: “a beaver district, more remarkable, perhaps, than any other of equal extent to be found in any part of North America.”

Morgan went on to study these beavers for years, resulting in his 396-page tome “The American Beaver and His Works.” Published in 1868, it included a map of 64 beaver dams and ponds spread across roughly 125 square kilometers (48 square miles) near the city of Ishpeming, Michigan. And now, almost 150 years later, a fresh look at Morgan’s map has revealed that most of the beaver dams are still there.

Other research has hinted at even longer resilience. A 2012 study, for example, found that some beaver dams in California date back more than 1,000 years. One of those dams was first built around 580 AD, making it older than China’s Tang Dynasty or the earliest-known English poetry. Later evidence shows the same dam was in use around 1730, when beavers apparently made repairs to it. It was finally abandoned after suffering a breach in 1850 — some 1,200 years after its initial construction.

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