Those 'Dam' Beavers

by Rod Walton
Fermilab Facilities Engineering Services Section

Along with bison, beavers typify the movement west by Americans during the mid-1800s. Along with bison, beavers were there first.

Pioneers found beaver colonies along virtually all the streams and rivers in the plains, including Illinois, and soon professional trappers followed to reap the rich harvest of hides to satisfy the appetites of Easterners and Europeans. Hides were used for vests, trimmings, and most importantly, top hats.

Before that time, Native Americans held the beaver in high regard. An early Indian word to describe the trait of affability was “beaver-like.” Beavers are, indeed, an affable species. Perhaps they are just too busy to be unpleasant.

Beavers are the largest rodents in North America, weighing in at 30 to 40 pounds, although individuals over 80 pounds are not uncommon. They are vegetarians and eat the young bark and wood of willows and cottonwoods that grow along streams and lakes. Unlike many large mammals, their aquatic habits limit their range to these surface waters.

Beavers have earned their reputation as busy and ambitious engineers. They harvest prodigious amounts of wood for food, then use the leftovers to build lodges and dams. They’ve been known to gnaw partway through a large tree and then wait for a passing wind gust to blow it over for them. Typically, they build their lodges first, with the small entrance at water level, then go a short way downstream and build a dam of small saplings and limbs, fortified with mud. The rise in water level behind the dam then obscures the entrance to the lodge, creating a safe and secure winter refuge for the beaver families.

The dam-building has fortuitous consequences for the rest of the ecosystem as well. The water backed up behind a beaver dam creates diverse habitat for many other plants and animals. The water can even help to recharge groundwater in underlying aquifers.

However, beavers and the technical handiwork of humans don’t always mix well. Beavers can be a nuisance when they gnaw down trees or shrubs that humans appreciate, or inundate a “desirable” plant community, or when their dam-building activities create high water in places where we don’t want it— places like an accelerator enclosure, for instance.

Because Fermilab has lots of surface water and very flat topography, beavers frequently build dams here, and they can have profound effects upstream. A typical dam that holds back a foot of water can result in acres of previously dry land becoming a pond bottom. Sometimes the extra water invades our technical spaces or just seeps into basements, causing various amounts of damage.

Jim Kalina, of the Roads and Grounds department in Fermilab’s Facilities Engineering Services Section, occasionally must trap beavers whose earthworks are causing problems for laboratory operations.

“When we first started removing beavers, in the mid-1970’s, we trapped as many as thirty animals each year,” Kalina said. “But now, we only have to remove around six.”

Beavers are a protected species, and the Illinois Department of Natural Resources requires a permit to trap problem animals. Kalina pointed out that Fermilab has about six large beaver families in Lake Law and the Main Ring Ponds.

“They have no adverse effects,” Kalina said, “so we leave them alone.”

Formerly endangered, these natural engineers are plentiful in Illinois— and on the Fermilab site

Kalina also noted that Fermilab is constantly experimenting with other remedies. Sometimes a long pipe that penetrates the dam, and continuously “drains” the beaver pond downstream, works—at least for a while. But often the beaver just moves 50 feet downstream and builds another dam. Various wire and mesh “beaver-deceivers” have been designed to exclude beavers from choice dam sites while allowing water to flow freely. Unfortunately, the industry and imagination of the beavers usually exceeds that of the humans, and such measures have met with inconsistent results.

“You have to give them a lot of credit and respect,” Kalina said. “They are very good engineers and builders, and they don’t give up.”

Beavers were actually exterminated in Illinois in the late nineteenth century, but attempts at re-introduction during the thirties were eventually successful. The state’s beaver population is healthy again, as Fermilab can attest.