Beaver dams have been found that measure up to 850 meters (0.5 mi) long and 5.3 meters (17 ft) high. How do beavers build such large structures?

 Beavers start by cutting down and trimming trees typically less than 30 centimeters (1 ft) in circumference. They push hefty branches into the streambed to form vertical columns and then weave saplings and reeds between the branches. The beavers anchor the structure by weighing it down with large rocks and attaching each end to a tree or large boulder. They also add reinforcing poles on the downstream side for extra bracing against the weight of oncoming water. The beavers seal the dam with twigs, weeds, and mud on the upstream side. In addition, they build spillways on both ends of the dam to divert excess water and enlarge them during heavy rains.

 Researchers have found that beavers’ efforts to build dams can positively affect the surrounding environment. For example, beaver dams reduce soil erosion, improve water quality, and raise groundwater levels. In addition, streams that previously ran dry in the summer often begin to flow year-round. The dams also reduce flooding by creating wetlands that absorb water during heavy rains. These wetlands may even serve as fire barriers, often remaining an oasis of green while surrounded by wildfires. The wetlands created by the dams also attract abundant wildlife.

 In one study, researchers concluded that “beaver dams resemble engineered structures.” In fact, researchers have built many imitation beaver dams in North America. For good reason, beavers are often called ecosystem engineers.

 What do you think? Did the beaver’s construction ability evolve? Or was it designed?