“Flooding becomes a crisis after unusually heavy rain or snow enters bodies of water with significant blockages. While beaver dams sometimes contribute to this type of flooding, they can also store water during periods of drought and slow down the movement of water from land to river systems, thereby preventing more serious floods and significant financial damage downstream (https://www.humanesociety.org/resources/what-do-about-beavers ). ”
To understand how beaver dams prevent serious flooding we must compare unchecked stream flow to that of flows that encounter beaver dams. First, stream flows without beaver dams can cause incised stream beds – where the stream bed has lowered below the floodplain significantly, sometimes until it reaches bedrock. This is caused by the increased water flow velocity that picks up and moves soil particles and rocks as it works it way down. In comparison, terraced beaver dams slow the water such that any sand/particles/silt fall to the bottom of the bed, a soil deposition.
Now that the water is slowed it accumulates in ponds behind each dam and spreads out laterally. Beavers create channels throughout the system of ponds in order to create a safe navigation route to their food source and other dams which they maintain continually. These channels help protect the beaver from predators during their daily life of eating willows and securing their dams and also increase the storage capacity of the pond system.
The abundance of water is prime habitat for may species of plant and animals to thrive. Willows are coppiced by the beavers for use in making the dams, as well as cottonwood and other similar tree species. Willows then sprout from the dam itself and the coppiced tree grow more vigorously resulting in an abundance of trees along the stream’s course where beavers make their residence. Aquatic plants grow thickly that are both food and homes for other animal species (cattail, tule, mule fat, etc.). These plants hold the stream banks intact to reduce erosion during high stream flows and become an impediment to increased stream velocity.
The overall effect of beaver dams is to slow the velocity of water, spread it our laterally, and sink water into the soil rather than allowing it to just pass through quickly. Flooding capacity is much reduced because of these combined principles resulting from terraced beaver dams along the course of a stream acting like a sponge to absorb much of the water from storm surges:
- Slow down (attenuate) the flow of downstream water and reduce peak flows after heavy rain
- Increase water storage capacity
- Increase number of channels
- Increase number of trees and plants
From one study in the United Kingdom the researchers found 30 percent of water from rain events remained within the pond system as measured by water volume, evidence that terraced dams buffer flood events. “Instead runoff vanished into ponds, dispersed laterally into wetlands, or sank into the ground (Eager Beaver, 211).”