Burrowing Animals: Rodent Control
Dam cross-section showing animal burrow holes
Rodents such as the woodchuck, muskrat, and beaver are attracted to dams and reservoirs and can be quite dangerous to the structural integrity and proper performance of the embankment and spillway. It is essential that these animals and their activities be controlled to ensure proper functioning of a dam.
Beavers will instinctively try to block spillways and intake structures. Research shows that beavers react to the sound of flowing water. Such actions can raise the water level in a reservoir, which could cause upstream flooding, reduce the spillway discharge capacity (potentially causing a dam overtopping which can lead to failure of the dam), or produce sudden high outflows from the dam should the beaver structure suddenly fail. Upstream beaver dams can generate large quantities of floating debris that can clog a dam's intake and outlet structures. Beaver activity downstream can raise the tailwater elevation, which in turn can reduce the discharge from the dam or erode the downstream toe of the dam. Beavers have also been known to burrow into the upstream face of embankment dams, below the water line. Traffic can then collapse the burrow and create a partial breach at the water line.
Beavers will try to plug spillways with their cuttings. Dam owners should routinely remove the beaver cuttings from spillways and intakes. Trapping beavers near dams should be encouraged, and may be done by a licensed trapper during the appropriate season, typically during the fall and winter. (These are listed on DEC's web-site.) During the closed trapping season, permits to control destructive beavers may be issued by DEC's regional wildlife personnel. Contact regional wildlife personnel for further assistance.
Woodchucks burrow into the downstream face of a dam. When burrowing into an embankment, woodchucks stay above the phreatic surface (upper surface of seepage or saturation) to stay dry. The burrow is rarely a single tunnel. Their burrows are usually a network of tunnels and chambers with multiple entrances. It is usually forked, with several side passages or rooms from 1 to 12 feet long.
Occupied woodchuck burrows are easily recognized in the spring due to the woodchuck's habit of keeping them "cleaned out." Fresh dirt is generally found at the mouth of active burrows. Half-round mounds, paths leading from the den to nearby fields, and clawed or girdled trees and shrubs also help identify inhabited burrows and dens.
Woodchucks will be discouraged from inhabiting an embankment if the vegetation cover is kept mowed and is properly maintained. Control methods should be implemented during early spring when active burrows are easy to find, young woodchucks have not scattered, and there is less likelihood of damage to other wildlife. In later summer, fall, and winter, game animals will scurry into woodchuck burrows for brief protection and may even take up permanent abode during the period of woodchuck hibernation. Woodchucks may be destroyed by trapping or shooting any time of the year.
Muskrats burrow into a dam's upstream face. Muskrats make their homes by burrowing into the banks of lakes and streams or by building "houses" of bushes and other plants. Their burrows begin from 6 to 18 inches below the water surface and penetrate the embankment on an upward slant. At distances up to 15 feet from the entrance, a dry chamber is hollowed out above the water level. Once a muskrat den is occupied, a rise in the water level will cause the muskrat to dig farther and higher to excavate a new dry chamber. Damage (and the potential for problems) is compounded where woodchucks or other burrowing animals construct their dens in the embankment opposite muskrat dens.
Barriers to prevent burrowing offer the most practical protection to earthen structures. A properly constructed riprap and sand/gravel filter layer will discourage burrowing. The filter and riprap should extend at least 3 feet below the water line. As the muskrat attempts to construct a burrow, the sand and gravel of the filter layer caves in and thus discourages den building. Heavy wire fencing laid flat against the slope and extending above and below the water line can also be effective. Eliminating or reducing aquatic vegetation along the shoreline will discourage muskrat habitation. Where muskrats have inhabited the area, trapping is usually the most practical method of removing them from a pond, and should be encouraged. Muskrats injuring private property may be taken by the landowner, his or her family or employee, or their written designee at any time in any manner. Contact regional wildlife personnel for further assistance.
Eliminating A Burrow
One method of backfilling a burrow in an embankment is mud-packing. This simple, inexpensive method can be accomplished by placing one or two lengths of metal stove or vent pipe in a vertical position over the entrance of the den. Making sure that the pipe connection to the den does not leak, the mud-pack mixture is then poured into the pipe until the burrow and pipe are filled with the earth-water mixture. The pipe is removed and dry earth is tamped into the entrance. The mud-pack is made by adding water to a 90 percent earth and 10 percent cement mixture until a slurry or thin cement consistency is attained. All entrances should be plugged with well-compacted earth, and vegetation re-established. Dens should be eliminated without delay because damage from just one hole can lead to failure of a dam or levee.
Please note that a permit may be required to remove or disturb an animal. You should contact DEC's regional wildlife offices to inquire about controlling muskrat or beaver.
For Questions or Comments Contact:
New York State Department of Environmental Conservation
Bureau of Flood Protection and Dam Safety, Dam Safety Section
625 Broadway, 4th Floor
Albany, New York 12233-3504
Telephone: (518) 402-8185
For more information, please refer to the following resources:
- "Impacts of Animals on Earthen Dams," FEMA 473, Federal Emergency Management Agency, September 2005. This publication can be downloaded by clicking on the "Links Leaving DEC's Website" in the right column on this page.
- NYSDEC publication entitled "Owners Guidance Manual for the Inspection and Maintenance of Dams in NYS".
- NYSDEC. Best practices for nuisance wildlife control operators. This document can be downloaded by clicking on the "Links Leaving DEC's Website" in the right column on this page.