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How To Keep Beavers from Plugging Culverts

Waterflow Devices

Beavers are particularly adept at manipulating their environment to suit their needs. They instinctively build dams to raise water levels and increase the area covered by water. Beaver ponds provide security for beavers while they move from one feeding site to another and allow them to transport construction materials easily. Although beaver ponds have many desirable features, flooding can be a problem, especially when it affects roads. Often, attempts to lower water levels are futile. When beavers hear running water, they add debris to block the flow. Any opening in a manmade or natural structure that produces the sound, appearance, or feel of escaping water will cause beavers to make repairs. Daily efforts to unclog manmade structures (such as culverts) or to make openings in beaver dams simply inspire beavers to make nightly repairs (figure 12).

Photo of a beaver dam in front of a culvert.
Figure 12—Beavers repeatedly repaired this dam that they
had built in front of a culvert.

Waterflow devices prevent beavers from perceiving the cues that cause them to build and repair dams. Designs for waterflow devices range from basic to complex. Recommended materials vary from natural components to plastics and metals. Site conditions, available resources, and management objectives will determine which device is best for a given situation. Incorporating the best features from several designs into a custom device that fits your specific conditions may yield the best results.

Certain criteria are important when installing any waterflow device. The water should be at least 3 feet deep at the device's intake. Alternatively, a trench, at least 4 feet deep and 8 feet wide, can be excavated beneath the intake, or along the entire length of the device's tubing that will be in the pond. The device should be sized to pass the required flow. If the waterflow device can accommodate no more than normal streamflow, short periods of flooding during high streamflow must be acceptable.

Use caution when working around culverts. Generally, it is not wise to constrict waterflow through a road culvert. Install the sizes of culverts needed to handle streamflows, based on engineers' estimates. Culverts need to be inspected regularly and cleared of debris so they will remain open.

It is dangerous to stand in the downstream channel while unblocking a culvert or to crawl inside a culvert while clearing debris. Always check the water levels on the upstream side. High water levels may exert pressure on blockages inside culverts, causing them to give way suddenly when they are disturbed.

Waterflow devices tend to fail because beavers plug the pipes or pull them from the dam whenever they can figure out how to do so. Pipes must be laid securely and fixed in place. The water intake area needs to be protected so beavers can't reach it.

Some users have claimed that waterflow devices do not require maintenance. Perhaps this is true under some circumstances. However, regular inspections and maintenance will reduce potential failures and possible flooding. High-water events can damage the devices.

Devices probably should be checked a couple of times a year, or at least before seasons when high water is expected. Maintenance generally requires removing debris and mud from intake areas, repairing breaks, or reconnecting pipes. If mud or silt has filled in around the intake areas, considerable effort or heavy equipment may be needed to clear the silt so the water can be restored to its original depth. These repairs are much easier during low-water periods, but beavers also have better access to build their dams during those periods.

Corrugated or Perforated Tubing

Corrugated tubing installed through culvert inlets or beaver dams can allow water to flow without creating the cues that cause beavers to make repairs or build dams. Corrugated plastic tubing can be obtained from most stores that sell construction materials. The appropriate diameter depends on the discharge flow. If the streamflow is known, the proper size can be determined by comparing the streamflow with standard rates of discharge for different diameter tubes (table 1). Several pipes can be used to increase the flow.

Table 1—Approximate water discharge rate for select tubing.
TUBING (whether perforated or not)
Tube diameter (inches) Water discharge (gallons/minute)
3 13.5
4 15.3
5 27.4
6 44.9
8 98.7
10 152.6
12 246.8
15 439.8

Perforation varies, but flow can be increased by drilling holes in the tubes. Small cuts made by the manufacturer often clog when silt and algae build up. Drill a 1-inch hole in every other high point of the last 10 feet of corrugated pipe nearest the intake point. The holes will increase the waterflow in deeper water away from the dam where it is less likely to be detected by beaver. These holes also allow air to escape from the corrugations, reducing the tubing's buoyancy and making it easier to anchor in place. Chicken wire or welded wire (2- by 2 ½-inch mesh or smaller) can be wrapped around the tubes to prevent beavers from damaging them. The wire also adds weight to the tubing, making installation easier. Tubes can be joined using a coupler or by sliding a split tube over adjoining tubes and using wire to bind the split tube. The intake end of the tubing should be covered with welded wire to prevent beavers from inserting sticks or other debris into it.

Installing the tubing is fairly straightforward. If the culvert is relatively small, it is best to use tubing the same size as the culvert. However, culverts generally are larger than most tubes. A grate mounted on the intake end of the culvert will prevent beavers from plugging the inside of the culvert. The tube is inserted through an opening cut in the grate. The unperforated end of the tube is inserted in the culvert. The perforated end should be held in place with a series of metal posts driven on either side. Wire tied above and below the tube will hold it at the desired depth. Beavers may cover tubes with mud if the tube is lying on the pond's bottom, so it is best to keep the end of the tube off the bottom.

Installing several tubes will increase the potential discharge. If beavers block the culvert, the amount of discharge will be reduced to whatever can pass through the tubes. If that happens, additional smaller culverts or a larger beaver guard can be installed to restore the needed flows.

The techniques for installing tubes through a beaver dam are similar to those for installing tubes through culverts. A notch is cut in the dam. The tube is laid through the notch with the intake end fastened as described before. The outlet end is extended some distance downstream from the front of the dam below the dam's base, reducing the risk that the beaver might plug the outlet or build another dam immediately below the first one. The tubes need to be checked periodically and maintained if necessary. Maintenance usually consists of adjusting the tubes' anchors or posts and raking mud and debris away from the openings in the tubes. The extent and frequency of the maintenance will depend on beaver activity in the area.

Forest Service Experience—Several respondents reported success using the perforated pipe. In the South-western Region, perforated pipe was extended about 4 to 5 feet beyond the culvert opening through a beaver dam, allowing water to flow until a road maintenance crew could remove the dam. The pipe prevented the road from being damaged by flooding. The Pacific Northwest Region has used corrugated pipe with 3- to 4-inch holes, with success. The pipe is extended 8 to 10 feet in front of the culvert inlet. The Northern and Rocky Mountain Regions also have had success using this method.

Both regions described removing a dam, laying perforated drain pipe about 4 feet into the culvert, and then extending the pipe 20 to 40 feet on the bottom of the stream. Wire fence panels were fastened to posts about 10 feet from the culvert and on top of the perforated pipe. This system allows beavers to try to build a dam against the fence panel while water continues to flow through. Maintenance may be needed to clean the perforated pipe or to remove debris from the fence panel, but that maintenance is far easier and less costly than cleaning the culvert. Both regions have had several years of success.

Several other respondents reported trying this method with little success. Beavers plugged the pipe with mud within days or, in one case, beavers used sticks to plug individual holes in the perforated pipe.

Clemson Beaver Pond Levelers

The Clemson Beaver Pond Leveler (figure 13) was designed to keep beavers from detecting where water is escaping by eliminating the sound of rushing water or by moving the sound downstream from the dam site. The leveler's intake device and reducer sleeve slow water movement Devices through the system (figure 14). The intake device is constructed from 10 feet of 10-inch-diameter schedule 40 polyvinyl chloride (PVC) pipe that has been perforated with 160 evenly dispersed 2- to 2 ½-inch holes. The upstream end is capped, while the downstream end is fitted with a reducer sleeve that connects to an 8-inch PVC pipe. Ten times as much water can flow through the perforations as can flow through the 8-inch outlet pipe. The reduced rate of flow reduces the noise as water enters.

Drawing of a Clemson Beaver Pond Leveler.
Figure 13—This photo shows a Clemson Beaver Pond Leveler
with the water intake pipe enclosed in wire mesh and a riser
to control the water level.—Drawing by Dr. Jeanne Jones

Photo of the Clemson Beaver Pond Leveler.
Figure 14—The Clemson Beaver Pond Leveler uses a
perforated PVC pipe for water intake and a reducer to limit
waterflow through the output pipe.

The intake pipe is suspended inside a 30-inch-diameter cylinder formed from rolled sheets of 2- by 4-inch galvanized welded wire mesh. The wire cylinder is closed at the capped end of the intake pipe and fitted around the reducer sleeve with wire mesh. The cylinder can be held in place with wire attached to four sides of the pipe and extended out to the cylinder. An alternative but more expensive approach is to insert 3⁄8- by 30-inch threaded rods through the intake pipe, locking them in place with washers and nuts. Additional washers and nuts placed on either side of the cylinder hold it in place. Two rods adjacent but perpendicular to each other at both ends of the intake device and about every 5 feet along it stabilize the cylinder. The wire cylinder prevents beavers from getting close enough to pile debris on to the perforations. A couple of metal posts, one on either side of the cylinder, are generally adequate to hold the device in place.

Installing the leveler is fairly straightforward. First, open one trough through the dam for each leveler to be installed. The appropriate number of levelers would be the number needed to drain water out of the pond at a rate equal to or faster than the rate at which water flows into the pond. If road culverts are plugged, all debris should be removed from them before the leveler is installed. If beavers return and plug the culvert, waterflow will be restricted to the amount of water that can pass through an 8-inch pipe.

The perforated intake pipe is installed upstream of the dam and oriented parallel to the stream channel, if possible. The intake pipe should be installed so that it will submerged to the greatest extent possible when the pond is at its lowest level. The outflow end of the intake device needs to be slightly lower than the capped end and should be no closer than 5 to 10 feet from the dam or culvert. Additional 8-inch pipe can be added to move the intake pipe farther away from the dam, if necessary. An elbow can be inserted in the outlet pipe if the dam or culvert was constructed at a bend in the stream.

The outlet pipe is laid through the trough in the dam so that it slants downward slightly from the reducer sleeve. The outlet end should extend at least 20 feet beyond the dam or completely through a culvert. It is best if the end of the outlet is underwater.

A T-joint at the terminal end of the outlet pipe allows the water level of the pond to be manipulated. The T-joint is positioned so that a standpipe on one side of the T (the riser) can point upward. A plug is inserted in the bottom side of the T. When the bottom of the T-joint is open, the pond will drain. When the bottom of the T-joint is plugged, water will rise to the height of the riser. An alternative is to use an elbow with a standpipe. When the standpipe is laid down, the pond will drain. When the standpipe is turned upright, the pond will drain to the height of the standpipe. The standpipe can be wired to a metal post to keep it in place.

Forest Service Experience—The Clemson Beaver Pond Leveler has been used successfully in the Southern and Eastern Regions on the Superior, Wayne, Francis Marion and Sumter, and Nantahala National Forests, among others.