Dam Owner’s Guide 

To Animal Impacts 

On Earthen Dams

FEMA L-264 / September 2005



Twenty-five states across the U.S. can write headlines 

of unsafe dam operations caused by nuisance wildlife 

intrusions, and many dam owners find the struggle 

to adequately manage nuisance wildlife at their dams 

a never-ending story. The tasks of proper and timely 

wildlife damage observation, species identification 

and management, and dam repair often prove to 

be daunting responsibilities that sometimes go 

unchecked, despite the good intentions of dam 

owners. The damages caused by nuisance wildlife 

often appear minor and small burrows, shallow dens, 

and limited erosion may not trigger a dam owner’s 

concern. However, the news in many states is that 

“minor” damages such as these are often at the core 

of unsafe dam operations or outright dam failure.


The dam owner is the first line of defense against 

unsafe dam operations. As such, the Federal Emergency 

Management Agency (FEMA) published this brochure 

to help the dam owner manage nuisance wildlife and 

wildlife damages at earthen dams. To reduce the risk of 

dam failure due to wildlife intrusions, this brochure 

provides information on nuisance wildlife damages, 

wildlife observation during routine inspections, 

wildlife identification, and basic damage repair. Most 

simply, this brochure provides dam owners with a 

process for observing and managing wildlife intrusion 

damages. Regardless of which species or damages 

are encountered at the dam, coordination with the 

State Dam Safety Official and State Wildlife Manager 

is recommended to ensure that appropriate and 

lawful dam repair and wildlife management occurs.

Small Animals 

Can Cause Big 

Problems 

Earthen embankment dams are used by private 

landowners and State and Federal agencies to 

store farm water supplies, city water supplies, 

recreational waters, flood waters, and wastewater 

lagoons. Earthen dams rely on a thick placement of 

compacted soils to withstand the water pressure of 

the pool contained behind the embankment. Often 

constructed outside of developed areas, the earthen 

dam environment is usually near a water source 

and can contain a variety of vegetation; given these 

characteristics, earthen dam environments can be 

naturally conducive to use by wildlife. Through their 


Wildlife species that 

routinely damage earthen 

dams include: Muskrat, 

Beaver, Mountain Beaver, 

Woodchuck, Pocket 

Gopher, North American 

Badger, Nutria, Prairie 

Dog, Ground Squirrel, 

Armadillo, Livestock (cow, 

sheep, horse, pig and 

wild pig), Crayfish, Coyote, 

Moles and Voles, River 

Otter, Gopher Tortoise, 

Red Fox and Gray Fox, 

Canada Goose, American 

Alligator, and Ants.

Wildlife interacts with the dam environment as if it were 

natural habitat. However, pocket gopher burrows and beaver 

dams can lead to disaster in the earthen dam environment.

natural desire to create dens, search for food, or 

escape predators, wildlife burrow, graze, root, and 

traverse the embankment as if it were natural field 

or forest. These activities cause a host of damages to 

an earthen dam and can even lead to dam failure.

The first step in fortifying a dam against unsafe 

operations caused by wildlife damage is to 

understand what could go wrong if wildlife 


damage is left unchecked. While a dam owner may 

observe a few small burrows on the upstream and 

downstream slopes, it is important to understand 

that potential problems, like those burrows, often 

run deep below the surface. In general, there are 

three main serious effects that wildlife activities 

can have on earthen dams: hydraulic alteration, 

structural integrity losses, and surface erosion.

Hydraulic Alteration

This is the most significant and often least 

obvious impact of wildlife intrusions. Burrows 

on the upstream and downstream slopes can 

dramatically alter how a dam controls the water 

pooled behind the dam. Dramatic changes to the 

designed hydraulic function of a dam include:

burrow

theoretical

phreatic surface

altered phreatic surface

process continued 

over time

Burrows can lead to piping within an embankment. 

Burrows can alter dam hydraulics by shortening seepage paths.


Shortened seepage paths; 

Increased seepage volumes; 

Increased probability of slope failure; and 

Internal erosion of embankment materials 

(piping), which is a progressive condition 

that can rapidly lead to failure of the dam. 

In addition, beaver mounds may block principal and 

emergency spillways and riser outlets, resulting in: 

Increased normal pool levels and reduced 

spillway discharge capacity;

Sudden high discharges from the 

dam if the beaver dam fails; 

Clogged water control structures as debris from 

an upstream beaver dam floats downstream; and 

Erosion of the downstream toe as a result of 

elevated tailwater caused by beaver activity.

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groundhog den

soil movement

into burrow


Structural Integrity Losses 

Many species excavate dens and burrows within 

embankment dams, causing large voids that weaken 

the structural integrity of the dam. Typical voids can 

range from the size of a bowling ball to a beach ball 

and much larger, and can cause many adverse effects: 

Localized burrow collapse can occur due 

to heavy rain and snow melt, or heavy 

equipment or vehicle use on the crest. 

Collapsed burrows can progressively 

lead to sinkholes or depressions on 

the embankment surface. 

Collapsed crest soils can result in a 

loss of freeboard, thus endangering 

the dam during storm events. 

Massive slope instability can result from 

collapsed burrows and soil losses. 

Surface Erosion 

Wildlife that graze or traverse areas of open 

vegetation associated with dam embankments 

can cause a widespread loss of vegetative cover. 

This increased feeding and traffic pressure on 

the dam’s vegetative groundcover can lead to: 

Erosion paths; 

Decreased soil retention on the 

dam’s crest and slope; 

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collapse

theoretical seepage line


Increased rates of soil erosion because of the lack of 

stabilizing vegetation from grazing and trafficking;

Irregular surface erosion and the 

formation of rills and gullies; and 

Reduction in freeboard and loss of cross 

section, and in turn, an increase in the 

dam’s vulnerability to damage from high 

water during large storm events.

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Livestock can remove stabilizing vegetation through grazing 

and hoof traffic.

Burrows can collapse, leading to formation of sinkholes and loss of structural integrity.
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Routine 

Inspection 

With A Twist

Upland Areas. Many species live in the upland 

areas, away from the water. Even the downstream 

slope, abutments, and groin areas of the dam 

can be considered upland in terms of habitat. 

Forest Fringe. The zone between two 

environments (the edge) is the best place to 

observe those species living at and around 

the dam. The more habitat types at the 

dam, the greater number of species likely 

to inhabit the dam. Mountain beaver or 

armadillo prefer forested/wooded areas.

Emergency Spillway. Beaver often dam the 

spillway, causing the pond water levels to rise. 

Left Abutment Contact.

Inappropriate Vegetation on Embankment. 

Many dams contain vegetation other 

than mowed grass. Improper vegetation 

provides cover and food supply, which 

encourage animals to inhabit the dam. 

Downstream Slope. This area is often the 

location where groundhogs, coyote, and 

fox excavate burrows. Canada geese will 

feed on the downstream slope, which could 

cause loss of protective vegetative cover 

and associated erosion. Species that prefer 

upland areas could be found in this area.

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6.

The second step toward fortifying a dam against 

the effects of nuisance wildlife damage is to 

inspect the dam from both engineering and 

biological perspectives. While the dam 

inspection is focused primarily on 

seepage, deformation, and structural 

deficiencies, dam owners should 

supplement their regular dam 

inspection (as required by dam 

safety) with wildlife behavior, 

hunting, and burrow location 

preferences so that routine 

inspections can be representative 

of the full range of potential 

performance problems. 
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Left Groin.

Discharge Conduit and Outlet Channel. 

Beaver can dam the outlet structure. Aquatic 

species may inhabit this area depending on 

water flow and availability of vegetation.

Toe of Embankment and Right Groin.

Erosion Pathways on the Embankment.

Livestock traverse the embankment 

creating erosion pathways. 

Right Abutment Contact.

Crest. Livestock traverse the crest, which 

creates ruts. The ceilings of beaver and 

muskrat burrows in the upstream slope 

are often just below the dam crest. 

Aquatic Fringe. The zone where the bank meets 

the pond usually contains aquatic vegetation 

preferred by many animals such as nutria.

Upstream Slope. Beaver, muskrat, and nutria 

prefer the upstream slope for burrow excavation. 

Alligators, otters, and turtles usually live in the 

shallow waters near the upstream slope. 

Principal Spillway (with riser and trash 

rack). Beavers can block principal 

spillways by constructing dams.

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Given the dynamic nature of wildlife and its desire 

to avoid human interaction, a dam owner will 

seldom witness wildlife causing damage to dams, 

and as such, accurate wildlife identification may 

be difficult. Misidentification of a wildlife species 

may result in inadequate mitigation, which could 

allow damage to continue, and perhaps lead to dam 

failure. As a general management approach, dam 

owners should become familiar with wildlife at 

their dam and the damage the wildlife can cause 

so that they can identify and mitigate wildlife 

damages before dam operations are compromised.
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Wildlife 

Identification And 

Damage Repair

Once dam owners 

identify wildlife damages 

and narrow down 

which species may be 

responsible, they should 

coordinate with their state 

dam safety representative 

(www.damsafety.org) 

and their state wildlife 

agency (http://offices.

fws.gov/statelinks.html). 

At this point, precise 

wildlife identification 

and assessment and 

repair of the damage 

must occur to protect 

dam operations and 

prevent further damage. 

Complex repairs and 

preventive action design 

requires the services of a 

professional engineer and 

should not be attempted 

by the dam owner
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Dam owners should seek positive wildlife 

identification either through their state wildlife agency 

contact or though a professional trapper (www.

nationaltrappers.com). A dam owner who uses A Dam 

Owner’s Guide to Animal Intrusions (FEMA 2004) is likely to 

positively identify the responsible species depending 

on the field clues available. However, professional input 

will provide the most accurate species identification 

results, and will be required should it be necessary 

to remove the species from the dam environment. 

Appropriate repairs to the dam would be made 

once a dam owner identifies the damage and the 

species responsible. In some cases, basic repairs 

can be conducted by the dam owner following 

coordination with the dam safety representative. 

However, complex repairs and preventive action 

design (such as installing rip-rap on the upstream 

slope) requires the input of a professional engineer 

and should not be attempted by the dam owner.
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Monitoring 

The next step to 

maintaining safe dam 

operation is to monitor 

the effectiveness of the 

repair or preventive 

action. In many 

cases, regular dam 

inspections and swift 

damage mitigation will 

adequately preserve 

safe dam operations. 

However, it is possible 

for a dam to become 

overrun by a nuisance 

species, or for several 

species to cumulatively 

compromise safe dam 

operation. In these 

Frequent owner inspection 

after animal damage repair 

can ascertain the extent of 

wildlife activity and guide 

future management actions.
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cases, repair actions are only partial solutions. 

Monitoring can help the dam owner determine 

whether wildlife mitigation is necessary. 

In general, it is recommended that the dam owner 

inspect the dam once every three months after 

first finding and repairing animal damage. At this 

frequency, the dam owner will be able to confirm 

that the animal has not returned to the dam once the 

damage has been repaired. As a guideline, if the dam 

owner finds new animal damage, such as burrows, 

in the dam on two consecutive inspections following 

repair and preventive actions, then implementing 

a wildlife control strategy is probably necessary 

to maintain safe dam operation. Coordination 

with the state wildlife agency is required prior to 

removing wildlife from the dam environment. 
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Wildlife Mitigation Measures

Wildlife mitigation measures typically include 

habitat modification, trapping, fumigants, toxicants, 

frightening, repellants, or shooting, used singularly 

or in combination. Wildlife mitigation in the 

dam environment can be beneficial and at times 

necessary to protect human populations from the 

disastrous effects of dam failure. However, applied 

indiscriminately, mitigation methods can adversely 

affect the dam environment, protected wildlife 

species, and even human populations. For this 

reason, nuisance wildlife management practices 

should be implemented only with coordination and 

input from appropriate State and Federal agencies 

and the county agent responsible for toxicant and 

fumigant registration and application. Coordination 

with these agencies will allow the dam owner to 

determine the most appropriate wildlife mitigation 

method, and implement the method in compliance 

with applicable State and Federal wildlife and water 

resource protection laws, such as the Endangered 

Species Act, the Clean Water Act, and the Federal 

Insecticide, Fungicide and Rodenticide Act.

Protect water resources 

and special status 

wildlife by coordinating 

with appropriate State 

and Federal agencies 

before initiating 

wildlife mitigation.
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Conclusion 

Routine dam inspection and management that is 

conducted with wildlife behavior, hunting, and 

burrow location preferences in mind allow for 

comprehensive dam management. A dam owner 

who is knowledgeable of the full range of potential 

performance problems—those stemming from 

wildlife behavior as well as engineering function—is 

the best line of defense to prevent unsafe dam 

operations or outright failures. FEMA hopes this 

brochure helps dam owners nationwide identify and 

mitigate wildlife damages before adverse effects occur. 
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For information on obtaining a print, CD, or 

online version of this publication or the technical 

manual go to www.fema.gov/fima/damsafe.