Who else thinks trees should not be allowed on Earthen Embankment Dams?



Probable Causes and Possible Consequences

Recommended Actions

  • Trees and other brush obscure visual inspection of the dam and harbor rodents.
  • Large tree roots can create seepage paths through earthen dams.
  • Large trees can blow over during a storm and damage the dam, which may cause a breach.
  • Do not allow trees to grow on the dam. Remove trees and other brush annually. Treat the stumps of deciduous trees with herbicide to prevent regrowth.
  • Remove large trees and their roots and backfill holes with well compacted soil. Before removing the roots, the reservoir should be lowered to reduce the risk of serious seepage problems developing during removal.
  • If possible, remove trees at the toe of the dam to provide a15-foot buffer around the dam.

trees on both sides of a sidewalk path

Trees should not be planted or allowed to grow on dams. Tree root systems create pathways for water and can damage dams if they are uprooted.

Seepage through dam damaged by tree roots

Seepage through dam damaged by tree roots. Trees should not be allowed to grow on dams. (Photo source: ASDSO)


nygov-logoProblems with Trees and Brush Near Dams

Trees and brush may be aesthetically pleasing and provide other benefits; however, the growth of woody vegetation on and near dams, including the downstream toe area, can lead to serious problems. Sudden uprooting of trees by strong winds can result in the movement of a relatively large amount of embankment material and create large voids in the embankment. This in turn can lower the crest of the dam, reduce the effective width of the dam, lead to instability of the embankment, and facilitate seepage. Falling trees can also cause structural damage to concrete, steel, stone, or timber structures.

The root systems of trees can be a potential hazard by allowing seepage pathways to develop through a dam. Trees eventually die and their roots decay and rot. The root cavity leaves a void within the dam through which water can enter and flow. This can ultimately lead to failure of the dam by piping (internal erosion). In general, a tree’s root system may extend to the edge of the tree canopy or tree drip line.

Brush and woody vegetation prevent the proper visual inspection of the dam surfaces. The observation of sinkholes, slides, animal burrows, seeps, and other irregularities can be obscured by trees and brush. Woody vegetation can also cause excessive shade which in turn can hinder the growth of sturdy, dense grass coverage. These affected areas are more prone to surface erosion.

Excessive vegetation can provide habitat for burrowing animals, which can create problems on a dam.

Erosion Control

Grass cover is a very effective and inexpensive means to prevent the erosion of embankment surfaces. The stems and root systems of grasses tend to trap fine particles of soil, thus inhibiting the migration of these particles. A good grass cover provides an excellent means against erosion due to runoff caused by rains, and may protect the embankment during limited overtopping. Extensive testing and well documented incidents have consistently shown that a good grass cover is highly effective in preventing erosion at dams.


Grass cover should be routinely cut to provide a surface that can be easily inspected. In general, the grass on a dam should be cut at least twice a year. Trees and brush should never be allowed to grow on or very near to a dam including the downstream toe area! Many older dams have very large trees growing on or near them. Trees at or less than 4 inches in trunk diameter should be cut flush with the ground as part of the dam maintenance program and monitored during routine inspections for any changed conditions around the stump. For very small dams, trees less than 4″ diameter may compromise the dam when removed. Removal of brush, and trees and roots larger than a 4 inch trunk diameter should be done under the direction of a qualified professional engineer knowledgeable about dam safety and maintenance, and may require a permit from the Department. When in doubt, consult with a licensed professional engineer.

femaTechnical Manual for Dam Owners

This manual  presents a combined sixty-five years of research and practice in dam safety engineering associated with tree and woody vegetation growth impacts on earthen embankment dams. It is presented in a manner to be beneficial to the entire dam safety community, dam owners, dam operators, dam safety inspectors, dam safety regulators, dam safety engineers and consultants. who can utilize this Manual as a reference for recommendations for proper maintenance of desirable vegetation growth, control of undesirable vegetation growth, and remediation dam design associated with the removal and control of trees and woody vegetation growth on earthen dams. Dam safety regulators and dam safety inspectors can utilize this Manual as a guideline for the inspection of earthen dams relative to tree and vegetation growth dam safety issues and for the direction of dam owners and operators in the proper method and procedures for maintaining earthen dams without detrimental vegetative growth. Dam owners and operators can utilize this Manual to establish proper operation and maintenance programs to promote the growth of desirable vegetative growth on earthen dams and/or remove and control the undesirable tree and woody vegetation growth on earthen dams.
The last verse in the famous poem Trees by Joyce Kilmer is as follows:
“Poems are made by fools like me / But only God can make a tree.” Again, the author will
paraphrase this last verse, not to debase the great works of Joyce Kilmer, but to make a
distinct point.
“Only God can make a tree / But not removing trees from dams / Is done by fools like me.”


“Problems with Trees and Brush near Dams. While trees and brush may be aesthetically pleasing, give off oxygen,and provide cooling shade, the growth of woody vegetation on and near dams, including the downstream toe area, can lead to serious problems. Sudden uprooting of trees by strong winds can result in the displacement of a relatively large amount of embankment material. This in turn can lower the crest of the dam, reduce the effective width of the dam, and facilitate seepage. Falling trees can also cause structural damage to concrete, steel, stone, or timber structures…………………………”

Those Dam Trees Have To  Go!

Spruce and aspen trees growing on the dam on the east side of the Thomas Reservoir in Aspen. The pond-size reservoir is on the hill behind the Castle Ridge apartment complex and Aspen Valley Hospital.

new hampshire
The present DES policy regarding trees and other woody vegetation is to restrict them from all sections of the dam structure, from within 15 feet of both abutments, and from within 15 feet of the downstream toe. If adhered to, this policy will in most cases protect a dam from detrimental effects that these types of vegetation can cause.


Policy on Trees on Dams

The Massachusetts Office of Dam Safety requires that earth embankment dams be maintained free of the existence of trees and woody growth.
Out of 48 states, 29 document problem vegetation as the cause of dam failures or unsafe dam operations in their states. The U.S.Army Corps of Engineers, the National Park Service,and the Natural Resources Conservation Service document several similar cases
at the federal level. State dam safety officials and other experts agree that trees and woody plants on dams are a major concern, one which can cause substantial and costly damage if not properly dealt with. Tree roots do not stabilize soil masses, but instead loosen embankment soils and create seepage paths. Dense, low-growing ground cover or tall grasses are not desirable embankment plants, and can actually obscure serious dam problems.


Vegetation, erosion and dams

A dense cover of low-growing grassy vegetation is recommended because it will provide protection from surface erosion, but its root structure does not penetrate the embankment so deeply as to create a potential path for internal erosion.

Grass on dams requires regular mowing. Although woody vegetation such as trees and brush may protect against surface erosion, such growth can cause other, serious problems. These problems develop over years and may go undetected until it’s too late. In addition, trees or brush can hide an embankment surface, making inspection difficult.

Many times, dam failure is described as a sudden event when in fact conditions leading to the failure went undetected or ignored for years.


A highly detailed report on all the problems with earthen embankment dams.
Why is it recommended that trees not be allowed to grow on dams?
While trees on a dam may be aesthetically pleasing they are not good for the dam. As trees grow they tend to shade out grasses that prevent erosion. They also develop large root systems, and one of the leading causes of dam failure is “piping” along the root system of a tree that has died. As the roots rot, conduits are formed allowing the passage of water through the dam. Also, trees uprooted from high winds can cause the loss of a significant section of the dam.

Repair Examples-
Replacement of or work to spillway structures. Grading and excavation of slopes, spillways or dam crest. Removal of tree roots and stumps. Installation of drainage controls (toe drains).

Trees and Vegetation. Deficiencies: (See FEMA-534)
Woody brush and tree growth on dams are undesirable.
Vegetation obscures inspection, can harbor rodents, or can topple in high winds.
Excessive vegetation or debris in spillways.

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