Forest & Grassland Health

Flooding & Beaver Damage

flooding is caused by beaver (Castor canadensis) damming activity or other stream channel disruption, or abnormally long periods of precipitaion precipitation or snowmelt combined with impermeable bedrock or saturated soils

Primary host(s) in Alaska:

all tree species in Alaska are affected by flooding; beavers tend to prefer hardwoods, but also damage conifers

Damage: abnormally high water can stress or kill trees by depriving tree roots of oxygen needed for cellular respiration; beaver feeding damage to bark and cambium wounds or girdles trees, and mortality occurs when stems are girdled or broken (beavers also feed on leaves, buds and roots)


Click on the image for a larger version.

Beaver damage to black cottonwood near Juneau.

Beaver feeding damage to black cottonwood near Juneau.

Significant beaver damage to shore pine.

Significant beaver damage to shore pine.

Beaver teeth marks and damage to Sitka spruce branch.

Beaver teeth marks and damage to Sitka spruce branch.

Beaver damage to yellow-cedar tree bole.

Beaver damage to yellow-cedar tree bole.

Beaver damage to Sitka spruce tree bole.

Beaver damage to Sitka spruce tree bole.



Shore pines killed by flooding damage resulting from beaver activity on Prince of Wales Island.

Shore pines killed by flooding damage resulting from beaver activity on Prince of Wales Island.

Flood Damage along the Tanana River west of Fairbanks, where significant flooding occurred in 2014.

Flood damage along the Tanana River west of Fairbanks in 2014.

Discolored tree crown and flooding on Prince of Wales Island.

Discolored tree crown and flooding on Prince of Wales Island.




Current Status in Alaska (2018 Update)

In 2018, 3,700 acres of flooding damage were mapped, similar to recent years but down considerably from the marked flooding that occurred in the Interior in 2014-2015. Flooding damage was widely scattered throughout the state (840 acres in Southeast Alaska, 500 acres in western Alaska and 2,300 acres in Interior Alaska). The largest area of damage (760 acres) was mapped near Tanana south of Tanana Island, with several flooded areas 200-300 acres in size mapped along the Yukon River near its confluence with the Chandalar River. Flooding damage is usually attributed to beaver dams and occasionally landslides, high precipitation, or snowmelt.

Symptoms, Biology & Impacts

The most obvious signs and symptoms of flooding include dead and dying trees with discolored and thin foliage and water around the bases of trees. Tree species vary in their tolerance to flooding; studies have drawn contradictory conclusions about species’ flood tolerance depending on research methods. The degree of damage that a tree sustains depends on the specific flooding situation (severity, season, duration), characteristics of the tree (species, height, crown class, age, vigor, root maintenance and activity), soils (aeration, pH, organic content), and environmental interactions with individual tree physiology. In general, white spruce is considered far less tolerant to flooding than larch. After just ten days of inundation, significant damage or even mortality can occur in flood intolerant tree species.

Beavers considerably alter riparian forests and waterways. Trees are killed directly for food and for use in dam construction and can also be killed indirectly by rising water tables and riverbank destabilization. Although there are negative impacts to individual riparian trees, stands, and understory vegetation, there are also many ecological benefits to beaver activity. Nutrients, sediment and organic materials are trapped in beaver ponds, filtering waters downstream and recharging underground aquifers. Beaver activity may help to stabilize disturbed riparian systems, improving habitat for fish, waterfowl, amphibians and other organisms. Beavers are distributed throughout most of forested Alaska.

Historic Activity

Significant flooding activity in Alaska last occurred in 2014, when 12,125 acres of flooding damage were reported. Much of this damage was located along the edges of rivers, sloughs, and lakes with no signs of impoundment; therefore, beaver activity is thought to have contributed to a relatively small proportion of the damage. Approximately 10,000 acres of flood related mortality were mapped in the Interior, consistent with record summer rainfall. For example, Fairbanks received 260% of normal rainfall in June, and 268% of normal rainfall in July, making it the wettest summer on record (according to the Alaska Climate Research Center). In late June 2014, more than three inches of rain in two days led to flooding and the evacuation of dozens of visitors to Denali State Park and Preserve. White spruce and paper birch were the primary tree species affected.

Some landforms are particularly prone to seasonal flooding, such as low-lying flatlands recently uplifted as a result of glacial rebound, such as the Yakutat Forelands. Other landscapes and ecosystem types, such as peatland bogs and poor fens, are wet most of the year, favoring tree species that are able to tolerate wet site conditions.


Beaver management can be complex. It can focus on the protection of individual or groups of high-value riparian trees using wire, metal sheeting, specialized paint or other materials on tree boles to discourage feeding. Beaver dams may block road culverts or other drainage structures, significantly contributing to infrastructure maintenance costs when dams must be repeatedly removed. Fences and barriers may be installed around culverts, drains, and structures and trees to keep beavers away. In some cases of extreme beaver damage to infrastructure or property, land managers may attempt to relocate animals if the practice is permitted.

Survey Method

Tree mortality from flooding is monitored through annual aerial detection survey, but information about the specific cause (e.g., beaver dams, abnormally high precipitation, etc.) is not recorded because of difficulty distinguishing the cause from the air.


Flooding damage to trees occurs throughout forested Alaska. 

Links to Resource & Publications

Living with Beavers, Alaska Dept. of Fish & Game webpage.

Beaver Damage Management. USDA Animal and Plant Health Inspection Service webpage.

Beavers. Internet Center for Wildlife Damage Management- Cornell University webpage.


Content prepared by Robin Mulvey, Forest Pathologist, Forest Health Protection,

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