Flooding & Beaver Damage

flooding is caused by beaver (Castor canadensis Kuhl) damming activity, 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)

Current Status in Alaska (2023 Update)

The Alaska Center for Climate Assessment and Policy reported elevated spring precipitation and total snow statewide in 2023 (19 and 25 percent above average, respectively). Despite these wet conditions, under 1,000 acres of flooding-related tree mortality were mapped statewide in 2023, comparable to 2022. In the Interior, we observed 650 acres of flooding, half of which were mapped near the Tanana River between Fairbanks and Tok. Last year, extensive areas of standing water along the Tanana River had not yet caused discernable symptoms. Elsewhere in Alaska, 150 acres of flooding were mapped across Southeast Alaska and 80 acres were mapped in Southcentral Alaska.

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

Nearly 14,000 acres of flooding were mapped in 2021, mostly in the Tanana and Yukon Flats, and along many rivers and low-lying areas in Interior Alaska. Flooded patches were typically about 100 acres, but the largest flooded area consisted of over 7,000 acres of low severity damage in a low flood plain along the Tanana River between Nenana and Manley Hot Springs. Personal communications with pilots in the area suggest that many areas were also flooded in 2020. Prior to 2021, significant flooding activity in Alaska last occurred in 2014, when more than 12,000 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.

Management

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.

Distribution 

Flooding damage to trees occurs throughout forested Alaska.