This technical memorandum presents a plan for the distribution and abundance of beaver (Castor canadensis) in reaches of the lower Owens River. Beaver are the river’s keystone species in that their dam building and use of willows affect the riparian habitat and flow system more than any other wildlife species. How beavers influence flow and riparian habitat also influences how other species of fish and wildlife use the river. Beaver alteration of river flow and riparian habitat must be an important component of present riverine-riparian ecosystem management and future adaptive management decisions.
Beaver is the largest rodent in North America, weighing up to 75 pounds. Beaver are highly specialized obligate riparian/aquatic rodents found in ponds, lakes, rivers and streams. Beavers are generalized herbivores: they consume a wide variety of plants, many types of plants (aquatics, forbs, grasses shrubs, trees), many parts of plants including, leaves, bark, twigs, rhizomes and flowers. While, beavers eat variety of foods they prefer, and are most dependent on woody riparian species such as aspen (Populus tremuloides), willow (Salix spp.), cottonwood (Populus spp.) and many other tree and shrub species1.
Beaver are native to some regions of California west of the Sierra Nevada, but are not native east of the Sierra crest. Beavers were introduced to many locations in during the 1930's and 40's as part of the Federal Aid in Wildlife Restoration Project California. Beaver in the Owens Valley probably belong to the Idaho subspecies, Castor canadensis taylori, Davis.
Beaver provide a striking example of how animals influence ecosystem structure and dynamics in a hierarchical fashion. Initially beaver modify stream morphology and hydrology by cutting wood and building dams. These activities retain sediment and organic matter in the channel, create and maintain wetlands, modify nutrient cycling and decomposition dynamics, modify the structure and dynamics of the riparian zone, influence the character of water and materials transported downstream, and ultimately influence plant and animal community composition and diversity2.
Beaver can influence wildlife, water quantity, water quality, fish habitat and fish populations, esthetics, and recreational opportunities, and the relationship of cattle grazing to riparian and streambank condition. Beaver influences interact with other land use and resource management practices along the lower Owens River. These interactions have had, and will continue to have a major role in the lower Owens River ecosystem. Consequently, beaver abundance and their distribution throughout the lower Owens River must be a key consideration with all ecosystem management plans.
Beaver ponds and associated flooding and high water tables create habitat diversity, edge effect, and vegetative changes that attract wildlife species that are not often found in non-beaver areas. Waterfowl, shorebirds, and songbirds that feed over open water are commonly attracted to flooded areas3. The higher water tables often create vegetative response that provide cover, forage, or edge effects that are attractive to a variety of wildlife. In healthy riparian ecosystems, beaver ponds generally provide unique and valuable habitat for many species of wildlife. Increased structural complexity and high interspersion of unique plant communities and habitat features are important factors influencing wildlife species presence and abundance. High breeding bird density, bird species richness and diversity, and total breeding bird biomass are typically associated with beaver ponds. Perhaps the most noticeable wildlife are the large ungulates. Elk and deer are commonly found associated with beaver influenced habitats in greater density than areas without beaver4.
The reduction of stream velocity as the Owens River runs through beaver ponds results in considerable amounts of sediments deposited in the ponds, depending upon the age and structure of the pond. Reductions in stream sediment of up to 90 percent have been documented5. Downstream resources or biological cycles that are adversely influenced by sediment often benefit from this reduction. On the other hand, sediment-dependent processes like bank building can be adversely affected by reduced sediment flow.
Beaver ponds in the lower Owens can also modify water temperatures. The ponding action tends to increase stream temperatures in the summer and reduce temperatures in the winter6. Fish populations existing in marginal water temperatures could be adversely affected. Beaver also have the potential to increase the level of pathogens downstream from their activities, resulting from beaver excrement. Beaver are one of many warm blooded mammals capable of transmitting the flagellated protozoan, Giradia lamblia. Unless the water is used in municipal supply systems, giradia transmission by beavers is generally of little concern.
The normal variability in seasonal stream flows is commonly reduced through the reservoir effect of bank storage adjacent to the pond. The amount of this storage capability is often quite low relative to the total amount of spring runoff flows; however, as this bank storage is released through the late summer, the additional amount of water may be important.
Beaver can have a dramatic effect on fisheries habitat, depending upon the natural channel size, characteristics, and endemic fish species. In flatter gradient streams like the lower Owens, beaver ponding covers streambed gravels, reduces habitat diversity, inhibits or blocks fish migration, and reduces fish spawning habitat for trout, but increases it for bass7. On the other hand beaver ponds often provide critical rearing habitat in steep gradient streams or in streams which cannot support much riparian habitat so that reductions in spawning success may be offset by increases in rearing space8.
Beaver have influenced the vegetative state of riparian and adjacent lands in the wetted reaches of the lower Owens River by the construction of ponds and the cutting of deciduous overstory as food supplies. Vegetation adjacent to ponds is often composed of more mesic types than those without ponds, due to the higher water tables and moister soils associated with ponding. The backwater effect of beaver dams can also promote the growth of tules.
This beaver activity can influence the distribution of livestock and the response of vegetation to livestock use. Ponds can create partial or complete livestock barriers to individual plants adjacent to water. Some plant species, such as deciduous shrubs, are vulnerable to severe damage or elimination by overgrazing. In such cases, a plant species such as willow continues to exist in a plant community subject to heavy grazing because of beaver-created isolation from livestock caused by flooding. If beaver are removed from the stream and the ponds recede, these plants become more vulnerable once again and can rapidly be eliminated from the plant community.
Beaver in some areas of the lower Owens have created elevated meadows through the deposition of sediments over several generations of pond maintenance that perched the meadow higher than the geologic knick point elevation. Streambanks formed through old beaver activity are generally low in rock content and are sensitive to mechanical damage. Heavy grazing can result in streambank collapse, accelerated meandering, enlarged channels, and losses of stream dependent resources. As a consequence, the loss of beaver in these circumstances can lead to the lowering of water tables and can ultimately lead to the change from a mesic meadow type to a more xeric plant community, thus resulting in reduced forage production9. In contrast, the amount of forage flooded by beaver ponds is more than compensated for in many cases by the increased production of adjacent lands resulting from higher water tables.
Water is often used to enhance the esthetic quality of a scenic area. The presence of small streams is often noticed only when a stream is in the foreground of a viewer; however, the larger expanse of water created by beaver ponds is noticeable from a greater distance, as is the vegetative diversity that is usually associated with the beaver pond. The variety that a beaver pond can create in an otherwise stream-oriented environment offers visual diversity.
The disproportionate numbers and diversity of wildlife associated with beaver ponds is attractive to recreationists, in part, because wildlife is typically more viewable in the semi-open setting. Beaver and sign of their activity is one of the more viewable forms of wildlife that can add measurably to a recreation experience. Beaver ponds create fishing opportunities because large open water bodies are easier to fish than adjacent streams reaches. This is particularly true in the lower Owens River where the density of future riparian vegetation may limit fisherman access to some river reaches. Anglers with small children find that beaver ponds are relatively safer for youngsters than a flowing stream.
Present condition of the lower Owens River ecosystem is, in part, a result of beaver impacts. Other interacting influences on environmental conditions include, water management and land use practices, domestic livestock grazing and grazing other large ungulates. One of the primary conditions of concern on the lower River is the widespread paucity of woody riparian vegetation recruitment. Survival and persistence of the numerous sprouts produced each year by willow also appears very low. Furthermore, many of the established shrubs and trees are showing signs of stress and are dying. These conditions are particularly evident in the reaches from the Islands to the Delta (reaches 4-7).
Contrary to some popular beliefs, beaver are not necessary prudent consumers. Beaver known over exploit their food resources causing habitat degradation and starvation of the colony. While we understand that beaver are one of many influences of habitat conditions in the lower Owens River ecosystem, we expect that given the relatively poor habitat conditions, beaver might suppress or inhibit the pace and magnitude of recovery. In a healthier state, the river system will be more resilient and beaver management will be carried out on a maintenance basis.
The lower Owens River beaver play both a positive and negative role in the ecosystem depending upon site conditions. Beaver activity occurs throughout the wetted portion of the river (from about Mazourka Canyon Road to the Delta), but is most pronounced in specific areas. The first major beaver dam downstream of Mazourka Canyon occurs at the Locust Spillgate. There are several older or smaller dams upstream of Locust (at Billy Lake for example) but these dams have less effect on the in-river ecosystem.
The Locust dam creates a significant backwater effect which promotes substantial tule growth in the river bed. The dam also provides important bass, bluegill, and other fish species rearing habitat. However, due to the high degree of depostion in the pond and backwater area, fish spawning habitat is very limited. Tree willows have grown around the pond margins as well as mesic plant species in the understory. While a definitive survey of beaver numbers has not been performed throughout the river, the Locust dam seems to support five or six lodges.
The next large beaver dam downstream of Locust occurs just below Georges Spillgate. Like Locust, the Georges beaver dam creates a large tule bed upstream in backwater areas but provides substantial fish habitat. An estimated 7 or 8 lodges are associated with this dam but several smaller dams occur upstream.
The island reach (Alabama Gates area) supports a substantial number of small beaver dams. Because of flow variation and shifting of stream discharge to different channels from time to time, larger and old beaver dams have not been developed. Consequently, beaver ponds in this area provide little fish habitat but do contribute to the development of mesic meadows and elevated meadows. It is unknown how many beaver lodges occur in this area.
Another substantial beaver pond occurs upsteam of the Lone Pine Pond area. This dam creates substantial fish habitat of deep water and heavy riparian habitat upstream for nearly a mile. The dam also creates a large tule bed upstream to the island reach. Another large beaver dam occurs downstream of the Lone Pine Pond and contributes to the formation of the backwater that makes up the Lone Pine Pond. Again, significant fish habitat is associated with beaver ponds throughout this reach as well as tule beds.
Although beaver activity has resulted in the removal of much willow and other shrub and woody vegetation and the dams create favorable tule conditions and reduce fish spawning habitat, they also provide important fish rearing habitat, mesic meadows, and promote the growth of other riparian species. It is most likely that the physical removal of beaver dams will result in more adverse environmental impacts than environmental benefits.
It is our conclusion that beaver dams should be left as they are, allow the natural forces associated with future out-of-channel and base flows to remove or incorporate them into the riverine ecosystem, but focus on controlling the number of beaver by reach through trapping as the management strategy.
The goal of beaver management in the lower Owens River is to protect the development and sustainability of riparian vegetation particularly willow and other shrub species. Thus management must be a function of the allowable number of beavers per acre of willow by river reach and vegetative condition. Beaver numbers based on colonies/km does not relate directly to available willow habitat and assumes that the number of animals per colony does not vary greatly throughout an area. In general, the number of beaver in a colony remains relatively constant in small areas of river; however, the lower Owens covers over 60 miles of river and past studies have shown than the number of animals per colony varies substantially over entire river lengths10. In time, with consistent flows, the riverine-riparian will become more homogenous with higher surface water elevation and discharge and some beaver may turn to bank dwelling rather than dam building.
Based on past experience in beaver control and riparian habitat development (W.S. Platts personnel communication), we suggest an allowable density of 1 beaver/29 acres of available willow habitat during the early seral stages of willow development. A final allowable density of 1 beaver/8 acres of available willow habitat may be possible when willow habitat reaches good condition. Table 1 shows the allowable abundance and distribution of beaver by river reach for each condition of willow development. Maintaining these densities of beaver by river reach and riparian condition will be achieved through trapping under the direction of the CDFG.
Beaver management is the responsibility of the California Department of Fish and Game. Monitoring of beaver activity and numbers will be an integral part of the longterm monitoring program and adaptive management. Numbers of beaver per river reach will be determined by fall cache counts as needed to guide adaptive management.
Table 1 -- Allowable beaver densities by river reach based on predicted
available willow habitat and condition.
| REACH | DESCRIPTION | LENGTH (miles) |
RIPARIAN WIDTH (feet) |
EXISTING BEAVER INFLUENCE |
PREDICTED WILLOW HABITAT (acres) |
ALLOWABLE # OF BEAVERS POOR WILLOW CONDITION (1beaver/29 acres of willow) |
ALLOWABLE # OF BEAVERS GOOD WILLOW CONDITION (1 beaver/8 acres of willow) |
1 |
Intake to 1 mile above Blackrock Ditch | 4 |
119 |
none | 26.8 |
1 |
3 |
2 |
Above Blackrock Ditch to Five Culverts | 14 |
65 |
none | 88.4 |
3 |
11 |
3 |
Five Culverts to Alabama Spill Gates | 18 |
153 |
heavy | 161 |
6 |
20 |
4 |
Alabama Spill Gates to Below "Islands" | 6 |
-- |
heavy | 50.1 |
1 |
5 |
5 |
Below "Islands" to Lone Pine Ponds | 10 |
163 |
moderate | 41.3 |
1 |
5 |
6 |
Lone Pine Ponds to Keeler Road | 4 |
107 |
heavy | 4.7 |
0 |
0 |
7 |
Keeler Road to Delta | 4 |
98 |
light | 11.5 |
0 |
1 |
TOTAL |
60 |
383.8 |
12 |
45 |