Animal Life in the Yosemite
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GOLDEN BEAVER. Castor canadensis subauratus Taylor

Field characters.—Body stout and heavy; head blunt; tail flattened, paddle-like, scaly; hind feet webbed; pelage dense, with long over-hair and plush-like underfur. Head and body 24-1/2 to 31-3/4 inches (625-805 mm.), tail 11-1/2 to 16-1/2 inches (295-420 mm.), hind foot 7 to 8 inches (180-205 mm.), ear (from crown) 7/8 to 1-1/8 inches (23-28 mm.); weight 34 to 50 pounds (15.4-21.8 kilograms). General coloration rich golden brown; tail blackish. Workings: Dams composed of brush and mud, backing up water and forming ponds; 'houses' composed of twigs and brush, located at the edges of ponds; gnawings on saplings and tree trunks; holes or burrows about 15 inches in diameter in banks of rivers.

Occurrence.—Resident in some numbers along Merced River at Snelling and along Tuolumne River below Lagrange. Inhabits slow-moving streams and sloughs. Nocturnal. Somewhat colonial.

Under original conditions, before the advent of the white man, the lowland streams of central California were heavily populated by the Golden Beaver, a race of beaver peculiar to the Great Central Valley of California. Trapping, especially in the early part of the nineteenth century, reduced the beaver population almost to the point of extermination. Happily, legislation of more recent years has afforded the animals the fullest sort of legal protection and the prospects for their perpetuation are now bright.

One of the regions where beavers are still to be found, and in some numbers, is the extreme western end of the Yosemite section, along the lower courses of the Merced and Tuolumne rivers west of the foothills. In fact, in 1920, in the neighborhood of Snelling, permission was granted to certain persons by the State authorities for the trapping of a number of beavers. The animals had become numerous enough to cause some trouble in irrigation.

The beaver is essentially an aquatic animal. It is to be found only in places where there are considerable and permanent bodies of water together with vegetation suited to the food requirements of the animal. The water serves the beaver as a place of escape in time of danger, and as a highway for travel and for transportation of the pieces of trees which it uses in its many operations.

The food of the Golden Beaver consists chiefly of young bark of the willow and the cottonwood, the two commonest trees along the rivers. To get at this material, the animal cuts down trees or shrubby growths and cuts off the branches, of which it eats the bark of the terminal, newer parts. The peeled wood and other remaining materials are often used for building dams and houses. In 1915, near Snelling, beaver work was seen on cottonwoods up to 2 feet in diameter, though stems and boles of much smaller size are more often sought.

One particular cottonwood 10 inches in diameter was seen east of Snelling on which a beaver had been at work for some time. The tooth marks showed that the beaver had turned its head sideways when cutting. The chips lying on the ground below the notch in the tree were from 1 to 2 inches long, 1/8 to 1/2 inch wide, and 1/16 inch thick. Each consisted of two or more flakes, loosely joined, indicating that several bites had to be taken before a chip was entirely severed. The cutting on this tree was on the side away from the water, yet the tree when downed would have fallen into the river.

In the neighborhood of Snelling, both above and below the town, much work of the beaver can be found along the Merced River and in the ponds and sloughs which were formed there in past years when gold dredgers were operating. On the Tuolumne River, also, within 3 miles below Lagrange, the industry of the beavers is manifest. Most or all of the work of the beaver is done under the cover of darkness, so that the activities of the animal have to be inferred from evidence of a circumstantial nature; but such evidence is in this case unmistakable. It consists of trees upon which the animals are cutting or which they have cut down, pieces of wood from which the bark has been peeled, refuge or nest holes in the banks of sloughs, broad runways beneath the stream-side vegetation or up over dikes, dams across the larger sloughs, and, lastly, houses in or on the banks of the ponds formed by these dams.

On January 9, 1915, two beaver dams near Snelling were examined in detail by the senior author. The first dam was across a narrow and shallow slough between a rock pile left by a dredger on the one side and a river-cut bank on the other. The dam, though small, was perfect, and was curved, with the convex side downstream. (See pl. 37b). The bottom of the pond just above the dam had been deepened by digging out rocks and mud to contribute to the dam. The dimensions of the dam were as follows: length along curve 12-1/2 feet (3.8 meters); radius of arc about 10 feet (3 meters); thickness of base at middle, 31 inches (0.78 meters); total height on lower side 19-1/2 inches (0.5 meter); depth of water just inside dam, 15-1/4 inches (0.39 meter); rise in water level of pond due to dam, 12 inches (0.3 meter). The dam consisted of two types of sticks, dead drift, and freshly cut green willow, some with the bark gnawed off. The pieces used were up to 2 inches (50 mm.) in diameter. Some freshly peeled sections of young willow stems and twigs averaging a yard in length were used. There were also whole untrimmed tops of willows, just as cut off, on the adjacent margins of the slough within 100 yards of the place. The interstices of the dam were filled in with small peeled willow twigs, grass pulled up by the roots, roots of other vegetation, and rocks up to 6 inches (150 mm.) in diameter. Some of these rounded rocks had been placed on the rim of the dam, on top of everything else, as if to weight down the mass. The whole structure was notably level-topped and symmetrical in curvature. Although newly made and relatively small, it already served to raise the water level in the slough.

The second dam, a much larger affair, was built across the mouth of a slough where it emptied into the main Merced. This dam was built by the beavers on a foundation of large rocks which some boys had placed to deepen the water in a swimming hole above; but the curvature (or rather the two arcs of the curvature) did not, evidently, relate in any way to the foundation which had been put down by the boys. One end of this dam had been carried away by flood water in the slough. It had been about 40 feet (12 meters) long originally; about one-third had been washed away. The width at base was about 5 feet (1.5 meters) and the height 27 inches (0.68 meter). The material used was much the same as for the first one described.

At one place in the bank of a sluggish stream a beaver's refuge hole 1-1/2 feet in diameter was found. This opened beneath the level of the water in the slough and led into a tunnel in the adjacent bank of hard packed sand. The tunnel when opened up was found to be about 10 feet in length, and the upper inner end was 4 feet below the surface of the bank but above the water level in the slough. It contained no nest, nor was there any branching to the passageway. Beaver runways were seen on the bank and there were fresh chewings on the new shoots rising from the trunk of a prostrate willow near by.

In one place a large 'bed' was found on a bank heavily overgrown by willows and other plant growths along the adjacent stream. This bed was about 4 inches in thickness and 5 feet in diameter and was composed of bark which had been stripped off from some large sections of willow trunk which were lying water-logged in the stream near by.

Beavers when present in a region stand in varying relations to the different persons who are carrying on agricultural operations there. For example, one resident at Snelling considered that the beavers, by raising the water level in the sloughs where their ponds were formed, were of aid to him in that they kept the water-table beneath his land high, and thus secured for him good subirrigation. On the other hand, certain farmers held the animals to be a nuisance, because they persisted in stopping up irrigation ditches. In former years, when the animals were numerous, their damming operations are said to have resulted in the frequent flooding of fields.

With the protection now afforded, we may hope that beavers will continue to live along these streams in goodly numbers especially in those places where their presence is not troublesome to the ranchers. From the standpoint of the naturalist and nature-lover, the beaver is one of the most interesting mammals in the fauna of the Yosemite region.


Animal Life in the Yosemite
©1924, University of California Press
Museum of Vertebrate Zoology

grinnell/mammals69.htm — 19-Jan-2006