226 University of California Publications in Zoology [Vol.12 



A stick "house" was located at the edge of the river a short dis- 

 tance below these cuttings. It resembled a mass of drift caught among 

 the snags resulting from the undercutting of the timbered bank. This 

 house was three and one-half feet in height, by fourteen and sixteen 

 feet in two diameters on the ground. It consisted of small branches 

 and broken saplings from the driftwood of the river and some brush 

 dragged in from the land side. A little earth and dead leaves had 

 been worked into the interstices. Less than a dozen beaver-cut sticks 

 were in evidence in the whole structure. There were no fresh signs 

 in or around this house. But a recently used "slide" was found on 

 the bank a few yards off. 



Opposite Eiverside Mountain, and below Ehrenberg in at least two 

 places, beaver cuttings were found on the Arizona side. And on the 

 California side from near Ehrenberg to below Palo Verde considerable 

 work was noted. In one place bark had been stripped within a day 

 or two from a cottonwood sapling lying in the water, where it had 

 recently fallen as a result of undermining by the river. 



It was thought that the continual undercutting of its timbered 

 banks by the river itself, thus precipitating many green trees into the 

 water, provided an immediate food-supply for the beavers, so that even 

 if present in numbers they would gain an easy living without them-_ 

 selves having to fell trees. It is possible, too, that the habitually 

 unsettled behavior of the river accounts for the loss of the dam-building 

 propensity, or at least for its futility on the part of the beavers of 

 the Colorado Valley. We saw no signs anywhere of dams such as are 

 reported by Mearns (1907, pp. 354-358) as occurring on the Verde 

 River, Arizona, a tributary of the Gila, which in turn flows into the 

 Colorado. 



A used "house" located at the margin of a slough near Palo Verde 

 was cut into by Stephens and Jones on April 3. It proved to be three 

 feet in height and twelve feet across at the base. It consisted of 

 branches and small saplings cut by the beavers when in full leaf, and 

 laid compactly. The mass was eight inches thick over the nest cavity. 

 The interior space was two and one-half feet high by four to five feet 

 across. At one side the bank had been dug away to make the floor 

 comparatively level, as the house was built on a sloping bank near 

 its top. There was an under-water entrance at one side, and an open- 

 ing through the wall above ground. A fresh willow sapling had been 

 hauled through this opening butt first. The beds were merely hollows 

 in the earfh floor. 



