American Beaver (Castor canadensis)
The image above taken by our HCO Scout Guard IR Camera 5MP Series SG550 on the night of February 12, 2009, shows a beaver near its den along the Middle San Pedro River near Cascabel. At this time and location, the river's water is running underground but near enough to the surface that the beavers can tap it and build canals.
Along this part of the San Pedro River, the river runs mainly underground during much of the year because of deep, permeable alluvium, water surfacing only where claybeds or other impermeable formations force it upward. When the river floods massively (as it does annually on various occasions; see San Pedro River valley), water sweeps over the current floodplain carrying along many objects including beavers, and this current beaver home is definitely recent. We know of their presence both in the Upper and Middle San Pedro, but our section of the river poses more severe flooding problems for their residential stability than does the valley near Sierra Vista.
Witness the circumstances of this recent den-forming operation. Below, the current river channel in the vicinity of the new dens, photographed on February 6, 2009 looking upstream:
A short distance downstream from the previous photograph, the streambed shifts and drops abruptly to this point below, where water lies very near the surface. Here David Omick stands before a canal currently under construction by the beaver(s). The den is located at far left in this image.
Below, a clearer view of the den, which has been dug into an embankment. Note the line of drying mud running vertically down mid-photo, marking a portion of canal levee under construction. During the night, beavers push fresh mud from underwater up along the growing edge.
Below, a second den lies nearby. Note the cut branches lying in the foreground.
Below, the beavers have already felled some fairly large cottonwood trees growing along the terrace above the current streambed, preparing to build their projected dam.
Below, a cut portion of a tree trunk photographed well upstream on the Three-links Ranch in April of 2004, evidence of a beaver population recently present in that area at that time. This group may have been swept further downstream by that date. Here, note the markings of the beaver's very powerful incisor teeth.
Thanks to Barbara Clark, we have more details regarding beaver activities in the 3-links ranch area. Residents saw beavers working at a site there following the September floods of 2006. A photograph taken in early fall of 2007 shows a completed beaver dam:
In this image, the very bright and light-colored material just above the dam is algae floating on the water surface.
By July 2008, the dam had been swept away:
In this image, the red line marks the approximate location of the dam.
As it happens, we have an image of the same location from March 13, 2007, well before the beavers started working there:
This enables us to infer that the beavers chose this site because the subsiding stream dropped its gravel load at this point (fall of flow energy), thus providing better purchase for dam-building materials laid here.
Our beavers are clearly expert dam-builders, but they can't compete with the massive forces driving our periodic floods. These new neighbors of ours are probably fated to be short-term transients, unless they can make their way to tributaries with less overwhelming flows.