There are three large rodent species that live in the watershed of Crystal Springs Creek. One is the North American Beaver (Castor canadensis), builder of dams and (sometimes) lodges, another is the ubiquitous nutria (Myocastor coypus) from South America which is easily seen out and about during daylight and is usually unafraid of humans but (sensibly) runs at the sight of a dog. The third species is the muskrat. More on muskrats later.
Beavers are shy of humans and mainly active in the hours of darkness or at twilight and in the early morning. Mostly they are unseen by humans but their works are often very evident as they are great modifiers of their habitat, constructing dams to raise and stabilize the water level, lodges for living quarters (and sometimes other purposes), and canals and slides (muddy trackways) for moving about unseen and to ease the transport of materials and food. Other evidence of their presence are the chewed stumps of trees and branches stripped of their bark seen floating in the water.
The nutria (which is called by the common name coypu in Latin America) is generally similar in appearance to the beaver, with the obvious exception of the tail, which is thin, scaly and ratlike, as opposed to the beaver's flattened paddle-like tail. This distinction is not always visible when the animal is swimming, and unless the viewing conditions are right the tail may not be visible at all under the water. Both animals have roughly similar fur with buff or even reddish-brown areas, but the beaver's is more generally brown. Both species have small rounded ears that are visible while swimming. The nutria's snout is somewhat more truncated and its muzzle and whiskers are whitish, and its front teeth more distinctly orange than that of the beaver. The two creatures resemble each other enough that they are often misidentified in photographs posted on the websites of government agencies and wildlife nonprofits, so don't feel bad if you are unsure of what you see!
The nutria/coypu were introduced into North America (including Oregon) in the 1930s as a replacement for the over-hunted and (then) rare beaver in the fur trade. Apparently the market for the pelts never developed and disgruntled nutria ranchers released the animals into the wild. Currently the nutria occupies the entire Willamette Valley, portions of the lower Columbia and many watersheds on the Oregon coast, as well as areas of Washington state. Here, as in other areas of North America, Europe and Asia (where they have also been introduced) they damage critical wetland plants, destabilize river and canal banks and levees, and compete with muskrats (native to North America but also introduced to Eurasia). Efforts have been made in some locations to eliminate them from areas where they have been introduced. They were successfully eradicated from California by the 1970s but have returned since. Eradication has failed in the Gulf Coast of the US, but has succeeded in Sweden. I am not aware of any ongoing effort to eradicate them in Oregon.
Muskrats also have a long history in the fur trade in North America (where they are native) and have been introduced into Europe and Asia and southern South America. We don't see them very often in the watershed probably for two reasons: a) like the beaver they are mostly active at night or in the twilight hours and b) (possibly) because they are in direct competition for food sources with the apparently more numerous nutria, which are also larger. Both species feed primarily on soft herbaceous vegetation, while beavers diet consists mostly of the inner bark of woody plants.
Okay, now for the muskrat in the park! I have not personally seen one in the Crystal Springs Watershed in a few years, and never managed to photograph one before, but this past Saturday I spotted this individual swimming underwater against the current at Westmoreland Park, south of the casting pond. It was using its tail as well as its feet for propulsion and did not surface during the minute or so that I viewed it.
I am fairly sure of my identification of this individual as a muskrat. It is clearly not a beaver, it uses its tail for propulsion, it seems to have a more pointed snout than a nutria, and the white whiskers are not apparent. I have uploaded the images to the website iNaturalist.org, and the identification has been confirmed by several others, but what do you think?
As always, if you see a muskrat or have other sightings, or if you have video or photos to share, please contact the CSP at firstname.lastname@example.org or https://www.crystalspringspdx.org/