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Found in a waterway in Butte County, CA. First thought it was a beaver kit, but it has no tail. What aquatic mammal has no tail? enter image description here

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Are you sure there is no tail? Muskrat? – kmm Jun 22 '14 at 20:11
Muskrats have a rat tail. This critter may have had a small knob of a tail, but the hair at the hindquarters was too wet to reveal much of anything. – greg Jun 22 '14 at 22:48
Your individual might have had an accident in the past! – Remi.b Jun 23 '14 at 9:25
What size is it? I'd agree with kmm and Remi.b that it's a muskrat which lost its tail. – har-wradim Jun 24 '14 at 19:08
What a great picture! – Oreotrephes Jul 1 '14 at 18:41

Looks like a mountain beaver (Aplodontia rufa) to me. Location, appearance, and minimal tail all match.

A bit more detail:

Where I'm from on Washington's Olympic Peninsula, they're not uncommon, but Butte County is near the southern end of their distribution (eol map page) and they may be less common there, and limited to cooler higher elevations (Beier 1989), and to wet habitats (their inability to concentrate urine (!) means they need to drink enormous quantities of water).

This white paper from USDA (Arjo 2007) is a great source, and more comprehensive than the the wikipedia page, which is also good.

Arjo, Wendy M. (2007). "Mountain Beaver: A Primitive Fossorial Rodent". USDA National Wildlife Research Center - Staff Publications. Paper 675.

Beir, Paul (1989). Journal of Wildlife Management 53(3) 649-654

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Hey, Oreotrephes, thanks for taking the time to respond with the correct answer. This is a video I shot of this animal, and it reveals that it definitely is the mountain beaver: – greg Jul 4 '14 at 5:17
Aw, what a cute little guy. Great footage! – Oreotrephes Jul 4 '14 at 16:31

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