The Trinity Alps Salamander is supposedly an immense amphibian, nine feet in length, that inhabits remote streams in the mountains of Northern California. It is said to be dark in coloration, with whitish spots along it's back.
I really don't know much else about this cryptid- how freqently it's sighted, or how credible the claims seem to be. From a theoretical standpoint, though, I think this one holds pretty firm.
For example, there are the Cryptobranchids- giant, aquatic salamanders that include the Chinese Giant Salamander and and Japanese Giant Salamander. In good conditions, they can reach lengths of up to six feet and weigh more than a small child. Like all other salamanders and newts, they like cool/cold water and thrive in the absence of human disturbances like habitat destruction and pollution (to which they are particularly vulnerable to). The Cryptobranchids are also found in the United States, via the Hellbender, and are believed by some to have even originated in North America. In my opinion, it would be no great stretch of the imagination to think that the Trinity Alps Salamander might be an undiscovered member of this family.