Help identify this worm-like thing
I am not too sure but this looks like Polycelis sierrensis.

Though it is apparent that the worm in your pictures is a tricladid planarian, I was not too sure about the species and the geographical distribution. 

However, from the Smithsonian list of freshwater planarians in North America, I deduced that this should be Polycelis sierrensis which is found in California and Nevada (Kenk ,1973). 

The EOL page on this species also concurs with the geographical distribution. Moreover, the shape and dimensions of the worm, as described in EOL, matches with that of the ones you have got. 

I could not find a picture of P. sierrensis but this is how a general Polycelis looks like: