Ted, Peridot is generally found as a gemstone in places with a prehistory of volcanic activity, alongside Olivine and Serpentine, which are related to it. I have no idea about the US, but down here I can pick chunks of any of the three I just mentioned out of rivers coming from Chimborazo, Rucu Pichincha, Imbabura, or any of a number of currently inactive volcanoes. I've never found it in the drainage of Tungurahua, though, which would suggest that it's present in complex with Andesite or Dacite and is only exposed after a period of erosion.
EDIT: xoxo, that could also be Nephrite, which is very common in the US and Canadian rockies. Jadeite is generally darker green and a bit cloudy, while Nephrite is a clearer, paler green.
The other super-longshot is Aquamarine or one of the beryl-class stones, but if that's the case it's lacking some of the telltale crystalline structures I'd associate with those.
Ted's absolutely right - knowing which mountains you got that from would help us ID it more closely....