Found in cool shallow spring, among leaf and detritus buildup at the bottom.
Taken home for my personal enjoyment. Can get more pictures.

Looks an awful lot like photos of Gammarus pseudolimnaeus. Range and habitat (superb spring water) seem to match as well.

Identified on iNaturalist as indeed G. pseudolimnaeus based on long setae on the basis of the 7th pereopod. A potential first for the guide. Once I get a decent enough picture (it was identified from a video) I will add that.
Will link the iNaturalist post and identifying video.