Clear fly lines
You must have been on the Lower Provo huh? There are only a few places where I have consistenly had fish spook by seeing my fly line-the lower Provo, Silver Creek, the Missouri and the RR on the Henry's Fork come to mind. All these places are high traffic spots where I think the fish make the connection that the thin thing moving above them means trouble. However, alot of the time, I had the feeling that they spooked because I was stripping the line in or moving the line unaturally against the current.
I do agree that the color of the fly line can make a difference, but I don't know if it has as big of an impact as the fly line manufacturers would have us believe. I've thrown the "new zealand grey" on the Henry's Fork and Silver Creek and felt like it did make a difference. SA makes a totally clear fly line that is an intermediate type I sink that I have fished in lakes and the sloughs of Silver Creek and enjoyed good results with-I really like how the SA clear line handles and felt that it did make a difference in not spooking fish. I've kind of gotten away from fishing different color lines in the last few years and concentrate more on better spotting the fish and covering them with my leader only, but I know that's not always possible. Hope this input helps, and good luck.