In general, the stream fishing is better below a dam. Especially since above the dam is usually a reservoir. The fishing is better below a dam, especially if it is bottom fed (creating a 'tailwater' fishery). These fisheries stay near a constant temp, allowing plants and insects to flourish, especially if the dam spills at a constant flow. As far as the dam on the South Fork Snoqualmie (two of them, actually, although the one you're referring to is the Weeks falls), it shouldn't affect fishing at all. I believe it's a run-of-the-river style dam that takes advantage of a natural falls. Therefore, no reservoir and no effect on flow or temp.