How accurate are these temperature gauges? Also, why does the ohiopyle one seem all over the place? How does the yough dip 10+ degrees in a day or two?

http://waterdata.usgs.gov/pa/nwis/uv?site_no=03081500

snowmelt,warm rain,cold rain,wind,clear day,cloudy day,time of day and many other factors will effect surfaced ,moving water constantly..
Only water under ground will have a uniform temp.

I find the the usgs temps gauges to be quite accurate.
There is no temp reading at the ohiopyle gauge - what you're seeing there is 2 flow readings - in gauge height, and cfs.
And FWIW - a 10 degree temp change in a day, on any stream, is nothing abnormal

dfg, there's a temp gauge, at least according to that link.

Agree, 10 degrees is nothing abnormal. Plus, it's a tailwater, so that adds another dynamic.

If you increase to a 30 day graph, you can see the daily variation clearly, which is only about 3 or 4 degrees, due to the tailwater dynamic I assume. The big macro-spikes correspond to spikes in flow, so I'd assume what you're seeing is a more constant cold water base from the dam, and then throwing varying amounts of warmer runoff on top of that.

Releases from the dam. But they must have the ability to release from different levels of the reservoir. If the water is close to 39 degrees at the bottom of the reservoir, which it should be at this time of the year, then you'll probably see swings in the temp during periods of rain, that's another possibility. We've had some very warm days.
It's kind of weird that the temps go up when the flows go up.

Well - thanks for the correction Pcray.
Somehow, the temp gauge got clicked off on the ohiopyle usgs link from my favorites list.

Anyway, the ohiopyle gauge is 10 miles downstream from the dam. And you also have a good bit of added inflow from the casselman river about a mile below the dam. So, yeah those readings can fluctuate a good bit

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I don't think so at all. I think the key you are missing Chaz is that only a small % of the flow at this gauge came out of the dam. Less than 30%. As dryfly said, it's 10 miles downstream of the dam, and there's a major tributary in-between.

All you are seeing is the different mixes between dam flow, direct runoff, and the influence of the Casselman.