Cold,
You are correct, that the legal requirement is that it be navigable in fact, i.e. CAPABLE of being used for commerce. That's the law. In actual cases to date, much of the proceedings revolved around showing that it HAD been used for commerce. If it HAD been used, then it most certainly CAN be used.
That said, your year round comment is off the mark. They've used the language "in it's ordinary state", though "ordinary" meaning for that particular time of the year. For instance, if at typical SPRING flows, it CAN be used for commerce, then it'd be navigable, even if it couldn't be used at typical late summer flows. But at the same time, you can't use an infrequent flood stage event to say, "see, I could float a boat!".
Another nugget is the language used in past cases. "If it's navigable in fact, it's navigable in whole". Meaning, if some sections are navigable, and some sections are not, the whole stream is still to be deemed navigable. For example, the Allegheny River is navigable in Pittsburgh, obviously. Well, way up in Potter County, upstream of Coudersport, it's a little brookie stream and there are places you can stand with a foot on each bank. It's still the Allegheny River. It's still considered navigable even there. Likewise, Penns Creek, if declared by a court to be navigable, would thus be navigable right on up to Penns Cave. (of course, a court could reverse previous language and say only parts are navigable).
But yeah, what counts for commerce is in question. So many streams have been used to build jack dams, and float logs downstream. Is that commerce? Or is that not it's "ordinary state" since it had to be dammed? I'd assume an individual kayaking would not count as commerce. But what if you were a kayak outfitter, and rented them out, dropped people off, and picked them up downstream? Is that commerce?
A lot of still unanswered questions by the courts. BTW, the DCNR does publish a list. They made it for gas purposes, but it is based off of the same criteria. Navigability = public streambed = public property, not private. The list isn't complete, so nobody's saying that streams not on the list AREN'T navigable. And since the DCNR has no authority, it's not saying most of these don't have to have a final determination by a court, either. It's just essentially the position of the DCNR. It is based off of PA legislature "declarations of public highway" in past legislation.
http://www.dcnr.state.pa.us/cs/groups/public/documents/document/dcnr_009715.pdf
In map form:
http://www.gis.dcnr.state.pa.us/maps/index.html?publicstreams=true
As for what's been verified by courts, I believe:
Allegheny
Mon
Ohio
Yough
D
Lehigh
Susquehanna (+ both branches)
Juniata
LJR
