A while back I tried looking into just how long it takes for the 'burgh's rivers to give up these lost souls. There's so many variables...locks and dams, river traffic, debris, etc. In the end there just wasn't any rhyme or reason to it.
A few examples (and they all went missing during Winter):
Joseph Harris, last seen 11/11/08. If memory serves he went into the Ohio at Brighton Heights. Found 2/13/09 at the New Cumberland locks. 3 months to go up to Monaca, take the left turn in the river and then all the way down to the locks at WV.
Michale Andres, last seen 12/30/10 in Beaver County, found in the Ohio River in Industry on 3/8/11, six miles from where he was last seen.
Molly Keats, missing 2/7/11 from Sandycreek. Found in the Allegheny 4/11/11 in Rockland Twp. As the river flows, a couple of miles away, if that.
Devin Minor, missing 1/20/11 from Beaver Falls. Found in the Beaver River near New Brighton on 5/5/11. Couple of miles away.
IF Paul went into the Mon, IMO, there's a very good chance he could still be relatively close to where he was last seen.
There is a rhyme and reason. Scroll down to "Body of Water" on this FBI webpage:
http://www2.fbi.gov/publications/leb/2006/feb2006/feb2006leb.htm
If the victim drowns, the body will sink until variables such as water depth, water temperature at the bottom of the river, food eaten prior to dying, etc., eventually bring bodies to the surface. If a victim dies of other causes and the lungs aren't filled with water, the body wont sink, unless it's weighted. That type of body will float right away shortly after entering the river, if it's not weighted down of course.
Now check out what it says about rivers and currents for movement, and you'll see our upcoming spring thaw will significantly move a body. Rivers will be at flood stages with fast moving water. There will be tons of ice moving along. There may even be trees and debris if the river floods the banks upstream. Those things help dislodge bodies and move them along.
Rivers differ from other bodies of water in two ways they are shallow and have currents. Depths of less than 10 feet do not have a high level of compression on the internal air spaces of drowning victims.
In extremely heavy currents, such as in flash-flood situations, the victims body probably will roll on the bottom for a considerable distancetrees or other debris also may carry along the corpse. During normal conditions in most rivers, this is not the case, and investigators usually will find victims on the bottom relatively close to the drowning site. However, after the body floats to the surface, it may drift due to the current before washing ashore or coming to rest in a back eddy.
Let's look at your examples. You stated they were all in the winter, as if that's similar. However, water temperatures at the bottom of the rivers will be warmer in November and December than January and February. Shallower rivers won't have the pressure of deeper rivers. And each winter is different. Some are more harsh and others are more mild. In that regard, it's noteworthy the two bodies in 2011 were both found approximately two miles away.
John Harris was lost and found in the Ohio River 59 miles downstream after 3 months. (PK has been missing for almost 3 months.) 59 miles is very far away when considering search areas.
Michale Andres. We don't know which river he started out in. He may have started in the Beaver River.
Molly Keats was lost and found in the Allegheny River farther north in the state where the river is much more shallow than it is down near Pittsburgh. Air temperatures are significantly colder in the area where she disappeared; as a result, water temperatures would be too.
Devin Minor was lost and found in the Beaver River. It's a much more shallow river.
Harris and Adreas are really the only ones comparable, and they're vastly different at 59 miles and 6 miles (really 7.4). That's like 50/50 (and I don't believe Adreas is truly comparable since he may have been in the Beaver River). How is that a good chance? It's looks more like an even chance.
I really think they would have found him if he was in the river near South Side when they were searching early on. They usually find people in the river when they do searches.
My BIL is a River Rescue paramedic and a search and rescue diver. The majority of bodies pulled from Pittsburgh rivers were only in the water for a few days to a few weeks. When they're found early on, they are often very nearby---like feet and yards away. These are much more common than ones that are there for months.
I'm compiling a list of bodies found in the Allegheny, Monongahela, and Ohio rivers. The Post Gazette is slowing me down by requirement a subscription (I hate the PG), but so far I have 21 that and the majority were found within a few days very close to where they went missing---one was 20 feet away. When I'm done with my list, I'll post it. But it's becoming clear to me that the longer they're in the water, the farther they travel.
Their not finding him nearby when they searched could be due to his not drowning, not being weighted down, and floating further away; or it could be due to the visibility in the Mon is the worst of the three rivers, and it's filled with a bunch of old cars and stuff that makes searching difficult.
I'm not saying PK isn't in the water somewhere. I'm just saying they need to widen the search this spring since it will be over three months by the time they can search again. In 6 more days, it will be three months since he went missing, and the river isn't going to be cleared of ice for a search by then.
Based on everything we've found, I was definitely wrong about the Mississippi, but I think he's most likely in the Ohio now.