Hey Ray! What Causes The Line At The Confluence Of The Three Rivers In Pittsburgh

PITTSBURGH (KDKA) — Sometimes in science, things mix well, other things don’t, and sometimes that mix takes time. That can be said for the Three Rivers, too.

You may have noticed something while looking down on the Ohio River at “The Point.”

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That is where the Monongahela and Allegheny River meet to form the Ohio River. There is a line created by the confluence of the rivers.


(Photo Credit: KDKA)


The fact that this line exists means there are some differences between the Monongahela and Allegheny Rivers, so what really goes into creating that line at the confluence?


For that, we talked with Fred McMullen, the coordination meteorologist at the Pittsburgh office of the National Weather Service which works closely with the Army Corps of Engineers and the United States Geological Society to which help to monitor, forecasts for and maintain our area river ways.


(Photo Credit: KDKA)


“The line is a function of what we call ‘rock bottom’, and it’s a thing with the two rivers” McMullen said.

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“The Allegheny and the Mon have different river composites. The Mon has a siltier, sandstone bottom, which is finer sand. When we get heavy rain, it gets stirred up more easily. That’s why you get a darker appearance to the Mon river versus the Allegheny’s bigger gravel and courser sand. The river doesn’t get stirred up as much, and therefore maintains the same color for the duration of an event.”


(Photo Credit: KDKA)


The rivers can take a while to mix all those different sediments into the water, too. McMullen tells us,


“We have different colors coming together because different sediments have been mixed up and they separate, and at some point, down the Ohio River they merge. Therefore, the river becomes one color again.”


(Photo Credit: KDKA)


Sometimes it is lighter, and sometimes the line is darker, too.


The greatest contrast at the point usually occurs when you get heavy rain at the West Virgina and Pennsylvania border and little to no rain in Northwest PA.

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That combination will cause the Monongahela to get stirred up, while the Allegheny won’t.