A River Runs Through, Then Runs Backward
April 29, 2008 8:49 AM   Subscribe

Is it rare, or just infrequent, to find a river branch whose confluence with a creek or other waterway, causes it to reverse direction?

I once visited a former mill-converted-inn around Pendleton County, West Virginia where an old map of the nearby (Potomac?) river was pinned on the wall. The overseer of the inn described how its previous use as a Mill was enabled by the meeting of "South Branch" with (of?) a creek such that it virtually "reversed direction" geographically and thus powered the mill effectively. However, I'm not finding direct references online to this phenomenon. Vaguely recall this person insisting "Only here and not many other places do you find this sort of thing" like a creek/river switching directions or flowing first one way then another.
Anyone have familiarity with what he was talking about? Thanks.
posted by skyper to Science & Nature (8 answers total)
I'm not sure I understand the question. Are you saying the confluence with the river cause an oscillation in direction of the river? If so, that can only be the result of an oscillation in one of the inputs. (One can model this something like a circuit, where the flows represent currents).

Otherwise, a steady-state, equilibrium flow is almost certain. Of course, flows in rivers and streams are not constant, but it would be quite unlikely to see frequent reversals.
posted by JMOZ at 8:57 AM on April 29, 2008

As long as it's flowing downhill, it can go any direction it wants.
posted by electroboy at 9:06 AM on April 29, 2008

I'd heard, but can't find a citation, that the River Ouse (which runs through York) can change direction. The Ouse's flow is from one river to another, and so its flow direction (I was told) depends on the relative level of each river.
posted by scruss at 9:08 AM on April 29, 2008

Could the place you visited have been Octoraro Creek, on the south branch of the Potomac? I am asking because if so the phenomenon described on the Wikipedia page might help others understand what you are asking. See the paragraph titled "Octoraro Creek in Maryland".
posted by procrastination at 9:14 AM on April 29, 2008

If you built your mill on a stream just upstream of its confluence with a river, then you could dig a channel from the river to a point on the stream just above your mill to divert some of the river flow into the stream bed. That would give you much more water to work with than the original stream flow afforded, plus much more control of water flow, and could only be done near a confluence. That's my guess.
posted by jamjam at 9:16 AM on April 29, 2008

There are a number of circumstances in which a river can reverse direction. Flood events and tidal bores can reverse a current direction, but they are not an ongoing phenomenon. True, ongoing flow reversal would be a result of stream capture or stream piracy when a fluvial system captures another one, such as the Atchafalaya River capturing the Mississippi in the future.

In my Fluvial Sedimentology book they call your example a "pressure eddy" (pg. 113-115). The authors write that a river hitting a confluence or an abrupt angle change greater than 90 degrees can create what is called a power pressure eddy wherein flow goes upstream.

So, this would be different that a regular eddy. You see regular eddies all the time in streams; an eddy by definition is created by reverse flow created by a void space, but most stream eddies are on a small scale from hitting a stream bank or a boulder. A pressure eddy might create the power necessary in your example.
posted by barchan at 9:22 AM on April 29, 2008 [1 favorite]

Could the place you visited have been Octoraro Creek, on the south branch of the Potomac?

I know for sure this was in West Virginia near the border with Virginia. It's the Potomac River, or a branch of it, meeting "Thorn Creek" -- which I located a reference to online, yet mostly when I Google these names I only get hunting pages with maps or water pollution sites. Nothing regarding the distinction where they meet as this man had indicated -- although barchan's answer sounds close to what he must have meant.
posted by skyper at 10:22 AM on April 29, 2008

The Mekong River in Cambodia causes the tributary Tônlé Sap River to reverse flow in the wet season. During the dry season the Tônlé Sap Lake drains via its river to the Mekong, in the rainy season it reverses flow and the water flows from the Mekong to the Tônlé Sap. This wiki article mentions it, as well as this NPR article.
posted by cftarnas at 5:34 PM on April 29, 2008

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