River Rocks for Dummies

Rounding of river rocks during saltation and traction

A not insignificant portion of the online posts the staff turn up in their searches for DotD nominees contain a mixture of fact and misinformation. In many cases, we can track the misinformation to a misinterpretation of original source material; usually because the writer is ignorant of the topic. That's pretty much what happened today: we looked at the references supplied by Athena Hessong in her Sciencing.com post, "How Are River Rocks Formed?" and we saw what she did...

Hessong, here making her ninth trip to the podium, performed a minute or two of research and turned up a source called "How Rocks are Formed: Erosion" at the site RocksForKids.com (the only one of her three references that still exists). That's where Athena plucked out wording that she turned into,

"Erosion occurs when the pieces broken from the rock get carried away by the river. These bits of rock create the sand and silt along the river banks and at the mouth of the river." Well, yeah, we'll go with that... except that the question was about "river rocks," which are the smooth, rounded pebble and cobble-sized stones used in landscaping. Which means that the OQ wanted to know first, why are they smooth and rounded and second, why are they that size? Hessong was rather vague about the size question (it's mostly a matter of transport distance, Athena) but pretty much hosed the questions of smoothing and rounding. According to Athena,

"The formation of river rocks requires moving water and smaller rocks. Rocks easily eroded by water more likely form river rocks. Typical rocks with jagged edges can fall into the bottom of a river or stream bed or remain on the river bank. The speed of the river determines how quickly the rock becomes a river rock."

That paragraph contains within it the information we need to explain river rocks, but Hessong couldn't put it together. What she did assemble is this:

"In the river, water flows constantly over the rocks. The movement of water itself does not weather the rocks, but that water carries with it smaller pieces of rocks, sediment and silt with it. These tiny bits of broken stones hit the rocks at the bottom of the river, breaking off pieces of them, which the river carries away. The faster the water moves, the more sediment flows over the river rocks, hastening weathering."

In reality, that's a lot closer to the way that wind creates ventifacts, sand-blasted rocks on the desert floor. If you think it through, however, that just means that any rocks sitting on the riverbed will eventually have smooth tops – but it says nothing about the side that is buried! Oh, and by the way, Athena? Both "smaller pieces of rocks" and silt are sediment, so your rule of three is actually just two.
What Hessong should have said is that as jagged rocks roll and bounce along the bed of a fast-moving river (the processes of traction and saltation), thousands of impacts with other moving and stationary rocks chip off projecting points. The overall effect is rounding and smoothing by removing any high points. That's all Athena needed to say, but she didn't... Hence her continuing reign as Dumbass of the Day.

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