It’s obvious that streams and rivers carry cobbles, gravel and sand downstream. Geologists know streams move a lot of material at flood stage but, even during calmer water flow, streams account for considerable movement of Earth materials.
One of the fun experiments I did for a class in graduate school depended on the use of a flume, or an artificial stream set up in a laboratory. A big pump moved water rapidly through a long box made of plexiglass so we could see what went on within it. At the bottom of the flume, we had a layer of sand. We varied the depth of the water and the speed of the current and studied the wavy ripples that were soon superimposed on the top of the sand. Geologists use information from such “bed forms” when interpreting ripples preserved in solid rock like sandstone. It’s just yet another way we have of inferring conditions that were present in a particular environment back in the depths of geologic time.
I was reminded of my youthful flume experiences when I read an article in the journal Science about what some geologists are doing to better study sediment in streams and rivers in the real world. Their studies are inspired in part by the need engineers have to protect levees and bridges from stream currents and materials moving downstream in the water. Another application is for ecologists planning restoration work along rivers.
Using modified or artificial rocks, some geologists are finding ways to trace movement of material not in indoor flumes, but in natural streams. One set of rocks is painted for identification and they contain radio tags in holes drilled into the stone. Higher-tech “smart rocks” are made of metal with internal electronics that can measure how the rock tumbles downstream, taking data on such movement several hundred times per second.
Joel Johnson of the University of Texas at Austin is one expert doing work with the new devices. The smart rocks, Johnson said to Science, are a “killer app” in the realm of stream and sediment research.
What do researchers do with the smart rocks? Essentially they drop them into a stream, go away for a while, then come back and try to find them again. In April of a recent year Johnson and a graduate student dropped four smart rocks into Reynolds Creek, a stream in Southern Idaho. They waited for springtime floods to wax and wane. Then they went back and looked for their smart rocks, ultimately finding two downstream of where they’d been tossed in. The other two special rocks were not to be found.
The two smart rocks they retrieved had batteries that pooped out before the rocks had moved. That disappointment has led to a redesign with batteries that last for months. The team is also constructing the rocks differently.
It’s still the early days in the ongoing tale of smart rocks. But studies in natural systems rather than flumes are starting to blossom.
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