Bed of Armor: Large rocks hold fast in flooding streams

Gravel-lined streambeds remain relatively unscathed by many floods, according to a new model. This finding may make ecological and hydrological effects of deluges easier to predict.

The surface layers of gravel-lined streambeds often have higher proportions of large rocks than does the underlying sediment. This armor of stones provides a habitat for aquatic insects and juvenile fish, says Peter R. Wilcock, a geomorphologist at Johns Hopkins University in Baltimore. It also influences how much sediment can be swept away from the streambed as well as the rate at which water is exchanged between the stream and the porous strata beneath and beside it.

The faster water flows, the more sand and stones it can carry, and the larger those stones can be. Therefore, scientists have speculated that streambeds lose their pavement of large stones during floods but regain it as water levels and flow speeds return to normal. There’s been, however, a dearth of field data supporting or refuting that presumption, says Wilcock.

Although it’s easy for scientists to collect samples from a calm stream, it’s more difficult—and sometimes dangerous—to do the same in a torrent.

Now, Wilcock and his Johns Hopkins colleague Brendan T. DeTemple have developed a mathematical model of the pattern of streambed sedimentation. It suggests that bottom armor may persist even during moderate-to-large floods. The model was based on data gathered during laboratory experiments and field observations measuring the amount and size distribution of particles kicked up into suspension by different flows of water.

The new analyses predict that the median size of particles incorporated within a streambed’s surface rocks changes little over a wide range of water speeds, says Wilcock. Although an individual stone may be swept to a different location, it’s often replaced by a rock of similar size. This constancy could simplify the task of scientists who formulate models to predict how floods will disrupt streambed habitats, he notes. Wilcock and DeTemple describe their findings in the April 28 Geophysical Research Letters.

The new model concurs with field observations showing that many large stones hold their streambed positions even during moderate floods, says Michael A. Church, a hydrologist at the University of British Columbia in Vancouver. In one of his studies, only 30 percent of the larger-than-average stones in a streambed moved during a flood of the size typically experienced there once every 2 years.

Some creatures that live in streams have evolved to tolerate occasional ecological disturbances such as floods, says Martin Doyle of the University of North Carolina at Chapel Hill. The new research suggests that bottom-dwelling organisms, as well as young fish that hunker down along the riverbed, “may have more-permanent homes than we thought they did,” says Doyle.

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