Sunday, August 15, 2010

On gravel bars

As late summer sets in, paddlers on two- to four-day floats on Ozark rivers start looking for the perfect gravel bar around 7, maybe 7:30 pm (enough time to make dinner and set up camp, but late enough for a long day on the river). The requirements are the same for most people: the sound of rushing water to sleep by, ample firewood for s'mores building, a relatively level space in which the larger rocks can be swept out of the way with a paddle for a comfortable sleeping spot, maybe a big bluff across the river or, for some, a good fishing spot for catching breakfast.

But on a couple of our Ozark rivers, one can paddle for miles without finding a gravel bar that meets all the requirements. And really, even though it may not seem like it at the time, that's a good thing. In the past 200 years, gravel has filled in the rivers at a very rapid rate due to land clearing and grazing and, in very recent times and to a slightly lesser degree, excessive wave action from motorboats (which causes significant shoreline erosion). As gravel levels increase in the rivers, large rain events can cause mass migration of gravel deposits. Gravel bars are naturally occurring, of course, but historically there probably weren't as many as there are today, and they were doubtless smaller than they are now, at least in certain cases.

Interestingly, however, plant communities have adapted to living on gravel bars in a natural community called Gravel Wash. Very little soil fertility exists here, as the substrate is gravel and sandy alluvium; gravel washes tend to occur on point bars and can mound into high terraces (good for tents if flood levels threaten). Dominated by shrubs, small trees and sparse populations of perennial forbs, they bear the brunt of high water events which inevitably shape their structure. These are not the protected, safe environments of high, north facing cliffs, many of which have never even been surveyed by botanists due to the inaccessibility.

Among the plants in bloom on gravel washes in early August in the Ozarks is the lovely, multibranching ironweed, particularly appreciated by butterflies this time of year with its bright purple flowers. Willows and vernal witch hazel are common on gravel washes, as are buttonbush and mallow. Sand grape (Vitis rupestris) is really common on Missouri streambanks, though not so much in other states. Water willows bloom in May each year here, elegant flowers that almost resemble orchids.
In some intact gravel washes, one may find little swales that possess populations of Helenium flexuosum or this peculiar Penthorum sedoides, which I'll place at the end of the page. The occasional canoe at the edge of a gravel bar isn't a problem, but ATVs, trucks and gravel mining operations can really wreck these areas.

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