September 7, 2007

#57: The Right (River) Shoes for the Job:
Geomorphology at Preddy Creek

Today is my third day of walking streams, not a bad way to spend a hot and humid summer morning – and I have finally mastered the footwear problem. The first two days, I wore my Chaco water sandals. No problem getting them wet, of course, but every step was an opportunity for pea-sized gravel to become wedged between the sole of my foot and the sandal, resulting in a pointedly painful step that reduced my progress to an awkward hobble until rectified – and just as soon as I had dislodged the offending rock, another would take its place. But now, I’ve discovered that an old pair of cheap canvas hiking boots not only keeps the gravel out but provides support and traction on the slippery rock outcroppings of Preddy Creek where today, I am working as a volunteer on a river morphology study with StreamWatch.

Getting up close and personal with the sand and gravel is exactly what this work is all about. We are here to classify the stream according to a system devised by Dave Rosgen that will help scientists and managers in our watershed understand better how the tributary streams are performing as streams – in other words: can the creek efficiently move its collected waters downstream? Are its banks relatively stable, or are they eroding in such a way as to alter the channel’s form? Is the stream in some state of equilibrium with its floodplain?

The analytic tools used to answer these questions include the Rosgen classification method – and this in turn requires that we measure the shape and curviness of the channel, the width of the floodplain, the slope of descent, and the distribution of the size of particles– the sand, gravel, cobble, boulders, and bedrock that make the channel what it is, in this moment of time. From these measurements, a host of ratios are derived – and finally a classification.

Today, we’re taking measurements along the stretch of Preddy Creek immediately upstream of the StreamWatch biological monitoring stations – as are all the sites selected for this geomorphic study. Tributary to the North Fork of the Rivanna, Peddy Creek originates in the rolling hills on either side of Route 29 where Albemarle and Greene Counties meet. Though the stream seems to adequately support aquatic life, downstream stretches have been designated as impaired by the Virginia DEQ due to excess bacteria. For a lot of reasons, it’s an area to keep an eye on.

Turning onto Route 670 at the big red dome of the Sheetz gas station, we have a front row view of acres of land cleared and graded, the contours draped with rows of truck-sized boulders lined up to check the flow of runoff during the construction of the retail and residential buildings that are on their way. Branching roads named Hickory, Fir and Willow, feed into the subdivision’s main road, Preddy Creek Drive. Access to the creek is in down the gas-line right-of-of way, a grassy swale that is mowed to the edge of the creek. We clamber down the bank into the rough cobble laid to protect the pipeline and start upstream to take our measurements, soon finding ourselves in the shaded cover of trees. My boots gush expelling water with each step and I am grateful for their heavy protection, even as I sink to my knees from time to time in small pools.

We work with measuring tape, stadia rod, and transit – gathering the data which in turn will be compiled with other data, such as land use, impervious surfaces cover, and habitat assessments, to see what correlations can be made between the health of Rivanna’s creeks and streams and the way the surrounding land is being used and changed.

River morphology is defined as a tool for diagnosis – for understanding how the life history of a river and its watershed has influenced – and is influencing the conditions we currently find. Webster’s goes further, saying that the science seeks a genetic interpretation of land and water features. This intersection of terminology between the science of rocks and the dynamic world of flowing water affirms for me, once again, that the river does have a life of its own, a purpose and a role – to collect and convey water and materials downstream. To be the instrument of erosion, collaborating with gravity and weather, to work the land into new shape and form.

I suppose this is what we’re trying to replicate with our earth-moving equipment and engineered stormwater management. Like naming our streets for trees and our subdivisions for creeks, we often fall terribly short of the real thing. Even giving Preddy Creek a stream-type classification, which will be the result of today’s work, will only go so far in understanding what the stream is all about. Perhaps that small piece of rock that was worrying my foot is another, equally important way to know a river.