#86 Legacy Sediment
May 1, 2008
This show originally aired on March 7, 2007 and then again on May 1, 2008 on “The Rivanna Rambler,” a weekly public affairs show airing every Thursday at 11:55 a.m. on WTJU 91.1 FM or wtju.net.
It’s the time of the year when rivers run high and brown here in Albemarle County. Some well-placed rain events, brought water levels to seasonal highs. Sediment from surrounding floodplains and other sources colored the water various shades of brown, from slick and bubbling chocolate during the first flush, to a steely brown that mirrored the gray March skies. I find myself wondering, again, where does all that dirt come from?
It turns out that here in the Rivanna Watershed, as elsewhere, the answer to that question is not as obvious as it might seem. It is clear that some of the earth is washed into the river from adjacent lands – sheet flow traveling over fields, lawns and parking lots picks up soil that is not firmly rooted. Construction sites, whose bare earthen slopes are theoretically protected by black and orange plastic woven silt fences, are another source especially in severe rainfall, no matter how conscientious the contractor. But there is another source of sediment in the rivers that scientists are just starting to quantify – and this is called legacy sediment.
Legacy sediment has its origins in the earliest days of European settlement of the colonies. With few restrictions save the terms of the Land Grants, settlers immediately set to work clearing the Piedmont hills to make way for pasture, row crops, and especially, tobacco. At the same time, rivers and creeks of all sizes were dammed to provide hydropower and to aid in navigation. The Rivanna River was, by 1840, a series of long flat impoundments between dams built in key locations: coming up from the James, a traveler would come first to Rivanna Mills, then Palmyra Mills. There were mills at Bernardsburg, now called, Crofton , at Stump Island , and at Shadwell, to name a few. The present day dams at the Woolen Mills and on the North Fork at Advance Mills are remnants of that time gone by. US Census figures reveal that by 1840 there were 65,000 water-powered mills operating in the eastern United States.
That this happened at the same time as widespread removal of trees meant that massive amounts of sediment from hillsides cut bare washed down, across fields and into the rivers. And much of that sediment was trapped behind the dams, occasionally washing downstream in floods. And though a good many of these dams have been removed, their legacy is layers of sediment, sometimes 2 to 4 feet thick spread out across the floodplains above the site of the dam.
Now a river’s job, some would say, actually is to move dirt. If you take the long view – the one over centuries and eons – we can thank water and its erosional power on the landscape for the layers of sand and clay that ultimately form sandstones and other sedimentary rocks. And it is the river bursting out of its channel in high water and rain that spreads sediments, fine and rough, downstream to build floodplains, wetlands, and marshes on out to the deltas of rivers as they reach into the ocean. And it is precisely when we disturb this normal flow by damming the river or by forcing it into concrete channels and between floodwalls in our cities, that the river becomes disconnected from its floodplain. And lacking the floodplain to absorb the excess volume of water in peak flows, the river cuts stronger and deeper into its channel.
It’s a vicious cycle and one that is now being studied to learn whether sediments layered across floodplains in colonial times are actually the source of much of the sediment in our now unbridled rivers as they slice through steep river banks such as we find here in the Rivanna Watershed. Add to the equation, stronger flows, augmented by high velocity runoff from our urban and suburban developments, and you have the muddy water you see today in virtually all Piedmont streams in all by the lowest of flow conditions.
Which sediment is which? The US Geological Survey uses a sophisticated method of sediment “finger-printing,” which can put the age of river sediment either before or after the 1940s, when trace amounts radioactive fallout from nuclear testing help determine the origin of sample. Whether the sediment is from modern day sources, resulting from inadequate stormwater controls or agricultural practices – or from a much earlier era, aged in floodplains, and slowly eroding away to destinations downstream. Next time you see the waters turn brown, its fair to say that at least some of what it is carrying is a legacy from our forebears and how they lived on the land over two centuries ago.