#115 South Fork Rivanna Reservoir Stores Our Dirt, Too
December 11, 2008
There’s a lot of different ways to look at our diminishing resources — running out of clean water, clean air, and …. good dirt? We might do well to look past the problem of the South Fork Rivanna Reservoir filling up with dirt — and try to understand the causes of — and consequences of losing dirt from the landscapes upstream.
This show originally aired on December 11, 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.
There is slow steady winter rain that’s keeping temperatures hovering around forty degrees and the skies dark with winter gloom. But the rain is good — for our groundwater, for our reservoirs, and it is good for the plants and animals that need this most essential resource to survive. This rain is also filling our rivers – and I would wager – sending a good amount of water into the South Fork Rivanna Reservoir, along with a healthy amount of dirt.
Now, that dirt is slowly but surely filling the reservoir – each year, decreasing its capacity from 1 to 5 per cent since it was completed in 1969. In another example of our human short-sightedness, like many public works installations of the era, the design life of this reservoir was only fifty years, at which time the reservoir would be filled to over 50% of its capacity. This was the “water supply plan” back then – and it is, in part, this plan that has us where we are today, scrambling to find a way to maintain the usefulness of a reservoir that was poorly situated with an uncharacteristically large watershed from which to drain – and one that, in the conventional wisdom of the day, was always expected to fill up with sediment. Hence, the current discussions about dredging.
Since the cost of disposal of the dredge material is generally agreed to be the most expensive part of any dredging operation, this got me thinking about the value of the sediment itself and whether we are, once again, missing the mark in the way we think about our natural resources – letting the pocket book drive the decision without consideration for other factors that, because they are ecosystem benefits and difficult to quantify, don’t often get put into the cost benefit analysis.
There have been some creative responses by contractors interested in dredging the South Fork Reservoir – and in most cases, we don’t know the ultimate destination of the sediment, it’s just part of the economic equation upon which the offers are being built. They range from a method of opportunistic, selective dredging of just sand and gravel when market makes it economically profitable – to more complex projects proposed, including using the fill to extend the Airport’s runway – or to fill a nearby quarry, presumably for reuse later on.
The US Army Corps of Engineers has been thinking about the use of dredged materials for years. Charged with keeping our waterways clear for navigation, the Corps promotes the use of dredged materials for creating wetlands and improving fish and wildlife habitats, as well as the construction-related kinds of uses, such as land creation for runways, buildings, and other human strudtures. And dredge materials can also be used to improve the soil structure of poor agricultural lands, creating topsoil or serving as the base structure for soil amendments.
Dr. David Montgomery is a geo-moprhologist and a 2008 MacArthur Foundation Fellow, who has spent his career looking at the complex relationship between rivers and the soils that they transport – historically tone of the primary land changing processes. His recent book, Dirt: The Erosion of Civilization, provides a sobering description of the consequences of squandering “good dirt” – the topsoil in which we grow our food, the topsoil that takes generations to create. Montgomery’s book looks at the repeated cycle of failure of civilizations – Mesopotamia, ancient Greece, the Roman empire, China, European colonialism, central America, and the American westward development – in each case, expansion beyond the carrying capacity of soil to produce enough food for growing populations.
Much of the erosion the along South Fork Rivanna River and its tributaries occurred in the 17 and 1800’s when the forests were stripped bare for agricultural land. This sediment has likely been deposited downstream below the dam, over time creating new and elevated floodplains. So it would be a mistake to think that the rich topsoil of our ancestors is lodged behind the reservoir’s dam. But while the reservoir is losing precious storage space for drinking water, it is also retaining, quite literally, land of a significant portion of our upper watershed.
The caution about the growing scarcity of and destruction of topsoil worldwide may seem a distant thing for many of us in our watershed, but consider this. We are, as David Montgomery says, “slowly remodeling then planet without a plan,” today moving more dirt around the planet than other biological or geologic process. The caution for us here is to look carefully at how we can prevent the soil from filling the reservoir to begin with – and carefully consider the possibilities for keeping our soil, like our drinking water, in our own watershed.