Newly created marshes near the mouth of the Mississippi River, West Bay, Louisiana
Of all the anthropogenic transformations occasioned upon the North American continent, few garner less attention than the shift in the dynamics of sediment transport: in essence, in the large-scale movement of huge quantities of earth. Yet the impacts of this profound alteration are all around us — literally, around the North American littoral — and what is at stake is nothing less than the survival of coastal places and spaces, and the peoples and processes that depend on them. Some background, both historic and technical. For millennia the movement of sediment was governed by natural processes; wind and water would erode and mobilize mineral particles across the topographic surface, and then deposit a share of these particles into the currents of rivers. The coarsest sediments would settle out upstream, while the finer particles would either disperse along the rivers' broad meander belts, forming rich riparian zones in alluvial valleys, or else would move downstream suspended in the water column or tumbling along in the bedload. As the rivers disembogued into the seas, their currents would slow, lose kinetic energy and dump their sediment load at the continent’s edge. There, the billions of tons of sand, silt and clay particles would accumulate in the form of deltaic lobes and coastal wetlands, or get swept sidelong by offshore currents to accrete on beaches, dunes, salt marshes and barrier islands along curving bights. The pedological loss of the continent’s interior thus begot the geomorphological and ecological gain of its edge. The resulting littorals — including the delta of America's greatest river — have long ranked among the most productive environments for an extraordinary range of life forms, including humans. Fast-forward to modern times, and much has changed.
We came to view the natural tendency of rivers to store surplus water laterally as an intolerable problematic — “flooding,” we called it — and so we strait-jacketed the channels within levees and floodwalls. We denuded forests, broke prairie sod, replaced biodiversity with monoculture, and augmented the fluvial and aeolian erosion of fertile topsoil. We routed water from wet to dry places via aqueducts, canals, pipelines and reservoirs, to be used for hydroelectricity, irrigation, municipal and industrial purposes, and upon these systems we built the world’s largest economy. Getting this abundance to market required efficient navigation, and that meant straightening and stabilizing key arteries like the Mississippi and its tributaries, building locks and canals to allow vessels to step-ladder upriver, and dredging constantly to maintain requisite depths. And on deltaic plains such as that of the Mississippi River in southern Louisiana, we scored and scoured the landscape to enable vessels to call more efficiently at ports, and to extract fossil fuels more effectively.
America’s sediment budget did not get knocked off balance by sinister or incompetent forces, but rather as an unforeseen consequence of promethean engineering projects that have produced great wealth for American society for generations. No more can we decommission dams and locks for the sake of increasing sediment supply than we can remove levees from the lower Mississippi for the sake of fast-tracking coastal restoration; millions of Americans rely on these engineering structures, and they are here to stay. Nevertheless, we should recognize that they come with a cost, and strategically augmenting the sediment load of rivers as they flow to coasts, particularly by maximizing the beneficial use of dredged sediments, is a good way to reduce that cost. Human intervention skewed North America’s sediment dynamics, and only human intervention can rebalance them.
Read more:
http://places.designobserver.com/feature/beneficial-use-sediment/37651/
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