The Life and Death and Consequences of Canals and Spoil Banks in Salt Marshes
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APPLIED WETLAND SCIENCE
The Life and Death and Consequences of Canals and Spoil Banks in Salt Marshes R. Eugene Turner 1
&
Erick M. Swenson 1
Received: 19 February 2020 / Accepted: 6 August 2020 # The Author(s) 2020
Abstract We describe the consequence and demise of levees (spoil banks) built from dredging canals in Louisiana salt marshes using morphometric measurements made over 30 years, soil collections on the spoil bank and in the salt marshes behind, and complementary observations from other areas. These measurements were used to determine the temporal bounds of how long spoil banks last and if salt marsh soils remaining in salt marshes are affected. If the rates of changes in spoil bank morphology continue, then the estimated life time of the shrub-tree vegetation at a representative spoil bank is 81 years, the spoil bank width is 89 years, and the dredged channel will erode to the center of the spoil bank after 118 years. The soils in marshes behind the spoil bank have a higher bulk density than in reference marshes, accumulate more mineral matter per year, have lower root mass and are weaker. These observations are compatible with measurements of spoil bank width, vegetative cover and soil compaction, and the conversion from wetland to open water on a coastwide scale. Keywords Salt marsh . Soil strength . Waterlogging . Belowground biomass . Land loss . Louisiana
Introduction Canals and levees have been built for many reasons for thousands of years and with diverse outcomes. Extensive canal networks existed for irrigation agriculture by 3000 to 2400 B.C. in Mesopotamia and were plagued by salinization (Jacobsen and Adams 1958), wetlands were drained throughout the Roman Empire (Allen and Fulford 1990; Rippon 2000), and extensive farming systems with levees and canals were within Amazonian swamps before European colonization (Mann 2008). In 1599 King Henri IV ordered a national effort to drain wetlands around Poitou, west-central France; colonists from there or nearby came to Nova Scotia, Canada, in the early 1600s, and began converting the Bay of Fundy salt marshes into agriculture (Butzer 2002). Parts of the Fens in England were drained by the Romans, but it was not until the * R. Eugene Turner [email protected] Erick M. Swenson [email protected] 1
Department of Oceanography and Coastal Sciences, Louisiana State University, 1195 Energy Coast and Environment Building, Baton Rouge, Louisiana 70803, USA
seventeenth century that it was massively altered when a confluence of Crown support, large landowners, and investors built large canals (Ash 2017). The soils then dewatered and oxidized to subside at up to 10 cm year−1 in the first few years, and became slower over 100+ years at rates closer to sea level rise (Turner 2004). Eggelsman (1976) showed that these subsidence rates were greater with nutrient availability. Soil subsidence could be caused by the smallest man-made canals, even if water flow rewets a wetland during tidal cycles. One example is from the 90% of the pre-colonial tidal wetlands between
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