How oilfield canals changed Pecan Island’s marsh

The oil boom left Pecan Island with a new network of waterways. In a 1983 study, the U.S. Geological Survey found that oilfield canals in its mapped Pecan Island area had grown from more than 30 kilometers in 1951 to 135 kilometers in 1968. The change was recorded in maps of the marsh itself, not just in a tally of wells. (USGS, page 33)

The study, by Donald W. Davis and John L. Place, placed that expansion alongside an older working landscape. In 1951, the most extensive local canal network was still the trainasse: narrow passages cut through marsh grass for pirogues and trapping. The authors counted 499 kilometers of those trails in the Pecan Island area that year. Their map distinguished trapping trails, transportation canals and petroleum-related waterways. (USGS, pages 23, 31 and 33)

That distinction matters to the history of the place. The USGS account describes trappers cutting access routes long before the oilfield network reached its later extent. Some small trails widened under repeated use, storms and currents, and some eventually carried petroleum-industry boats. Industry entered a marsh that people had already worked and altered; it added a much larger kind of access route rather than inventing marsh travel. (USGS, page 23)

The canals also changed more than the route a boat could take. In its account of coastal Louisiana, the 1983 report describes excavation turning land into water and dredged soil accumulating in continuous banks beside channels. It identifies altered water movement and chemistry, including saltwater entering formerly fresh areas, among the consequences of canal construction. Those are coastal processes described by the study, not a calculation of how much Pecan Island marsh any one canal destroyed. (USGS, pages 20–24)

Pecan Island also differed from other oil-producing marshes. The authors reported that soils on the Chenier Plain could support drilling equipment on timber board roads. Where those roads could reach a drilling site, a canal was not necessary. They described a lower density of manmade canals there than in the Barataria example examined in the same report. The ground beneath an oilfield helped determine the infrastructure built above it. (USGS, page 33)

The resulting map is a record of a particular area and period. Its 135-kilometer figure is neither a Vermilion Parish total nor a present-day canal inventory. It does, however, show the scale of the oilfield network already in place around Pecan Island by 1968—and why the history of development here is also a history of roads, channels and the marsh between them. (USGS, Figure 12 and page 33)