The Lower Atchafalaya River and the GIWW spread Floodwater to the Coastal Marshes

THE LOWER ATCHAFALAYA RIVER

The Atchafalaya River exits its basin through a complex series of lakes and passes that carry it through the Teche ridge, which protects its basin from saltwater intrusion from the Gulf of Mexico.

The Lower Atchafalaya River at the GIWW at Berwick

Lower Atchafalaya River at the Intracoastal Waterway: Berwick

St. Martin Parish, Louisiana

The Gulf Intracoastal Waterway (GIWW) runs across the entire Louisiana gulf coast, cutting across distributaries in the Deltaic Plain and running parallel to the border between the uplands and the wetlands. Between the Rigolettes, which flows out of Lake Pontchartrain on the east, and the Sabine River, which forms the Louisiana-Texas border on the west, the GIWW captures water streaming out of the uplands and carries it both east and west. It captures water from the Lower Atchafalaya River where it crosses it just south of Berwick. It does the same where it crosses the Wax Lake Outlet. During spring floods the Intracoastal Waterway distributes freshwater and sediment to the coastal wetlands. It links the various interdistributary basins in the Deltaic Plain and at certain times of the year it delivers freshwater and sediments to the wetlands to its south. If we better understood the flow of water and sediment through the waterway and its relationship to the Lower Atchafalaya River, we could formalize the waterway’s role in refreshing the wetlands.[i]

Louisiana’s coastal waters are on the average never more than a foot above sea level and usually less, river stages along the Atchafalaya rose steadily in the last sixty years of the twentieth century, an average of two feet higher at its crossing with the Intracoastal Waterway. With this head the Lower Atchafalaya drives water in the Intracoastal Waterway to the east as far as eighty miles from their intersection and to the west as far as fifty miles. Even at mid-summer when the Atchafalaya is running low, it drives flows in the waterway opposite directions from this intersection.[ii]

The Lower Atchafalaya River at the GIWW

The relationship between the Lower Atchafalaya and the GIWW explains to some extent how water from the Morganza Spillway into the East Atchafalaya Floodway will spread both east and west through the Louisiana Coastal Marshes.

Floodwater from the Atchafalaya Floodway spreading east and west through the Coastal Marshes

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[i] Swarenski, Christopher M., Surface-water hydrology of the Gulf Intracoastal Waterway in South-central Louisiana, 1996-99, U.S. Geological Survey Professional Paper 1672, 2, http://pubs.usgs.gov/pp/pp1672/pdf/pp1672.pdf.

[ii] Swarzenski, Christophe, “The Intracoastal Waterway as a distributary of Freshwater and Sediments to Coastal Louisiana, “ paper delivered to AUG, 1998 Meeting, http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=sm98&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Fsm98%2Fsm98&maxhits=200&=”OS42B-02.”