Discharge, suspended sediment, and salinity in the Gulf Intracoastal Waterway and adjacent surface waters in South-Central Louisiana, 1997–2008

Discharge, suspended sediment, and salinity data collected between 1997 and 2008 indicate that the Gulf Intracoastal Waterway (GIWW) is an important distributary of river water and suspended sediments to coastal wetlands in south-central coastal Louisiana. Following natural hydraulic gradients, the GIWW passively distributes freshwater and suspended sediments from the Atchafalaya River to areas at least 30 to 50 miles west and east, respectively, of Morgan City. The magnitude and reach of the discharge in the GIWW increase as stage of the Wax Lake Outlet at Calumet and Lower Atchafalaya River (LAR) at Morgan City increase. The magnitude and duration of discharge vary from year to year depending on the flow regime of the Atchafalaya River. Annual discharge of water in the GIWW was greater during years when stage of the LAR remained anomalously high throughout the year, compared with average and peak flood years. During years when Atchafalaya River flow is low, Bayou Boeuf, a waterway draining the Verret subbasin, becomes a major source of water maintaining the eastward flow in the GIWW. The GIWW is the only means of getting river water to some parts of coastal Louisiana.

The length of time stage of the LAR at Morgan City exceeds a given height has increased from the 1940s to 2008. This shift has increased the length of time the GIWW functions as a predictable distributary of river water each year. Similar shifts in the future could be expected to increase the duration and amounts of river water reaching coastal Louisiana wetlands through the GIWW.

Median suspended-sediment concentrations in the GIWW to the west of Morgan City were around 160 milligrams per liter (mg/L). In the GIWW east of Morgan City, median concentrations were 120–160 mg/L, except in Bayou Boeuf at Railroad Bridge in Amelia and the parts of the GIWW between Bayou Boeuf and the Houma Navigation Canal; median concentrations here were around 100 mg/L.

River water penetrates much of the Louisiana coast, as demonstrated by the large year-to-year fluctuations in salinity regimes of intradistributary basins in response to differences in flow regimes of the Mississippi and the Atchafalaya Rivers. This occurs directly through inflow along the GIWW and through controlled diversions and indirectly by transport into basin interiors after mixing with the Gulf of Mexico. The GIWW plays an important role in moderating salinity in intradistributary basins; for example, salinity in surface waters just south of the GIWW between Bayou Boeuf and the Houma Navigation Canal remained low even during a year with prolonged low water (2000).