environmental setting, water quality, and ecological indicators of

QUANTITY, IN MILLION GALLONS PER DAY
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Aquaculture
Ground Water
Aquaculture
Surface Water
Rice Irrigation
Ground Water
Figure 5. Sources and quantities of water used in the Mermentau River Basin, southwestern Louisiana, 2000.
and Bayou Queue de Tortue enters the river from the east.
The five bayous divide the Mermentau River Basin into
a series of broad, flat areas ideal for agriculture. These
areas are separated by bottomland hardwood riparian
corridors that vary in width from only a hundred feet to
several miles. Land-surface elevations are less than
100 ft above NGVD 29 in most of the basin and less than
25 ft above NGVD 29 along the Mermentau River mainstem.
Thus, the Mermentau River Basin is characterized
by a low gradient and dendritic river system that drains
to the Gulf of Mexico.
Hydrology in the Mermentau River Basin is complicated
by the extensive use of ground water for irrigation
and by modifications for agriculture and navigation.
Water used to flood ricefields may enter the Chicot aquifer
system, be reused downstream for agriculture, or
drain to the Gulf of Mexico. During the study described
in this report, a dam was constructed upstream from one
of the long-term surface-water sites, Des Cannes,
because a train derailed and spilled chemicals (National
Transportation Safety Board, 2002). Modifications of
streams for navigation and water-control structures are
common in the basin.
The Mermentau River Basin is characterized by
free-flowing streams in the north. The low gradient and
low topographic relief in the southern part of the basin
create both bidirectional (downstream and upstream) and
interbasin flow, with water velocities typically less than
10
Rice Irrigation
Surface Water
WATER-USE CATEGORY AND SOURCE OF WATER
Public Supply
Ground Water
1 ft/s. The river is tidally affected, and most upstream
(negative) flow occurs in the summer and fall. Negative
flow can be caused by natural events such as storms and
sustained winds from the south in conjunction with
unusually high tides. These negative flows can be
increased in magnitude and duration during low-discharge
periods, such as occurred during the drought of
1999-2000 (fig. 6). Pumping of surface water for irrigation
may cause bidirectional flow, but typically only in
the smaller tributaries.
The naturally low gradient and human-made canals
and waterways allow interbasin flows. The primary flow
route is the Gulf Intracoastal Waterway (GIWW), constructed
in 1940 (fig. 3). The GIWW crosses the Mermentau
River north of Grand Lake and is a major route
for east-west barge traffic along the Gulf Coast. A complex
series of gates and locks is operated by the U.S.
Army Corps of Engineers (USACE) to manage the
waterway. The GIWW hydraulically connects the Mermentau
River Basin with the Calcasieu River Basin to the
west and the Teche-Vermilion Basin to the east.
In the 1970’s, the natural mouth of the Mermentau
River was bypassed with the construction of a separate
4.6-mi navigation channel from Lower Mud Lake to the
Gulf of Mexico. This navigation channel, the GIWW,
and other hydrologic modifications throughout southwestern
Louisiana are managed by numerous water-control
structures. One such structure on the Mermentau