Lessons Learned from Freshwater Diversions in South Louisiana
Barton D. Rogers
Donna R. Rogers
GEC, Inc.
9357 Interline Ave.
Baton Rouge, Louisiana 70809
Brogers@gecinc.com
225 278-0037
Coastal land loss in Louisiana has reached critical levels, leading to the degradation
and loss of this unique ecosystem. This “Lessons Learned” effort was commissioned by
the Louisiana Coastal Area (LCA) Science and Technology Program (S&T). The LCA
S&T Program is a joint effort by the U.S. Army Corps of Engineers and the Louisiana
Office of Coastal Protection and Restoration to provide sound science and technology
for the planning of coastal restoration. Freshwater, nutrient, and sediment diversions
from the Mississippi River is one important technique being currently used and more
diversions are under consideration. Effects and knowledge of existing diversions is
critical to the planning and design of proposed diversions, as well as to understand the
effects of diversions for flood protection. This is most important when considering the
Flood Fight of 2011 where openings of Bonnet Carré and Morganza Spillways are
critical to protect the infrastructure of south Louisiana.
Nearly 1,300 documents, covering a wide array of topics associated with river
diversions and their effects, were reviewed and evaluated to assist in the planning and
design of proposed diversions. The major types of diversions include large-scale
freshwater diversions, crevasses, siphons, and flood control spillways. The diversions
that appeared to build the most land were those that diverted a significant amount of
sediment, such as the Atchafalaya Delta, Cubit’s Gap, and the Delta Wide Crevasses
Project. Freshwater releases through the Caernarvon diversion have increased the
amount of freshwater and intermediate vegetation. Average aboveground biomass
production was greater at sites near Caernarvon when compared to sites in the lower
basin. Freshwater releases through the Caernarvon and Davis Pond Diversions have
increased the amount of submerged aquatic vegetation. Increased phytoplankton
growth has been observed after Bonnet Carré Spillway openings; algal blooms may
result in fish kills during some diversion years. The abundance of most fisheries
species increased slightly with diversions. Wildlife populations, particularly alligators,
muskrats, and waterfowl, generally increase with freshwater diversion operation. Peak
discharges of diversions during periods of brown shrimp larval recruitment may result in
reduced harvest. Invasive exotic species such as Asian carp and zebra mussels may
be introduced from the Mississippi River by diversions and endangered species such as
pallid sturgeon may be relocated from the river.
Recommendations from the literature include coordinating and sharing information to
maximize benefits on a project-by-project basis and carrying information learned from
past projects into future project development. Gated structures provide for greater
flexibility in operations. Sediment diversions would be more successful if they are
designed after natural crevasses. Structural, hydrological, and human constraints
currently determine the placement and operation of Mississippi River diversions.
Conflicts among user groups have affected diversion construction and operation and
existing diversions are operated conservatively to reduce conflict, limiting their
effectiveness to restore land.