Restoring the Natural Flow: Diversions Rebuild Wetlands

SWOLLEN WITH waters drained from nearly half of the continental United States, the Mississippi River in flood stage embodies the terrifying and destructive force of nature. In its wake it can leave a trail of devastated fields, uprooted trees and ruined buildings.

To protect human life and property from the ravages of flood, a 1,600-mile system of levees and control structures now constrains the river and funnels its muddy waters into the Gulf of Mexico. But, while shielding cities and industry from disaster, these barriers of earth and concrete are starving the wetlands to death. Deprived of the floodwaters' rejuvenating nutrients and sediment, the coastal wetlands cannot combat the degrading effects of subsidence and saltwater intrusion and are vanishing. To restore the benefits of flooding without the risks of flood, wetland proponents advocate the strategic breaching of the levees, allowing river water to flow into selected marshes and bays. The hopes and challenges associated with this powerful technique are the subject of this issue of Watermarks: Revitalizing Louisiana's Coastal Wetlands.

Fresh water flooding into the marshes delivers nutrients and sediment vital to a thriving wetlands ecology.
Courtesy of David Seawell

River Water Builds Coastal Lands

For eons the Mississippi River has shaped the coast of Louisiana through cycles of growth and decline. Jumping its natural banks and engulfing the delta, the river pushes back saline water driven toward land by wind and tide while depositing nutrients and sediment collected from upstream. Particle by particle, the sediment settles into the alluvial soil of bayous, swamps and marshes. Plants, nourished by river water, take root, trap more sediment, die and decay to create a biomass that traps yet more sediment. Incrementally, wetlands emerge, supporting marsh vegetation, providing habitat for fish and wildlife, and buffering the coast from storm surge and sea. As land accretes, the river inevitably seeks a shorter route to the Gulf, eventually carving out a new channel and abandoning the old wetlands. Deprived of an influx of nutrients and sediment, wetlands sink, succumb to salt water and disappear.

Though today's levees and control structures successfully restrain floods, they also interrupt the age-old process of wetlands formation. The Mississippi, constrained within its banks, is forced to carry its load of land-building sediment into the Gulf of Mexico, where it tumbles to the ocean floor and lies useless for restoring land. Without the rejuvenating sediment and nutrients of the river, existing wetlands decline. No new wetlands emerge. Salt water, advancing rapidly through navigation channels and pipeline canals, invades the estuaries. Vegetation recedes, marshes erode into the ocean, and coastal Louisiana vanishes at the rate of one acre every 35 minutes.

While historically threatening to human life and property, river floods play an essential role in building and sustaining healthy wetlands.
Courtesy of NOAA

Reclaiming Benefits by Mimicking Nature

The best antidote for the decline of Louisiana wetlands may lie within the Mississippi River itself. Purposeful breaching of the levees, known as diversions or river reintroductions, can mimic the natural overflow of the river, routing fresh water, nutrients and sediments into marshes to nourish vegetation and to deposit the material needed to build emergent land.

Although diversions differ in construction and scale, the primary goals of all diversions are to supply marsh-building ingredients and to alter the saline content of the outflow area, boosting critical plant growth and providing a healthy habitat for fish and wildlife.

River Water Builds Wetlands

1. Floodwater carries nutrients and sediment. 2. Sediment settles. 3. Plants take root. 4. Plants trap more sediment. 5. Biomass increases. 6. Wetlands emerge.

Expanded Purpose for an Old Practice

The first levees along the Mississippi River were built in the early 18th century. In the 1830s the first intentional breaching to enhance oyster yields was recorded, and diversions to irrigate crops and stimulate fish production became common. The idea of using diversions to restore wetlands emerged over half a century ago, and is now a leading strategy as agencies affiliated with the Coastal Wetlands Planning, Protection and Restoration Act recognize the Mississippi River as their most powerful ally in combating the current crisis of Louisiana's coastal land loss.