First in a series
The flooding in the Atchafalaya Basin has ruined crops and soaked hundreds of camps, but the water could also change the swamp and bolster one of the few areas of the state’s coast that is not washing away.
Scientists who study the Basin say it’s too early to know for certain what the overall effect will be.
Wildlife, such as deer and the threatened Louisiana black bear, could take a hit, and the fast-moving flood waters could shift the landscape, eroding some areas while building up others.
But the flooding should also flush out stagnant swamp water and carry sediment down to the coast to help build new marsh.
“From our viewpoint, if the marshes could speak, they would be singing hymns of praise,” said D. Phil Turnipseed, director of the U.S. Geological Survey’s National Wetlands Research Center in Lafayette. “Floods, in general, are very beneficial to wetlands on the coast.”
The swollen Atchafalaya River, combined with the Mississippi River water being diverted into the Basin at the Morganza Spillway, is moving a heavy load of sand, silt and clay though the swamp and to the coast below Morgan City.
For the past few decades, the sediment being carried by the Atchafalaya River has actually been building land in that area even as other parts of the coast wash away.
One delta has been growing at the mouth of Atchafalaya River. Another delta is spreading at the mouth of the Wax Lake Outlet, a channel off the main river that was dug in the 1940s to redirect some of the Atchafalaya River west of Morgan City to reduce the flooding threat there.
The deltas first began emerging from the water after the flood of 1973 — the last time the Morganza Spillway was opened. The current flood could mark a new period of growth, said Paul Kemp, vice president of the National Audubon Society’s Louisiana Coastal Initiative.
“It may have been building underneath there for a long time, and then it will pop up after an event like this,” Kemp said. “We would expect to see a significant pulse in the deltas after an event like this.”
The Wax Lake Delta had been growing at a rate of little more than half a square mile per year in the 1980s and 1990s, said Yvonne Allen, a U.S. Army Corps of Engineers research ecologist.
Growth tapered off in the past decade, Allen said, but the new flooding could kick-start another period of expansion.
“I would expect and hope that it’s going to be delivering a lot of sediment with that water,” she said.
The load of sediment being carried through the swamp is not entirely welcome.
Along the southern stretch of the Atchafalaya River through Morgan City, extensive dredging will be needed to keep the river navigable for ship traffic, said Morgan City Harbor and Terminal District Executive Director Jerry Hoffpauir.
“There are going to be sandbars in the river,” he said.
Dredging is an annual chore for the port, but Hoffpauir estimated double or triple that effort will be needed this year.
Within the Basin swamp, federal and state agencies have worked for years on projects to keep sediment from filling in some of lakes, swamps and bayous. Some people worry that the flooding could create new land where it’s not wanted.
“We are losing the entire Basin to sediment, and this is going to aggravate the problem,” said Dean Wilson, director of the non-profit conservation group Atchafalaya Basinkeeper.
The flood now coursing through the Basin could lay down anywhere from a few inches of sediment to possibly a foot in some areas, said Thomas Doyle, chief of the National Wetlands Center Wetland Ecosystem Branch.
The ultimate effect is impossible to know until the water recedes, he said, and different areas of the Basin will see different impacts.
But even with possibly unwanted sedimentation, Doyle said that flooding is part of the natural cycle.
“Generally the flood, other than its threat to human life and infrastructure, is part of the lifeblood and dynamics of the flood plain that rejuvenates it,” Doyle said.
Kemp likened a major flood in the Basin to a “reset of the system,” pushing freshwater over the levees and ridges to flush out stagnant, oxygen-poor areas of the swamp known as “black water.”
Many of those areas seldom see much fresh water because the Atchafalaya River — through dredging and levees — has been managed more to quickly move water to the Gulf of Mexico rather than to flood the swamp.
“Areas will be flooded that have not seen high water in many years,” Kemp said.
In addition to flushing out stagnant water, the floodwaters might also wash away some of water hyacinth and other invasive aquatic plants that have spread throughout the Basin and clogged waterways, said Daniel Kroes, an ecologist and Basin specialist with the U.S. Geological Survey.
The effect on wildlife in the Basin could be a mixed bag.
Crawfish and anything else that lives in the water will likely flourish in the coming years, but deer, bears and other land animals could suffer, said Kenny Ribbeck, chief of the wildlife division for the state Department of Wildlife and Fisheries.
Of particular concern, he said, is the Louisiana black bear, a threatened species with a range that includes parts of the area flooded by the Morganza Spillway.
“We know we have some bears up in trees in the spillway, and we are monitoring them,” Ribbeck said.
He said black bears can survive up a tree for a few weeks, so most should be OK if the water recedes quickly. But bears typically reproduce less in major flood years, he noted.
The state has been working for several years to rebuild the black bear population.
“Any significant loss that would come from this event could set back the black bear effort,” Ribbeck said.
He said the flood could also harm the Basin’s deer population to the extent that Wildlife and Fisheries might need to adjust the length of future hunting seasons to allow for a rebound.
Many of the deer have found temporary homes on levees and ridges or in the woods and farmland on the edge of the Basin. Wildlife officials have opted to leave them alone rather that risk scaring the tired animals back into the water, Ribbeck said.
He said the best approach is to let the animals use their instincts to respond to the natural event.
“Those animals have all evolved based on the habitat they live within,” he said.
About the series
TODAY: The flooding in the Atchafalaya Basin has ruined crops and threatened homes and camps, but that water also could change the swamp and bolster one of the few coastal areas not washing away.
MONDAY: Some coastal groups are calling for a better way to manage the Mississippi River — and the sediment it carries — for the benefit of coastal wetlands and navigation.