The omission of mallards from this list of most common species may prompt some "mallard purists" to dismiss the coastal marshes, but that would be a mistake. The Louisiana coast provides wintering habitat for more than three-quarters of a million mallards, although mallards are outnumbered there by several other duck species. The Lower Mississippi Alluvial Valley, arguably the most important wintering area for mallards in North America, has wintered between 1 million and 2 million mallards in recent years.
The Louisiana coast will not winter 9 million ducks every year. Some years there are more, and some years less. Breeding success, weather, and habitat conditions throughout the flyways may reduce or increase the number of waterfowl wintering on the coast in any given year.
The number in some years may increase substantially. Picture a scenario where breeding habitat conditions and waterfowl production have both been excellent, consequently pouring millions upon millions of ducks into the funnel. Now picture harsh winter weather settling over much of the Midwest, driving the ducks south. If habitat conditions in key areas like the Mississippi Alluvial Valley happen to be below average, then we have the makings for a "perfect storm" of ducks pouring into Louisiana's coastal marshes. Once there, they depend on its rich habitat for winter survival so they can return north to produce a new generation of waterfowl to travel the flyways.
As early settlers moved west, they found abundant resources in the hardwood forests and prairies of the midcontinent. That abundance was linked to the fertile soils that today are the foundation of North America's agricultural breadbasket. Runoff from storms and winter snowmelt constantly erodes these soils and washes the rich sediment into creeks and rivers. These transport fresh water, sediment, and nutrients into the Mississippi River, which carries them downstream to the Gulf of Mexico.And this in turn fuels the productivity of Louisiana's coastal marsh.
The Mississippi River originated at the end of the last ice age when melting glaciers gave rise to a river system that drains more than a third of the continent. A map of North America reveals that the Mississippi's watershed is funnel-shaped, the very funnel that directs midcontinent waterfowl south in fall and guides them north in spring. The origins of the Louisiana coastal marshes, therefore, are in faraway places like western Pennsylvania, where the Allegheny and Monongahela rivers meet to become the Ohio.Far to the northwest, the Milk River brings sediment from southeastern Alberta and northern Montana's prairie country to the Missouri, where it makes its way ever so gradually to the Mississippi and eventually to the Louisiana coast.
When the Mississippi reaches the Gulf of Mexico, fresh water meets salt water in massive quantities. Tidal counterforce applied by the Gulf of Mexico slows the velocity of the river. When the velocity drops, sediment settles and forms land—not just any land, but rich, fertile deltaic marshland made from some of the most productive, nutrient-laden soils on earth. Delta formation occurs on almost all the world's major rivers, but not all rivers receive sediment as rich as that carried by the Mississippi.
Historically, the Mississippi River delta was dynamic. Each pulse of floodwater reshaped the delta. Over the last 7,500 years or so, the mouth and lower main channel of the Mississippi River have shifted, sometimes to the east, sometimes west, and sometimes by as much as 150 miles. Picture the river as an out-of-control fire hose, its end swinging wildly from the pressure of the water it discharges. These channel shifts occurred because deltaic processes built land that eventually impeded the river's flow. Not to be denied access to the Gulf of Mexico, the river sought alternate routes. If there was no other channel to follow, the river would overrun low-lying marshland, and the force of the water would make a new channel. These channel shifts might have been gradual, or they may have occurred dramatically during major flood events.
In any case, the deltaic processes and associated river channel shifts were critical to the formation and maintenance of the Louisiana coastal marsh. Each time the river channel shifted, its sediment-laden water built new marsh. Meanwhile, marsh created around the abandoned channel began to subside (sink) and erode. Marsh creation was more or less equal to marsh loss during the past 7,500 years or so. The river would build delta, and the raised deltaic marshland would force the river to shift channels. The new channel would build new marsh. The marsh built by the old channel but now cut off from its source of sediments subsided as organic materials decayed and soils compacted. Wind and wave action further eroded these areas.