The U.S. Mid-Atlantic Coast (especially Chesapeake Bay, Delaware Bay, Currituck Sound, and Pamlico Sound) historically wintered large numbers of waterfowl, although changes in these estuaries reduced their attractiveness to ducks during the 1900s. Sea level rise is likely to reduce further the amount of suitable shallow water habitat. The latest assessment report for this region predicts a relative sea level rise of 7.5 inches by 2030, and 26 inches by 2095. Chesapeake Bay salt marshes do not receive sufficient sediment and organic matter to keep pace with current rates of sea level rise, and the discrepancy between sea level rise and sediment accumulation rates is likely to widen in the coming years.
Changing climate also may affect stream flows that could, in turn, affect local salinity, nutrient loading, and aquatic food webs. Maintaining or improving water quality in the Chesapeake Bay and other Mid-Atlantic estuaries will be challenging given the projected regional growth in human populations. Species that are able to move to agricultural lands for food may face fewer limitations than the diving ducks dependent upon aquatic foods.
Mississippi Alluvial Valley
Uncertainty about future precipitation and runoff also clouds predictions for the Mississippi River Basin, the third-largest drainage system in the world. More than half of the land area of the basin is devoted to cropland, much of that former bottomland hardwood forests. Wetlands in the upper basin provide important breeding and staging habitats for Mississippi and Atlantic flyway waterfowl. The lower basin is the most important wintering area on the continent for mallards, and supports large numbers of other dabbling ducks and wood ducks.
The extent of winter flooding in the valley affects body condition and winter survival of mallards. Currently, however, different climate models offer contrasting predictions about future river flows, leaving us with little ability to predict future flooding patterns in the valley and, thus, the future suitability of the region as waterfowl habitat.
Great Lakes shippers struggled last winter as water levels from Duluth to Montreal continued to recede. Now some 39 inches below 1997 levels, the lakes are edging ever closer to record low levels. Reduced precipitation and runoff, coupled with warm temperatures (which means reduced ice cover and more lake-effect snows) are keeping it that way. If climate modelers are on track, the Great Lakes of 2001 will be more the norm than an anomaly. For waterfowl, this could be mixed news. Certainly, in the short term, lakeside wetlands could suffer from lower water levels. Reduced water volume could also concentrate nutrients and pollutants entering the lakes, degrading waterfowl food supplies. On the other hand, if we can muster support for far-sighted coastal zone planning, there may be opportunities for enhancing and protecting new shallow-water habitats for wildlife as water levels gradually recede.
Central Valley of California
Until the 19th century, the Central Valley of California contained one of the largest complexes of wetlands in the United States. Drainage for agriculture and human settlement eliminated some 95 percent of those wetlands, although many basins have been restored in the last 20 years, and flooded rice fields provide thousands of acres of supplemental habitat. The densities of waterfowl wintering in California are generally the highest to be found in the United States, so DU views any threat to the integrity of these wetlands with concern.
Recent studies predict that warmer temperatures will cause more precipitation in the Sierra Nevada Mountains to fall as rain. More rapid runoff and earlier snowmelt would lead to higher winter flows and reduced summer flows in most California rivers. Decreased summer stream flows would intensify competing demands for water. Moderate flooding in the Central Valley probably benefits wintering waterfowl by increasing the amount of feeding and refuge habitat available to the birds, while simultaneously reducing crowding and the likelihood of disease transmission. So, if future winters are wetter than today, waterfowl may benefit. The value of this flooding, however, depends critically on underlying land use. If the extent of rice culture in the valley were replaced by less-wildlife-friendly crops or if it is reduced in the future (for instance, if irrigation water becomes too costly), then winter flooding of agricultural land would be of little value to waterfowl.
Along the California coast, sea levels are projected to rise by eight to 12 inches in the next century. Shallow tidal habitats could be reduced substantially because human development will limit inshore "migration" of coastal wetlands. Increased winter stream flows following decreased summer flows to the Delta and San Francisco Bay are predicted to result in higher concentrations of contaminants in the estuary. Diving duck habitats are generally more limited along the Pacific Coast than the Atlantic, so any deterioration of habitat quality would be cause for concern.