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Intro > Classification > Depressions > Unconnected Alluvial Depression
Unconnected Alluvial Depression
Wetland Class: Depressions
Wetland SubClass: Isolated Depression
Introduction: Unconnected Alluvial Depression
Landscape Setting and Hydrology
Unconnected alluvial depressions occur in major river floodplains that have been cut off from the channel by levees, and on terraces and relict alluvial deposits throughout the state. They are not within the 5-year frequency zone for overbank or backwater flooding, and their principal water sources are precipitation and associated runoff from surrounding land surfaces. Those in good condition may support a wide variety of plant species, and some are habitat for rare species such as pondberry. Many of these wetlands have been altered by agricultural activities including drainage works that either reduce or increase water storage within the depression.
The features designated as alluvial depressions differ from sand ponds and valley train ponds. Although all of these features are formed by flowing water, sand ponds and valley train ponds reflect glacial outwash processes while alluvial depressions are clearly the result of stream channel meander activity. They usually have an arcuate or sinuous shape and dimensions consistent with partially filled abandoned channel segments or the swales left behind by migrating channels. Soils are usually very heavy clays very gradually deposited by floodwaters when the depression was still part of the active river floodplain, although erosion from adjacent croplands has added considerable recent material in most instances
Where these wetlands occur in fairly extensive forested areas, their hydrology is controlled primarily by precipitation and local runoff. In heavily agricultural areas, however, drainage networks may terminate in depressions, create connections among formerly isolated depressions, and connect formerly isolated depressions to channel systems. In some cases depressions are used as reservoirs for irrigation water. All of these uses greatly modify the hydrologic environment of isolated depressions. Vegetation
Unconnected alluvial depressions are most commonly the remains of abandoned stream channels, or broad swales left behind by migrating channels. In either case, they typically have an arcuate or linear shape, with the deepest portion down the center line of the basin. Depressions that are deep enough to hold water year-round will have an open-water zone in the center, with baldcypress and buttonbush in areas that are rarely dry, and relatively narrow zones of progressively "drier" plant communities on both banks. In depressions that have largely filled with sediment, the deepest, central part of the basin may be wet for only part of the growing season, and the overstory may be dominated by species such as overcup oak and water hickory. The extended flood durations in alluvial depressions tend to reduce the density and diversity of vine species that are common on adjacent flats and floodplain sites. Ground cover is also restricted by the extended growing season inundation. Lizard's tail is a characteristic plant on these sites. Characteristic Plants
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| Carya aquatica | water hickory | | Fraxinus pennsylvanica | green ash | | Liquidambar styraciflua | sweetgum | | Nyssa aquatica | swamp tupelo | | Populus heterophylla | swamp cottonwood | | Quercus michauxii | cow oak | | Taxodium distichum | baldcypress | |
| Acer rubrum | red maple | | Carex spp. | sedges | | Cornus foemina | stiff dogwood | | Juncus effusus | soft rush | | Planera aquatica | water elm | | Saururus cernuus | lizard's tail | | Scirpus cyperinus | bulrush | | Toxicodendron radicans | poison ivy |
Distribution
Unconnected alluvial depressions are common throughout the Delta, Coastal Plain, and Arkansas River Valley Wetland Planning Regions. They are uncommon, but occasional elements on terraces of large streams in the Ozarks and Ouachitas. Examples of this type can be observed most readily in the vicinity of large rivers, particularly on State Parks and State Wildlife Management Areas.
Other Isolated Depression Community TypesMountaintop Depression
Sand Pond
Sinkhole
Valley Train Pond
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