Atlantic Coastal Plain Blackwater Stream Floodplain

This Atlantic Coastal Plain system, which is most abundant in the Carolinas and Georgia, occurs in floodplains of small streams that carry little mineral sediment (blackwater streams). These streams occur in low areas within sandy portions of the Coastal Plain. The water is usually strongly stained by tannins and other dissolved organics and has little suspended mineral sediment. Depositional landforms may be absent or present in limited variety and of small size. Soils are usually strongly acidic. The duration of flooding is long (semipermanent) in the wettest areas, and shorter in slightly higher gradient small streams. Some small blackwater streams near the Fall-line Sandhills have most of their flow from sandhill seepage and have limited fluctuation in water levels. But other blackwater stream channels may dry out during the late summer. In these cases, water tables are not far below the channel, and are high enough that the deeper depressions may still hold water. Vegetation varies from north to south, but generally consists almost entirely of forests of wetland trees, but occasional, small shrub-dominated sloughs may also be present. A variety of tree species may be present; wetter examples (especially toward the northern range limits of this system) are often strongly dominated by Taxodium distichum and Nyssa biflora. Other examples have mixtures of these species with Quercus spp. and other bottomland hardwoods tolerant of blackwater conditions. Species richness ranges from low to moderate, but is lower than in comparable brownwater systems. Flooding is an important ecological factor in this system and may be the most important factor separating it from adjacent systems. However, the high water table supported by inflow from adjacent areas also maintains these areas as wetlands. Flooding excludes non-flood-tolerant species. Unlike river systems, flooding tends to be variable and of shorter duration.

Vegetation in the wetter zones consists of forests composed of obligate wetland trees. Composition is more diverse and variable in other areas that are less prone to flooding. Wetter examples are strongly dominated by Taxodium distichum and Nyssa biflora. Other examples have mixtures of these species with Quercus spp. and other bottomland hardwoods tolerant of some flooding and drier conditions in the summer (Burke et al. 2003). Except in the very wet examples, understory, shrub, and herb layers are generally well-developed, and woody vines are also prominent. Species richness ranges from low to moderate but is lower than in comparable brownwater systems.

Examples of this system occur in floodplains of small streams of the Atlantic Coastal Plain that carry little mineral sediment (blackwater streams). These streams occur in low areas within sandy portions of the Coastal Plain (Smock and Gilinsky 1992). The water is usually strongly stained by tannins but has little suspended clay and is not turbid. Depositional landforms may be absent or may be present in limited variety and of small size. Soils are generally sandy in drier portions of the floodplain, mucky in wetter portions, or may be uniform organic soils. Soils are usually strongly acidic, but spring-fed rivers or streams may have local components with calcareous water and non-acidic soils. Flooding ranges from semipermanent in the wettest floodplains to intermittent and short in slightly higher areas and along higher gradient streams. Some small blackwater streams near the Fall-line Sandhills have most of their flow from sandhill seepage and have limited fluctuation in water levels, but other blackwater stream channels may dry out during the late summer. In these cases, water tables are not far below the channel, and are high enough that the deeper depressions may still hold water (Smock and Gilinsky 1992). Sediment oxygen demand is high in blackwater swamp areas which have long-duration flooding and high amounts of total organic carbon in the soil and sediments. Evidence suggests that blackwater streams may naturally be low in dissolved oxygen (Todd et al. 2010).

The fluvial features of riverine floodplains occur less frequently along small streams. These features, such as river terraces, oxbows, alluvial flats, point bars, and streamside levees, may occur, but on a smaller scale and sometimes are poorly developed. Fine-scale alluvial floodplain features may be abundant. In pre-European settlement forests, community diversity in these streamside systems was much more complex than in the modified landscapes of today. Fire and beaver activity created a mosaic whose elements included canebrakes, beaver ponds and grass-sedge meadows in abandoned beaver clearings, as well as the streamside zones and mixed hardwood and/or Pinus spp. forests that make up more than 95% of the cover that exists today. The most prominent evergreen south of Virginia is the shade-intolerant Pinus taeda, which manages to maintain itself by reproducing in larger (multi-tree) treefall gaps.

Flooding is an important ecological factor in this system and may be the most important factor separating it from adjacent systems. Flooding brings nutrients and excludes non-flood-tolerant species. Unlike river systems, flooding tends to be variable and of shorter duration. It is unclear how important aquatic fauna are when the system is flooded, but they may be important. The small flows, low gradient, and binding of sediment by vegetation limit channel shifts and sediment movement, but floods may cause local disturbance by scouring. The areas flooded for the longest durations tend to have accumulations of organic sediments which deplete levels of aquatic dissolved oxygen (Todd et al. 2010). Most of these forests would exist naturally as multi-aged old-growth forests driven by gap-phase regeneration. Windthrow is probably the most important cause of canopy gaps.

Fire is probably more important than in larger river systems, because distances to uplands are short and because stream channels and sloughs are smaller and less effective as firebreaks. However, most of the vegetation is not very flammable and usually will not carry fire. Some of these areas apparently were once canebrakes, which presumably were maintained by periodic fire. Fire-return interval varied highly in this system. Except in canebrakes, most fires were very light surface fires, creeping in hardwood or pine litter with some thin, patchy cover of bottomland grasses such as Chasmanthium laxum and Chasmanthium latifolium. Flame lengths are typically 15-30 cm (6-12 inches) (Landfire 2007a). Even so, fire-scarred trees can be found in most small stream sites except in the wettest microsites. Stand-replacement fires are unknown in this type. Except where Native American burning was involved, fires likely occurred primarily during drought conditions and then often only when fire spread into bottomlands from more pyrophytic uplands. Trees may be partially girdled by fire in duff, followed by bark sloughing. While fire rarely killed the tree, this allowed entry of rot, which, in the moist environment, often resulted in hollow trees, providing nesting and denning habitat for many species of birds and animals. Surface fires occurred on a frequency ranging from about 3 to 8 years in streamside canebrake, streamside hardwood/canebrake, or pine, to 25 years or more in hardwood litter. Low areas having a long hydroperiod, islands, and areas protected from fire by backswamps and oxbows were virtually fire-free. Fire effects were largely limited to top-kill of shrubs and tree saplings less than 5 cm (2 inches) diameter, and the formation of hollow trees.

The distinctive dynamics of stream flooding and protected topographic position dominate the distinctive vegetation of this system. The small watersheds and sometimes higher gradients on these streams may limit floods to fairly short duration. Flooding is most common in the winter, but may occur in other seasons. The sorting of plants by depositional landforms of different heights suggests that wetness or depth of flood waters has significance. In higher gradient streams, flood waters have significant energy. Scouring and reworking of sediment make up an important factor on the streambanks, and channels may occasionally change course. In addition to disturbance, floods bring nutrient input, deposit sediment and disperse plant seeds. However, because of the limited sediment transport, nutrient input is less in blackwater stream systems than in other floodplains. Stream flooding rarely leads to canopy tree mortality.

The most significant natural disturbance along small streams is wind. Winds create gaps, usually of small to medium size, in which trees regenerate and where smaller vegetation temporarily proliferates. Winds affect streamside forests because of wet sandy or mucky soils, and trees that are shallow-rooted. Canopy tree mortality was generally limited to tree-by-tree or small group replacement. Windthrow formed the primary cause of tree mortality in bottomlands. The frequency of these events equates with major hurricanes occurring at approximately 20-year intervals. Tornado tracks can be found passing across uplands and bottomlands, leaving narrow swaths of felled trees. The majority of windthrow seems to have been the result of hurricanes and tornadoes spawned by them. However, some of the most abundant tree species of Coastal Plain blackwater stream floodplains, Taxodium distichum and Nyssa biflora, are notably stable in strong winds. Susceptibility to wind mortality may depend on the species composition of a given community.

Beavers were once an important part of the dynamics of these systems, one which is returning to higher frequency in some areas. Beavers can dam the main channel of many small streams, and create ponds which can cover the entire width of the floodplain for a stretch (M. Schafale pers. comm. 2013). Ponds are often built in series, so that as much as a kilometer or two of the stream may be affected. Most of the crucial parameters of beaver dynamics under natural conditions are unknown or poorly known. Abundance of beavers, duration of a colony in a given place, and whether dam sites were chosen at random or whether specific favorable sites were repeatedly used would have had major effects on the ecology of this system. The existence of a diverse flora of native aquatic plants of ponds, which appear to take long times to colonize a pond (they are found in greater diversity in 100+-year-old millponds than in younger impoundments) hints that beaver ponds may have been long-lasting features. Impoundment drowns the lower strata of plants and displaces non-aquatic fauna, leading to colonization by aquatic plants and shade-intolerant marsh herbs and shrubs. However, Taxodium distichum and Nyssa biflora trees in the swamp forest may survive to provide a partial to complete tree canopy. When beavers abandon a pond, the dam will eventually breach, but sometimes remains and at least partially impounds the area for a long time. With the limited mineral sediment input, long-standing ponds fill with muck, sometimes developing boggy vegetation that may persist for many years (M. Schafale pers. comm. 2013).

The most critical anthropogenic threats are ongoing canopy removal from large-scale regeneration logging and intensive forest management; hydrological alteration, drainage and channel modifications; water quality impacts (eutrophication) from surrounding agriculture, range, development and urbanization; and water withdrawals. There are few, if any, remaining watersheds in the Coastal Plain that have not been significantly impacted by human activity (Smock and Gilinsky 1992).

Conversion of this type has primarily resulted from clearing of the forest cover, artificial impoundment, drainage through channelization, and levee building (Smock and Gilinsky 1992). These and other forms of ecosystem alteration continue to be extensive in these systems. Channelization isolates the channel from the floodplain, reducing the frequency and magnitude of floodplain inundation. In small floodplains, the spoil piles from channel digging can also cover a significant portion of the floodplain. Channel alteration, excessive scouring, and sediment deposition as a result of impervious surfaces and vegetation clearing in the watershed can be significant. Repeated logging disrupts the natural structure of the forests and may permanently alter their species composition. Feral hogs (Sus scrofa) conduct rooting of the soil, destroying native vegetation and soil-dwelling animals (Engeman et al. 2007).

Alteration of natural hydrologic processes through impoundment and channelization have severely disrupted the function, structure, and species composition of large areas of bottomland forest. In addition, the widespread introduction of Ligustrum sinense, Microstegium vimineum, and other exotic invasives has dramatically reduced native diversity in the understory of some examples. The most significant potential climate change effects over the next 50 years include alteration of waterflow, most likely periods of drought alternating with more intense storms. Ligustrum sinense, Microstegium vimineum, and other invasive exotic plants have dramatically reduced native diversity in the understory of some examples. The most significant potential climate change effects over the next 50 years include alteration of waterflow, most likely periods of drought alternating with more intense high-flow events associated with storms with heavy rain in areas of the watershed.

This system is potentially found throughout the Atlantic Coastal Plain north to about the James River in Virginia, but it is most abundant in the Carolinas and Georgia.