South-Central Interior Large River Floodplain

This floodplain system is found in the Interior Highlands as far west as eastern Oklahoma, as well as throughout the Interior Low Plateau, Cumberlands, Southern Ridge and Valley, and Western Allegheny Plateau, and lower elevations of the Southern Blue Ridge. Examples occur along large rivers or streams where topography and alluvial processes have resulted in a well-developed floodplain. A single occurrence may extend from river's edge across the outermost extent of the floodplain or to where it meets a wet meadow or upland system. Many examples of this system will contain well-drained levees, terraces and stabilized bars, and some will include herbaceous sloughs and shrub wetlands resulting, in part, from beaver activity. A variety of soil types may be found within the floodplain from very well-drained sandy substrates to very dense clays. It is this variety of substrates in combination with different flooding regimes that creates the mix of vegetation. Most areas, except for the montane alluvial forests, are inundated at some point each spring; microtopography determines how long the various habitats are inundated. Although vegetation is quite variable in this broadly defined system, examples may include Acer saccharinum, Platanus occidentalis, Liquidambar styraciflua, Populus deltoides, and Quercus spp. Understory species are mixed, but include shrubs, such as Cephalanthus occidentalis and Arundinaria gigantea, and sedges (Carex spp.). This system likely floods at least once annually and can be altered by occasional severe floods. Impoundments and conversion to agriculture can also impact this system.

Vegetation varies quite widely, encompassing shrubby and herbaceous communities, as well as forested communities with a wide array of canopy types. Examples may include Acer saccharinum, Platanus occidentalis, Liquidambar styraciflua, and Quercus spp. Understory species are mixed but include shrubs, such as Cephalanthus occidentalis and Arundinaria gigantea (= ssp. gigantea), and sedges (Carex spp.).

This system inhabits broad floodplains along large creeks and rivers that are usually inundated for at least part of each year. Flood frequency depends on precipitation patterns within the watershed and proximity to the main channel. Areas adjacent to the main channel or low islands within the channel can be flooded every year or even more than once per year. Those areas further from the channel on terraces or behind natural levees may only be flooded once every several years. Free-flowing rivers migrate across their floodplain, cutting new channels or eroding the bank on one side while building up the bank on the other, so the flooding regime of any one point in the floodplain will change over time. Flooding redeposits alluvium, eroding some areas and aggrading others, can bury or wash away small plants, and redistributes nutrients, especially in less frequently flooded zones where silt and clay tend to be deposited. These processes open up new areas for colonization.

Flooding dynamics are an important factor in the development and maintenance of this system. Flood frequency depends on precipitation patterns within the watershed and proximity to the main channel. Areas adjacent to the main channel or low islands within the channel can be flooded every year or even more than once per year. Those areas further from the channel on terraces or behind natural levees may only be flooded once every several years. Free-flowing rivers migrate across their floodplain, cutting new channels or eroding the bank on one side while building up the bank on the other, so the flooding regime of any one point in the floodplain will change over time. Flooding redeposits alluvium, eroding some areas and aggrading others, can bury or wash away small plants, and redistributes nutrients, especially in less frequently flooded zones where silt and clay tend to be deposited. These processes open up new areas for colonization.

This system has been heavily impacted by human activities. The flooding and channel migration that is important in maintaining this system has been affected by attempts to contain the channel in its current location through bank armoring (riprap or other bank stabilization techniques) and channelization (man-made levees, dredging, wing dams, closing dams). While these may not immediately affect large areas of this system the changes to the flooding regime have longer term impacts. Dams, built for irrigation, recreation, hydropower, or navigation, have immediate impacts by flooding the reservoir area and increasing the amount of open water compared to floodplain. They have longer term effects by changing the flooding pattern, reducing the amplitude of low water in the upstream pool and of high water both upstream and downstream. Dams also trap much of the sediment being transported by the river and reduce the erosion and deposition rates downstream (Johnson 1992).

Agricultural and urban development has affected many examples of this system. Direct conversion to cropland or pastures can destroy this system. Indirect effects of agricultural or urban development within or near the floodplain include increased sediment loads from erosion and chemical pollution from pesticides, herbicides, fertilizer, industrial activities, road maintenance, and other sources.

Grazing by native species was not likely an important factor shaping this system, but grazing or browsing by high white-tailed deer populations or domestic livestock can impact this system and lead to decreased cover of many graminoids and some sensitive forbs. Weedy invasives can dominate parts of the floodplain. Several herbaceous species are particularly aggressive and can dominate floodplain marshes, sometimes forming near mono-cultures. These include Phragmites australis, Phalaris arundinacea, Typha x glauca, and Lythrum salicaria. Other weedy species can become abundant in the understory of floodplain forests.

A serious threat to stands of this system that contain Fraxinus spp. is emerald ash borer (Agrilus planipennis). This exotic beetle has seriously affected Fraxinus spp. trees in southern Michigan and is projected to continue to spread throughout the range of Fraxinus spp. in the Midwest and Northeast by 2045 (DeSantis et al. 2012). After prolonged infestation, mortality of Fraxinus spp. is nearly 100% (Herms et al. 2010).

This system ranges from the Ozarks, Arkansas River Valley, and Interior Low Plateau to the Southern Blue Ridge and north into the Western Allegheny Plateau.