Despite the extensive study of bottomland hardwood forests in the southern United States, there is not a well defined set terms used to describe bottomland systems and the ecological zones which occur between the river channel and the upland environment. These zones are often separated by less than a few feet but the soil and hydrological conditions within each zone may be quite different leading to changes in vegetation. This section summarizes the development of bottomland systems and establishes a standard set of terms and characteristics which define the ecological zones within bottomland hardwood forests. These terms and characteristics will be used consistently throughout this guide and will be explained in greater detail in the Hydrology, Soils, and Vegetation sections.
Bottomland hardwood forests are one of many important riparian ecosystems in the United States. The term “bottomland hardwoods” was first used to describe forests of the southeastern US that occurred on river floodplains. Years later, Huffman and Forsythe (1981) used the term to describe floodplain forests throughout the eastern and central US that have the following characteristics:
- The area is periodically inundated or saturated by surface or groundwater during the growing season.
- Soil saturation occurs periodically within the root zone during the growing season.
- Common tree species in the area have morphological and/or physiological adaption(s) which allow them to survive, reach maturity, and reproduce in an environment where the soils within the root zone may become anaerobic during periods of the growing season.
Bottomland hardwood forests typically have distinct ecological zones at different elevations and flood frequencies. These zones are the result of an active river cutting its banks and forming new land (Putnam 1960). The soil which erodes from the bank is often deposited on the inside bend downstream on what is referred to as a Point Bar. As the erosion continues, sedimentation buildup increases the height and extent of the point bar and is considered New Land. Repeated flooding raises these bars above the normal high-water level creating what is referred to as Front Land.
Subsequent floods overflow the front land and deposit coarse sediment near the river edge forming high, well-drained Ridges or Natural levees. Beyond these ridges the waters eddy and slow, and between flood events these backwaters are inundated by tributaries. Fine sediment is deposited in these areas and low, broad stretches of poorly drained soils form. Minor relief features such as Low ridges, Flats, and Sloughs breakup the backwater areas into distinct zones - this relief is often small and variable and in many cases unrecognizable. Low ridges are formed by old, smaller watercourses and are generally more common than fronts or levees. Flats occur between ridges and are typically wide, with poor surface drainage. Sloughs are the remnants of former watercourses. These areas just described are often collectively referred to as First bottoms. They represent relatively recent or present floodplains and contain newer deposits of alluvium and less mature soils.
Within or beyond first bottoms, shallow Swamps may be formed when new fronts obstruct drainage from low flats. Areas beyond the first bottom where the river migrated at a relatively early stage and where the soils are much more mature are commonly referred to as Second bottoms, even if they are similar in elevation to the first bottom. Second bottoms are remnants of former floodplains left when a general uplift or tilting of the earth’s surface causing the riverbed to erode its way to a lower elevation. Beyond the second bottom, Terraces may form the primary parts of the valley along slow-moving river systems.
Figure 1 illustrates the different ecological zones within a bottomland system. As elevation from the aquatic ecosystem increases, flooding frequency and intensity often decrease. The illustration of ecological zones is useful from a descriptive sense and can be used to describe conditions along river floodplains in the Upper Midwest, however it is important to note that Figure 1 is not intended to be used as a management tool because it does not consider the many interactions which occur within and between zones and across the entire riparian area.
Click to enlarge
Figure 1. Cross-sectional view of a bottomland system.
Below are brief descriptions of the different zones in Figure 1 for more information on each zone and how it is formed see the Site Characteristics section of this guide or click on the zone below.
I. River. Open water at all times of the year.
II. Point bar. Sedimentation from river bank erosion upstream.
III. Natural levee or Front. High, well-drained ridges that form from repeated floods that overflow the front lands.
IV. First bottom. Relatively recent or present floodplains which contain newer deposits of alluvium and less mature soils.
1. Low ridges. Banks of older, smaller or less permanent watercourses.
2. Flats. Areas between ridges which are typically wide, with poor surface drainage.
3. Sloughs. Channels from former water courses.
V. Swamp. Abandon section of channel which becomes an oxbow lake and, and as sedimentation continues, a swamp.
VI. Second bottom. Remnants from former floodplains, left when a general uplift or tilting of the earth’s surface caused the riverbed to erode its way to a lower elevation.
VII. Low terrace. Form the primary parts of valleys of slow-moving river systems.