The Transitional Upland Fringe

The transitional upland fringe serves as a transitional zone between the floodplain and surrounding landscape. Thus, its outside boundary is also the outside boundary of the stream corridor itself.

Whereas stream-related hydrologic and geomorphic processes might have formed a portion of the transitional upland fringe in geologic times, they are not responsible for maintaining or altering its present form. Consequently, land use activities have the greatest potential to impact this component of the stream corridor.

There is no typical cross-sectional structure for this component. Transitional upland fringes can be flat, sloping, or in cases such as ravine walls, nearly vertical. Their width may vary substantially, and their outer boundaries are often indistinct. They can incorporate features such as hillslopes, bluffs, forests, and prairies, often modified by land use. All transitional upland fringes have one common attribute, however -- they are distinguishable from the surrounding landscape by some kind of greater connection to the floodplain and stream, such as the continuation of floodplain forests or other natural vegetation part-way up the hillslopes or on the edges of upland terraces.

An examination of the floodplain side of the transitional upland fringe often reveals one or more flat-topped benches. These landforms are called terraces. They are formed in response to new patterns of streamflow, changes in sediment size or load, or changes in watershed base level--the elevation at the watershed outlet. Although many terraces were formerly active floodplains, most have become isolated from periodic flooding by the stream in recent years. In some regions of the country, some of the higher, older terraces were formed by glaciation.

Terrace formation can be explained using the aforementioned stream balance equation. When one or more variables change, equilibrium is lost, and either degradation or aggradation occurs.

The figure at left presents an example of terrace formation by channel incision. Cross section A represents a nonincised channel. Due to changes in streamflow or sediment delivery, equilibrium is lost and the channel degrades and widens. The original floodplain is abandoned and becomes a terrace (cross section B). The widening phase is completed when a floodplain evolves within the widened channel (cross section C).

Vegetation Across the Stream Corridor
Vegetation is an important and highly variable element in the stream corridor. In some minimally disturbed stream corridors, a series of plant communities might extend uninterrupted across the entire corridor. The distribution of these communities would be based on different hydrologic and soil conditions. In smaller streams the riparian vegetation might even form a closed canopy above the channel.

Plant communities play a significant role in determining stream corridor condition, vulnerability, and potential for (or lack of) restoration. Thus, the type, extent and distribution, soil moisture preferences, elevation, species composition, age, vigor, and rooting depth are all important characteristics that must be considered when planning and designing stream corridor restoration.

Although vegetation patterns vary widely among different stream and river types, sizes, and regions of the country, the following are some of the more common vegetative patterns:

Submerged aquatic plants will grow in many types of channels if the substrate and stream velocity allow for rooting (see a, at left). Obligate hydrophytes (plants requiring submerged or saturated conditions) including floating rooted, free floating, and emergent herbaceous and shrub species can become established in the edges of slow-flowing waters of shallow- gradient rivers and deeper streams. Examples may include coontail, duckweed, pond lily, and cattail.

Gravel bars, shores and active floodplains within the bankfull channel (see b, at left) may support stands of herbaceous plants and small shrubs or tree seedlings temporarily, but these communities are displaced by moderate flooding almost yearly. These plants are often somewhat water-tolerant but are not necessarily obligate hydrophytes; examples include stargrass, smartweed, and seedlings of cottonwood, alder and willow.

In forested regions of the US, plant communities dominated by shrubs and trees are found on the floodplains just above the bankfull channel (see c, at left). These communities can vary with distance from the water. For example, alders and willows may dominate the edge while mature trees begin to dominate farther from the stream.

In the absence of land use disturbances, the transitional upland fringe will often consist of forested terraces and hillslopes in most regions. In desert and grassland regions, however (see d, at left), the fringe may mark the end of riparian woodlands or shrubs and the beginning of dry grasslands or desertscrub.