Geomorphic Origins

Streams in steep topography tend to undergo continual downcutting and, consequently, act as sources of fine and coarse material with little to no opportunity for deposition. Sediment loads are easily carried downstream because the high gradient of the channel imparts sufficient energy for water to retain particles. In many cases, as streams emerge from steeper terrain and move into flatter areas such as coastal plains, the gradient of the channel decreases and flows may spread and lose energy. This

(a) Nonincised stream Surface 3

Hydrologie floodplain

(a) Nonincised stream Surface 3

Hydrologie floodplain

Surface 3

Surface 2 Surface Surface 2 i v

Bankfull channel

(b) Incised stream (early widening phase) Surface 3

Hydrologic

Surface 3

Figure 3 Cross-sectional view of a floodplain topographic positions. Adapted from Hodges 1998.

Surface 3 Terrace

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Terrace

c?

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Incised, widening channel

Incised, widening channel

(c) Incised stream (widening phase complete) Surface 4

Surface 4 Terrace

(c) Incised stream (widening phase complete) Surface 4

Surface 4 Terrace

Bankfull channel

Figure 2 Terraces in (a) nonincised and (b and c) incised streams. In Stream Corridor Restoration: Princples, Policies, and Practices (10/98). Interagency Stream Restoration Working Group (15 federal agencies) (FISRWG).

Bankfull channel

Figure 2 Terraces in (a) nonincised and (b and c) incised streams. In Stream Corridor Restoration: Princples, Policies, and Practices (10/98). Interagency Stream Restoration Working Group (15 federal agencies) (FISRWG).

promotes the occurrence of overbank flow and creation of deposition surfaces or sediment sinks. However, a floodplain may shift between being a sediment sink or source depending on hydrologic changes induced by climate, anthropogenic activities, or other influences. Downcutting also occurs as older floodplains are abandoned by streams and become terraces which resemble stair steps in cross-section (Figure 2).

Sedimentation occurs as particles settle during sheet-flow and is highly variable both temporally and spatially. Sediment deposition or alluviation makes possible the high soil fertility that is generally associated with many floodplains although there are notable exceptions. Rates of sediment accumulation vary markedly among flood-plains and, in the southeastern United States for example, range from 1 to 6mmyr~\

Deposition and scouring may often occur simultaneously on different portions of floodplains and at different times in individual locations. Consequently, the scale at which sedimentation is assessed is very important in gaining an accurate assessment of net changes. The

Figure 3 Cross-sectional view of a floodplain topographic positions. Adapted from Hodges 1998.

result of the spatial irregularities is a pattern of swales and berms that generally runs parallel to the stream course. The convex and concave microrelief may represent elevation differentials of only a few centimeters. Nonetheless, those minor differences have major importance in defining soil environments for vegetation and in influencing the extent of contact between floodwaters and the floodplain surface. In many cases, the microtopography of major floodplains is somewhat predictable (Figure 3) and, similarly, drives spatial patterns of species composition and NPP of vegetation communities. However, changes in microreliefmay be much less apparent on some floodplains due to either prolonged or infrequent flooding.

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