Regulated Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 64

sand, silt, and debris closed the lower part of the channel. Clearing in the watershed continued in
the 1940s. During the 1960s, additional channelization extended the ditch downstream, realigning
and dredging the mouths of all major tributaries with the construction of erosion control structures.
The channel response was:

• Headward-migrating knickpoints in all tributaries

• Extensive streambank erosion

• Recruitment of large woody debris, and the deposition of sands and debris at the terminus

of channelization (Langendoen 2009)

A knickpoint is some location where there is a local, sharp change in the channel slope.
Knickpoints may occur, for example, where a tributary enters the channelized river or the chan-
nelized river transitions to a more natural river. Knickpoints are often marked by waterfalls, as
illustrated in Figure 3.22. Knickpoints often tend to migrate toward the headwaters of the channel
through the drainage network.
Streambank erosion is a natural process. However, accelerated streambank erosion, which could
occur due to channelization or other changes, is a major cause of nonpoint source pollution associ-
ated with an increased sediment supply (Rosgen et al. 2001), and often accounts for the majority of
total watershed sediment yields. For example, in his studies of a selected stream in the southeastern
plains ecoregion, Ramirez (2011) found that the sediment supply from incised streambanks repre-
sented up to 70% of the entire sediment discharge observed within the watershed. Other studies
have shown that the sediment from streambanks can account for up to 90% of watershed sediment
yields (Simon et al. 2002; Capello 2008).
Streambank erosion typically results from one of two processes: hydraulic action (also called
hydraulic erosion, luvial erosion, or tractive erosion) and gravitational mass failure (Ramirez
2011). These two processes are often linked where the hydraulic processes are often a precursor to
gravitational failures (e.g., a hydraulic-induced mechanism, such as streambank undercutting, can
cause a gravitationally induced collapse such as a cantilever failure [Ramirez 2011]) followed by
the subsequent cleanout of the failed material. Examples of hydraulic entrainment are illustrated in
Figures 3.23 and 3.24 and basal cleanout is illustrated in Figure 3.25. Figure 3.26 illustrates some of
the common types of gravitational failure.

FIGURE 3.22 Headward-migrating knickpoints in a tributary. (From Langendoen, E.J., Concepts:
Conservational channel evolution and pollutant transport system, Workshop presented at Mississippi State
University, 2009.)