Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 139

Fig. 3.18 Rough bedrock of the Black River in Shaanxi

3.2.3 Knickpoints
A high gradient of a section of bedrock channel may take the form of vertical or undercut waterfalls
several meters in height, or they may be short, steep sections of channel called knickpoints Bedrock
channels may also have downstream variability in gradient as a result of knickpoints. Knickpoints are
commonly most well developed in layered or jointed rocks; and are generally regarded as disequilibrium
features that either migrate upstream, erode toward a lower angle, or otherwise change in form or
location fairly quickly relative to the evolution of the stream channel. Knickpoints, are, thus, commonly
regarded as evidence of channel instability in the form of headcut migration. Figure 3.19(a) shows the
famous Kettle Waterfall in the middle reaches of the Yellow River in China. The fall is a knickpoint
formed in the channel. The knickpoint migrates upstream at a rate about 0.5 0.7 m/yr. The knickpoint is
maintaining a nearly vertical face as it migrates upstream.
The classic theory for a process of knickpoint retreat in stepped headcuts was first proposed by Gilbert
(1896, 1907) for Niagara Falls. This theory featured undercutting by erosion of less resistant strata due to
turbulence and abrasion in the plunge pool, creating an increasingly unstable cap rock that eventually
failed (Tinkler et al., 1994). However, the plunge pool at Niagara Falls is not undercut below the water
level, and the role of plunge pool erosion in knickpoint retreat remains unclear. Subsequent studies have
described knickpoints for which pothole erosion at the lip is an important component of headward retreat
(Bishop and Goldrick, 1992). The Kettle Waterfall on the Yellow River retreats due to the erosion caused
by the turbid water. It occurs often that many circle holes develop around the knickpoint, which are
called potholes. The mechanism of the development of potholes is yet to be studied. Potholes were
observed at the water fall. Figure 3.19(b) shows the pothole around the Kettle Waterfall. Figure 3.20 shows
developing potholes around a small knickpoint on a mountain stream in the Xiangjiang River Basin in
Hunan Province, China.
Knickpoints are the sites of the greatest concentration of energy dissipation along the course of a
stream. Most investigators have assumed that knickpoints retreat fairly rapidly, and have concentrated on
the mechanisms and rates of knickpoint retreat. Shear stress is highest just above and at the knickpoint
face (Gardner, 1983). Knickpoint retreat is proportional to discharge, and is a function of the balance
between the rate of downward wear on the steep reach and the rate of backward wear on the knickpoint
face, and of knickpoint height (Holland and Pickup, 1976). Very few actual measurements exist for rates
of bedrock knickpoint retreat. Exceptions come from the well-dated Niagara Falls region and from
Hawaii. Radiocarbon ages of clam shells at Niagara Falls suggest that the 46-m-high falls migrated very
slowly (0.05 0.70 m yr -1 ) along the narrow gorge section at Niagara Glen from 10,500 to 5,500 yr BP,