A cut bank is a steep, often nearly vertical, bank found on the outside curve of a winding river channel, known as a meander. This feature results from concentrated erosion by flowing water. Its formation is directly linked to the difference in water velocity between the inner and outer bends of the river, shaping the landscape through the continuous movement of water and sediment.
Defining the Cut Bank
The physical structure of a cut bank is recognizable as a sharp, concave indentation along the river’s edge. This bank is typically undercut at the base and has a steep, sometimes cliff-like, profile. The exposed surface often consists of unconsolidated sediment, soil, or bedrock, depending on the geology of the area.
A cut bank always forms on the outside bend of a meander, which is the concave side where the water is directed. Due to continuous erosion, the channel is deepest at the base of the cut bank, often corresponding to the thalweg, or the line of fastest flow. The constant undercutting destabilizes the bank material, leading to slumping or mass wasting, where trees and soil collapse into the river.
The Mechanics of Erosion and Formation
The formation of a cut bank begins with increased water velocity on the outside of the river bend. As water flows around a curve, centrifugal force pushes the highest velocity current toward the outer bank. This faster-moving water possesses greater kinetic energy and shear stress, increasing its ability to scour and erode the bank material.
A secondary flow pattern, known as helicoidal flow, significantly contributes to the erosion process. This flow is a corkscrew-like circulation that spirals across the river channel. The surface water is pushed toward the outside bank, where it spirals downward, scouring the bank’s base. The flow then moves along the riverbed toward the inside bank before rising to the surface, completing the helical motion and sweeping eroded material across the channel.
The Complementary Feature: Point Bars
The cut bank and the point bar define a meandering river. While the cut bank is a feature of erosion, the point bar is a feature of deposition, located directly opposite on the inside bend of the meander. On the inside curve, the water velocity is significantly lower due to the shorter path and reduced centrifugal force.
As the helicoidal flow sweeps material from the cut bank, it loses energy on the inside bend, causing the transported sediment to settle out. This deposition creates the point bar, which is characterized by a gentle, sloping surface and a shallow channel depth. The point bar gradually builds up, consisting of well-sorted sand, gravel, and small stones.
Role in Meander Migration
The continuous action of erosion at the cut bank and deposition at the point bar drives meander migration. The river channel constantly shifts its position laterally across the floodplain. The cut bank erodes outward and downstream, while the point bar builds inward, causing the entire meander loop to move.
Over extended periods, this migration can cause the neck of the meander, the narrow strip of land between two adjacent curves, to become thin. During a high-flow event, such as a flood, the river may breach this narrow neck, taking a shorter, straighter path. The abandoned meander loop is then cut off by sediment deposition, eventually forming a crescent-shaped body of water known as an oxbow lake.