Point bars are curved landforms made of accumulated sediment, or alluvium, that develop on the inside bend of meandering rivers. The formation of a point bar is part of the ongoing process by which a river moves across its floodplain, constantly eroding one bank while simultaneously building up the opposite bank. The presence of these bars is a clear indicator that the river is mature and actively developing its characteristic S-shaped bends.
Anatomy of a Meander: Location and Definition
A point bar is situated along the inner, convex bank of a river meander. This location is defined by the relatively slower water flow, which allows suspended and rolled sediment to settle out of the current. The bar is an arcuate-shaped deposit that follows the curve of the bend, storing sediment within the channel.
The point bar’s location is best understood in contrast to the opposite bank, known as the cut bank. The cut bank is the outside, concave side of the meander, where the water flows fastest and actively erodes the bank. This continuous cycle of erosion and deposition causes the meander to migrate laterally across the floodplain over time.
The surface of a point bar often has a gentle slope that inclines toward the channel, with an elevation close to the typical water level. Over time, the bar builds up through lateral accretion, meaning it grows outward and slightly downstream as the meander shifts. This accretion process is responsible for the overall morphology of the bar.
The Role of Flow Dynamics in Point Bar Formation
The physical mechanism driving point bar formation is directly related to how water velocity changes as it navigates a river bend. As water flows around a curve, centrifugal force pushes the main, fastest current toward the outside bank. This makes the water flow significantly slower along the inside curve, which is the region where the point bar forms.
This difference in speed creates a secondary, corkscrew-like current known as helicoidal flow. The water on the surface of the river is forced to the outside bank, but the water near the riverbed moves in the opposite direction, sweeping across the channel floor toward the inside bank. This spiral motion effectively transports sediment from the high-energy, eroding cut bank and redeposits it onto the low-energy, accumulating point bar.
The helicoidal flow acts as an efficient conveyor belt for bedload material, such as sand and gravel. This flow sweeps the sediment laterally across the bottom of the channel and up the shallow-sloping face of the point bar. The reduced velocity along the inner bend diminishes the water’s ability to carry this load, causing the sediment to drop out of the current and accumulate. The strength of this secondary current is a major factor controlling the architecture and size of the point bar.
The process of deposition is continuous, with material being added layer by layer as the channel migrates. The deposition occurs not just along the inner edge but also down the bar. This constant accretion allows the point bar to grow and maintain its characteristic shape as the river channel slowly shifts position.
Composition and Internal Structure
Point bars are primarily constructed from alluvium, which is unconsolidated material like sand, gravel, and silt deposited by the flowing water. The specific composition directly reflects the sediment load that the river is carrying. The material is typically well-sorted, meaning particles of similar size are deposited together.
The internal structure of a point bar is characterized by a distinctive pattern known as a “fining-upward sequence.” This means that if one were to examine a vertical cross-section of the bar, the coarsest materials, such as gravels or coarse sands, would be found at the base. The sediment would gradually become finer, transitioning to fine sand, silt, and clay toward the top.
This fining-upward trend is a result of the meander’s lateral migration and fluctuating flow levels. The coarsest material is deposited at the base in the deepest part of the channel, where the current is strongest. As the river migrates outward, the inner bank, and thus the point bar, builds up laterally, placing successively finer sediments over the coarser base as the water depth and energy decrease.
The internal layers of the point bar also exhibit stratification, most notably large-scale trough cross-bedding. These structures are created by the migration of underwater sand dunes and ripples, providing a visible record of the ancient flow directions and energy levels of the river. The final surface often displays a series of low, curved ridges and troughs, known as scroll bars and swales, representing successive positions of the river bank.