Bar (river morphology)
Elevated sediment deposits shaped by river flow and geometry.
A bar in a river is an elevated region of sediment, such as sand or gravel, deposited by the flow. Bars are important features in river morphology because they reflect sediment supply conditions and indicate where the sediment supply rate exceeds the transport capacity. Their locations are determined by the geometry of the river and the flow through it.
- types
- mid-channel bars, point bars, mouth bars
- common in
- braided rivers, meandering rivers, river deltas
- formation factors
- river geometry, flow, sediment supply
- associated features
- braid bars, cut banks, oxbow lakes
Lore & Background
Mid-channel bars, also called braid bars, are common in braided river channels, which are broad and shallow and found in areas with high sediment loads such as glacial outwash or mountain fronts. These rivers have high slope, sediment supply, stream power, and bed load transport rates, leading to complex and unpredictable channel patterns. Braided streams are often overfed with sediment, creating multiple channels separated by mid-channel bars. Anastomosing river channels also create mid-channel bars, but these are typically vegetated and more permanent than those in braided rivers, which change rapidly due to non-cohesive sediment and high stream power. Bars can also form mid-channel due to snags or logjams, where an obstruction causes local flow convergence and divergence, leading to deposition downstream and potential island formation.
Reader's Guide
Bars are fundamental to understanding river dynamics and landscape evolution. Point bars form on the inside of meander bends in meandering rivers, where shallow flow and low shear stresses cause sediment deposition. These crescent-shaped features are located opposite the cut bank, the area of erosion on the outside bend. Over time, the combination of point bar deposition and cut bank erosion can lead to oxbow lake formation. Mouth bars occur at river deltas, where the river widens and flow slows, causing sediment to settle mid-channel. These bars tend to prograde as flow pressure erodes the upstream face and redeposits sediment downstream. When water depth becomes shallow enough, flow diverges around the bar, leading to lateral sediment accumulation that can eventually block the river mouth. Bars thus serve as indicators of sediment transport and deposition processes in various river systems.
Did You Know?
- Mid-channel bars are also called braid bars and are common in braided rivers.
- Point bars are crescent shaped and form on the inside curve of meander bends.
- Mouth bars can prograde and eventually block the entire river mouth.
- Logjams can create mid-channel bars by causing flow convergence and deposition downstream.
Frequently Asked Questions
What is a bar in river morphology?
A bar is an elevated deposit of sediment—typically sand or gravel—that accumulates within a river channel when the flow loses energy and drops its load. It marks a zone where the incoming sediment supply outpaces the river's ability to carry it downstream.
What types of bars exist in river systems?
The main categories include mid-channel bars, point bars along the inside of bends, and mouth bars at river outlets. In braided systems they appear as braid bars, while meandering rivers often develop point bars opposite active cut banks.
How does a river bar form?
Formation depends on the interplay of channel geometry, flow velocity, and the rate of sediment delivery. When the flow slows or the channel shape forces deposition, sediment settles and builds an elevated surface that in turn reshapes the local flow pattern.
Where are river bars most commonly observed?
They are characteristic features of braided rivers, meandering rivers, and river deltas. Their specific placement is governed by the local channel geometry and the hydraulic conditions of the passing flow.
Why are bars significant in fluvial geomorphology?
Bars serve as direct indicators of sediment supply conditions, marking locations where deposition exceeds transport capacity. They also drive the development of associated landforms such as oxbow lakes, cut banks, and the overall channel pattern of a river system.
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