6.3.3. Floodplains, Natural Levees and Point Bars

Deposition develops a floodplain just as erosion makes valleys. Floodplain is a major landform of river deposition. Large sized materials are deposited first when stream channel breaks into a gentle slope. Thus, normally, fine sized materials like sand, silt and clay are carried by relatively slow moving waters in gentler channels usually found in the plains and deposited over the bed and when the waters spill over the banks during flooding above the bed.

Floodplains are areas adjacent to rivers that get flooded during high water events. They form mainly through the deposition of sediments carried by the river. When a river flows over gentle slopes, it slows down and starts to deposit large materials first, followed by finer ones like sand, silt, and clay. This process contributes to building the floodplain's structure.

Think of a carpet being laid on the floor. Just as the heaviest objects would settle first on the carpet before the lighter ones are placed on it, a river deposits larger sediments first before finer ones, contributing to the formation of the floodplain.

A river bed made of river deposits is the active floodplain. The floodplain above the bank is inactive floodplain. Inactive floodplain above the banks basically contain two types of deposits — flood deposits and channel deposits.

Floodplains consist of two types: the active floodplain, which includes the riverbed made up of recent river deposits, and the inactive floodplain that lies above the riverbanks. This inactive section collects sediments from past floods, categorized as either flood deposits from overflow events or channel deposits formed when rivers change course.

Imagine a playground after a rainstorm. The area directly under the swings (active floodplain) collects mud as the rainwater drains. Over time, the playground near the edges (inactive floodplain) may collect leaves and debris, indicating it hasn't received recent rainwater directly.

Natural levees and point bars are some of the important landforms found associated with floodplains. Natural levees are found along the banks of large rivers. They are low, linear and parallel ridges of coarse deposits along the banks of rivers, quite often cut into individual mounds.

Natural levees are elevated banks that form alongside rivers when sediment is deposited during flooding. When a river overflows, the fastest-moving water carries larger particles which settle near the riverbank, forming levees to create elevated ridges. This process occurs repeatedly whenever floods happen.

Consider how you might build a small barricade of sandbags to hold back water from flooding a garden. The sandbags resemble natural levees, as they catch and hold back the sediment and water, forming a raised area along the edge.

Point bars are also known as meander bars. They are found on the concave side of meanders of large rivers and are sediments deposited in a linear fashion by flowing waters along the bank. They are almost uniform in profile and in width and contain mixed sizes of sediments.

Point bars form on the inside curve of meanders where the flow slows down, allowing sediments to accumulate. These bars represent where the river loses energy, causing it to deposit sand and silt, thus forming new land. They often have a gentle slope leading from the river to the bar.

Imagine a lazy river where a raft floats downstream. As it curves, the water slows on the inside bend and drops bits of sand and pebbles, forming a small sandy beach. This sandy area is like a point bar in a river.

In large flood and delta plains, rivers rarely flow in straight courses. Loop-like channel patterns called meanders develop over flood and delta plains.

Meanders are sinuous curves that rivers form when they flow across flat plains. As the water flows, it erodes the outer banks where the speed is higher and deposits sediment on the inner banks, causing the river's course to bend and loop.

Think about how a water hose might create bends on the ground when you move it around. Each curve represents how the water moves faster on the outside and slower on the inside, gradually shaping the path into meanders.

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