Floodplains, Natural Levees and Point Bars

A river doesn’t only carve — it also builds. Just as erosion carves out a valley, deposition builds up a floodplain, and this floodplain is one of the major landforms created by river deposition. The whole process hinges on one simple idea: when a stream’s channel breaks onto a gentler slope, its speed drops, and the largest materials settle out first. That’s why the finer stuff — sand, silt and clay — is carried by the relatively slow-moving waters of gentler channels, which are typically found in the plains. These fine materials get deposited over the river bed, and when the waters spill over the banks during flooding, they get deposited above the bed as well.

This gives us two distinct parts of a floodplain. The river bed itself, made of river deposits, is the active floodplain — it’s the part that’s under water in normal flow. The floodplain above the bank is the inactive floodplain, because it only gets water when the river overflows. The inactive floodplain above the banks contains two types of deposits: flood deposits and channel deposits.

Here’s where it gets interesting. In the plains, channels don’t stay put. They shift laterally and occasionally change their courses altogether, leaving behind cut-off courses that gradually fill up. These abandoned or cut-off channels build up areas on the floodplain that contain coarse deposits — because when a channel is cut off, the heavier material it was carrying settles right there. In contrast, the flood deposits from spilled waters carry relatively finer materials like silt and clay, since the water that overflows the bank is moving slowly and can only hold fine particles. When a floodplain forms in a delta, it’s given a special name: the delta plains.

Two more landforms are closely tied to floodplains, and the book treats them together because they’re often found side by side.

Natural levees are found along the banks of large rivers. They are low, linear and parallel ridges of coarse deposits running along the banks, and quite often they get cut into individual mounds. Think of them as the river’s own raised embankments, built up over time as floodwaters drop their heaviest load right at the edge of the channel.

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