Natural levees are raised, elongated embankments of sediment that border a river channel and are the product of the river’s own flooding processes. These naturally occurring ridges stand in contrast to man-made levees, which are engineered structures built for flood control. The formation of a natural levee is a slow geological process, built over many flood cycles through the physics of flowing water and the subsequent drop of suspended sediment. They define the boundary between the main river and the surrounding floodplain.
The Physics of Overbank Flow and Sediment Drop
A river carries a load of sediment, including sand, silt, and clay, whose volume and particle size are directly related to the water’s speed, or velocity. During a flood event, the river’s water level rises and overflows the confined banks, a process known as overbank flow. The moment the water leaves the main channel and spreads out across the broad, shallow floodplain, its velocity decreases dramatically.
This sudden reduction in speed instantly diminishes the water’s capacity to transport its full sediment load. Consequently, the heaviest and coarsest particles—primarily fine sands and coarse silts—drop out of suspension almost immediately. These materials settle right along the edge of the river channel where the velocity first slowed, creating the foundation for the levee.
As the floodwaters move further onto the floodplain, the remaining flow continues to slow, dropping progressively finer material. This mechanism explains why the largest sediment particles are found closest to the riverbank.
Composition and Structure of Levee Deposits
The material that forms a natural levee is highly sorted, exhibiting a clear gradient away from the river channel. The crest of the levee, the highest point immediately adjacent to the river, is composed of the coarsest sediments like sand and very fine sand. This material settled first due to its weight and size.
Moving away from the river, the particles become progressively finer, creating a gentle slope known as the backslope. This lateral fining sees the sand transition into silts and then finally into fine clay particles, which are carried the furthest into the low-lying floodplain before settling out.
The levee’s internal structure often displays evidence of rapid deposition and multiple flood cycles, showing alternating layers of coarser and finer sediment. This distinct stratification is a hallmark of a natural levee, differentiating it from the uniform material of the adjacent floodplain.
Levee Growth Through Cumulative Flood Events
Natural levees are the cumulative product of many moderate and large flood events occurring over centuries. Each time the river overflows its banks, a new layer of sediment is deposited, incrementally raising the height and width of the existing embankment. This repetitive layering allows the levee to grow into a substantial topographic feature.
As the levee grows higher, the river channel itself also tends to fill with sediment, a process called channel aggradation. This means the riverbed effectively rises over time, causing the water surface elevation during normal flow to be significantly higher than the surrounding floodplain. The ever-rising banks and bed create a situation where the river is “perched” above the land it flows through.
The continued vertical growth eventually confines the river to its channel during smaller floods. When a major flood does occur, the consequences of a levee breach can be severe and sudden because the floodwaters are released from a height onto the much lower floodplain.