Streams are naturally ever-changing to their surrounding environment. The flowing water can carve channels, move material downstream, and occasionally change course, causing gradual bank erosion. Poor storm water management can cause bank erosion to occur more quickly, deteriorating the stream structure and its ecosystem.
Two main factors form stream banks: hydraulic force and debris load. The hydraulic force is the flowing water combined with debris load of sediment, rocks, wood, and ice carried in the water to shape the stream channel. The hydraulic force of flowing water moves in a three-dimensional pattern as demonstrated in the diagram here. The flow pattern of straight and winding sections can carve out the streambed and stream banks, moving a large amount of material downstream. The amount of water and sediment changes with the size of rain event and surrounding land types.
Most natural streams flow on a series of bends and curves that are formed continuously by the flow of water and sediment as shown here. The natural meander of streams have riffles, pools and different flow elevations.
Streams can function optimally, transporting water and sediment downstream without causing destructive erosion, when allowed to freely meander. When a stream is surrounded by development, the meandering boundaries are restricted and limiting the functionality. Thus stream bank erosion is often the result. As the bank erodes, the land above it is lost to the erosion as well. At this point, the stream needs to be restored for not only the health of the stream, but also to return functionality to public infrastructure.