When stream banks are resistant to the powers of waterflow, the channel tends to be narrow and deep. That becomes the foundation of a stable riparian area. When floods occur, higher water speeds are slowed by streamside vegetation. Vegetation helps build stream banks and riparian areas.
Vegetation helps keep streams flowing during dry periods.
A simple doubling of the speed of a stream's flow allows it to erode four times as much and to carry 64 times the amount of material. Streamside vegetation reduces the horsepower of a stream, slowing water down through friction. A two-inch deep rootmat resists erosion up to 20,000 times better than bare soil streambanks. As the percentage of roots in stream banks increases, erosion decreases.