How wide should a buffer be?
There is no single buffer width that fits all situations. Rivers in wide, flat valleys require much more space (wider buffers) than do rivers in narrow valleys with sloping sides. Where rivers move back and forth, or meander, buffers help protect people and structures from the consequences of channel movement.
Buffers should be as wide as the meander belt width. This is the width from the outside of right-turning meanders to the outside of left-turning meanders. Meanders move downstream or down valley, as well as across valleys.
In the past, people made the mistake of straightening river channels. A straightened river becomes steeper and faster with more erosion, causing a river to cut down or incise into the valley floor. Incised rivers focus the force of floodwaters in a deep, fast-moving channel. This accelerates erosion as the channel builds a new floodplain. As buffers become narrower, the lives and property of landowners, as well as the water quality and living river processes, are put at increasing risk. For an incised or straightened river, buffer width should be as wide as the meander belt width shown on very old maps made before channel incision occurred.
What about buffer width in steeper valleys?
In narrow valleys without meanders, buffers should be at least as wide as the flood zone for very large floods, containing about the flow of water expected in a 100-year flood or larger. Fortunately, flood widths are much narrower in these settings than in wide valleys, as the canyon walls contain the flows.
Another tool for setting buffer width is to determine the width of the riparian vegetation that taps into the abundant moisture in the floodplain area.