How can the same streamflow be a 100-year flood in one location and only a 50-year flood at another?
Recurrence intervals are based on the probability of the peak streamflow occurring at a given location in any year. As water flows downstream from point “A” to point “B” and the drainage area increases, the volume of streamflow increases. Given this, it may seem reasonable to think that peak flows would increase in the same manner, but this is not necessarily true. The flow at any particular point on a stream depends on local stream channel and floodplain conditions as well as on conditions upstream or downstream of the point, such as channel slope, floodplain shape, and any impoundments of streamflow. Downstream points on a stream will have always have greater total volume of streamflow resulting from flooding (except in certain very unique situations), but the rate of streamflow can be quite different from upstream points, and will often be less. In these cases, streamflow will remain elevated for a longer period of time. This phenomenon, known as peak attenuation, can be attributed
