Direct measurements of stream-incision rates are generally neither practical nor meaningful because bed-rock abrasion rates are so small, and the variability of average stream discharge and sediment load is so great. Field examples of bed-rock incision by streams rarely allow determination of incision duration, initial channel geometry, or history of flow parameters. Complications arising from threshold-response and complex-response behavior of drainage basins (Schumm, 1977) often obscure the very cause of incision. However, incision resulting from datable stream diversion or capture allows determination of the duration of cutting. In extraordinary instances, such as the one reported herein, initial channel geometry, hydraulics, and sediment transport can be estimated in addition to the time of diversion.
This paper develops the chronology of incision of a glacially diverted river over a time span of some 25,000 yr. Duration of this natural incision experiment was long enough that short-term perturbations may be ignored and meaningful average incision rates determined. The chronology, with accompanying estimates of channel geometry, hydraulics, and sediment-transport rate, thus provides data for the study of bed-rock incision processes in gravel-bed rivers (Foley, 1980a, 1980b).