Abstract
Reconstructing the incision history of bedrock rivers can provide evidence of formation processes in knickzones on longitudinal river profiles. In this study, we determined incision rates at the upstream end of a knickzone in the Oshika Gorge, Tottori prefecture, southwest Japan, by exposure dating of a series of granitic strath terraces using terrestrial cosmogenic 10Be. The measured 10Be concentrations increased with the height of the sampling surface above the present-day riverbed. Exposure ages were determined as 50.2 ± 3.9 kyr BP (Before Present) for the highest strath terrace, which was 11.0 m above the present riverbed, and 5.3 ± 0.7 kyr BP for the lowest at 1.6 m. Regression analysis of all data gave an average incision rate of 0.24 mm/yr, which is comparable to the uplift rate in this region. Results from the multiple sampling points suggest that the incision rate may have changed in a stepwise manner over the past 50 kyr BP, as follows: 0.05 mm/yr between 50.2 and 32.4 kyr BP, 0.53 mm/yr between 32.4 and 19.9 kyr BP, and 0.15 mm/yr between 19.9 kyr BP and the present. This pattern of an increasing and then decreasing incision rate is difficult to explain in the context of the retreating knickzone that is located just downstream from the sample site. The timing of the increase in incision rate corresponds to the beginning of decreasing precipitation and temperature toward the Last Glacial Maximum. Although the present study was unable to determine the reason for this accelerated incision, it possibly reflects the response of the channel to enhanced sediment supply from the surrounding hills during the glacial period.
