TRANSIENT BEDROCK INCISION OF THE FORTYMILE RIVER IN EASTERN ALASKA AND YUKON, CANADA: SLOW RESPONSE TO YUKON RIVER HEADWATER CAPTURE?
To quantify bedrock incision in the Fortymile basin, we analyze the 2 arc-second National Elevation Database digital elevation model, which covers the Fortymile River basin across the Alaska-Yukon border. Long profile and slope-area data reveal a broad knickzone ~225 km upstream of the Fortymile River outlet. This knickzone marks a boundary between the steep, linear downstream reach (ksn = 1120; θ = 0.009) and a ~85-km-long headwater reach that is low-gradient, highly concave and minimally incised (ksn = 228; θ = 0.620). Below the knickzone, the river incises a low-gradient (~15 cm/km) strath terrace to depths of ~280 m. A downstream projection of the headwater channel grades into the terrace profile, suggesting a relict landscape upstream of the knickzone that has not yet “felt” the transient incision.
Preliminary basin-averaged cosmogenic 10Be concentrations from seven modern tributaries between the Fortymile River outlet and headwaters increase linearly by a factor of two (from 290000 to 610000 atoms/gram). This suggests a slow tributary response to bedrock incision along the Fortymile River over the past 105 years. To test whether the incision represents a slow transient response to Yukon River headwater capture at ~2.6 Ma, we are measuring cosmogenic 26Al and 10Be concentrations in quartz-bearing terrace gravel clasts near the Fortymile River outlet and at the furthest upstream extent of the terrace. We will utilize the cosmogenic isochron burial dating technique to directly date the deposition of the terrace gravels and determine rates of bedrock incision and knickzone celerity averaged over the past 106 years.