TRANSIENT BEDROCK INCISION OF THE FORTYMILE RIVER IN EASTERN ALASKA AND YUKON, CANADA: SLOW RESPONSE TO YUKON RIVER HEADWATER CAPTURE?

Quantifying the transient response of bedrock rivers to base level fall represents a critical part of calibrating bedrock incision models. The Fortymile River flows from eastern Alaska to the Yukon River in Canada and provides an ideal setting to quantify transient bedrock channel response for two reasons. First, a single continuous gravel-capped strath terrace records the previous channel gradient along the lower ~175 km of the river. Second, a mechanism for abrupt base level lowering is implicit in the growing body of evidence that the Yukon River acquired its modern course via headwater capture upstream of the Fortymile outlet at ~2.6 Ma, after the first advance of the Cordilleran Ice Sheet.

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 (k_sn = 1120; θ = 0.009) and a ~85-km-long headwater reach that is low-gradient, highly concave and minimally incised (k_sn = 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.