Ksn Values Research Articles

Testing monsoonal controls on bedrock river incision in the Himalaya and Eastern Tibet with a stochastic‐threshold stream power model

Abstract10Be‐derived catchment average erosion rates from the Himalaya and Eastern Tibet show different relationships with normalized channel steepness index (ksn), suggesting differences in erosional efficiency of bedrock river incision. We used a threshold stream power model (SPM) combined with a stochastic distribution of discharges to explore the extent to which this observation can be explained by differences in the mean and variability of discharge between the two regions. Based on the analysis of 199 daily discharge records (record lengths 3–45 years; average 18.5 years), we parameterized monsoonal discharge with a weighted sum of two inverse gamma distributions. During both high‐ and low‐flow conditions, annual and interannual discharge variabilities are similarly low in each region. Channel widths for 36 rivers indicate, on average, 25% wider streams in Eastern Tibet than in the Himalaya. Because most catchments with 10Be data are not gauged, we constrained mean annual discharge in these catchments using gridded precipitation data sets that we calibrated to the available discharge records. Comparing 10Be‐derived with modeled erosion rates, the stochastic‐threshold SPM explains regional differences better than a simple SPM based on drainage area or mean annual runoff. Systematic differences at small ksn values can be reconciled with ksn‐dependent erosion thresholds, whereas substantial scatter for high ksn values persists, likely due to methodological limitations. Sensitivity analysis of the stochastic‐threshold SPM calibrated to the Himalaya indicates that changes in the duration or strength of summer monsoon precipitation have the largest effect on erosional efficiency, while changes in monsoonal discharge variability have almost no effect. The modeling approach presented in this study can in principle be used to assess the impact of precipitation changes on erosion.

Neotectonic characteristics along the eastern flank of the Central Range in the active Taiwan orogen inferred from fluvial channel morphology

The tectonic activity of the Taiwan orogenic belt has been studied by different methods over various timescales. Constraints on tectonic characteristics over timescales of 103–105 years, however, are rather limited. To further understand the tectonic forcing over this timescale, we analyzed 20 major river basins along the eastern flank of the Central Range, the mountainous core of the orogen. The results of river steepness index (ksn) values and the distribution patterns of knickpoints are consistent with the patterns of tectonic evolution in Taiwan. On both ends of the orogen, the rivers have low ksn values that indicate low uplift rates throughout the area. In the central part of the island where the orogen is under rapid collisional processes, the rivers are steeper and are characterized by many tectonic knickpoints. Our results show different patterns from both short-term and long-term uplift and denudation rate patterns. This suggests that these rates change temporally and spatially. Moreover, the steepness indexes of rivers in the Taiwan orogen show the same increasing trend with erosion rates as those in other active orogens but correspond to significantly higher erosion rates. This is likely produced by the frequent debris flow and landslide events in Taiwan, a character that makes the island distinctive from other active orogens in the world.

Correlations of fluvial knickzones with landslide dams, lithologic contacts, and faults in the southwestern Annapurna Range, central Nepalese Himalaya

We investigate the role of landslide dams, spatial changes in lithology, and rock uplift on faults in the formation of knickzones on bedrock rivers. We focus our analysis in the southwestern Annapurna Range of the central Nepalese Himalaya where detailed geologic maps, topographic data, field observations, and aerial photographs are available. We identified knickzones in our study area from normalized river steepness indices (ksn values) extracted from river longitudinal profiles derived from a 25 m digital elevation model we interpolated from digitized topographic map contours. We compared the location of these knickzones with (1) lithologic contacts and faults from a detailed geologic map of the Modi Khola valley and (2) inferred ancient landslide dam features mapped from field observations and aerial photographs. The steepest location on the Modi Khola occurs near the same latitude as the steepest reach on the Mardi Khola located directly to the east, potentially highlighting a major topographic transition across the Annapurna. However, we find that landslide dams once blocked the flow of the Modi Khola, and damming followed by incision after landslide breaching can explain the location of these knickzones without the need for active faulting near the Main Central thrust. We also conclude that (1) knickzones do not correlate with any spatial changes in lithology and (2) knickzones generated by rock uplift on unmapped faults cannot be ruled out. We emphasize that disentangling the processes responsible for knickzone formation remains challenging even when high‐resolution geologic and topographic data are available.