Quantifying the competing influences of lithology and throw rate on bedrock river incision

Abstract

River incision in upland areas is controlled by prevailing climatic and tectonic regimes, which are increasingly well described, and the nature of the bedrock lithology, which is still poorly constrained. Here, we calculated downstream variations in stream power and bedrock strength for six rivers crossing a normal fault in western Turkey, to derive new constraints on bedrock erodibility as function of rock type. These rivers were selected because they exhibit knick zones representing a transient response to an increase in throw rate, driven by fault linkage. Field measures of rock mass strength showed that the metamorphic units (gneisses and schists) in the catchments are ∼2 times harder than the sedimentary lithologies. Stream power increases downstream in all rivers, reaching a maxima upstream of the fault within the metamorphic bedrock but declining markedly where softer sedimentary rocks are encountered. We demonstrate a positive correlation between throw rate and stream power in the metamorphic rocks, characteristic of rivers obeying a detachment-limited model of erosion. We estimated bedrock erodibility in the metamorphic rocks as kb = 2.2−6.3 × 10−14 ms2 kg−1; in contrast, bedrock erodibility values were 5−30 times larger in the sedimentary units, with kb = 1.2−15 × 10−13 ms2 kg−1. However, in the sedimentary units, stream power does not scale predictably with fault throw rate, and we evaluated the extent to which the friable nature of the outcropping clastic bedrock alters the long-term erosional dynamics of the rivers. This study places new constraints on bedrock erodibilities upstream of active faults and demonstrates that the strength and characteristics of underlying bedrock exert a fundamental influence on river behavior.

Sedimentary characteristics and morphologic change of till-bedded semi-alluvial streams: Medway Creek, Southern Ontario, Canada

“…These short-term values are very high relative to geologic erosion rates for till, which is similar to results for bedrock channels (Stock et al, 2005;Kent et al, 2021). This may be caused by: (i) periods of rapid valley incision and of relaxation when the channel reaches hard substrate; (ii) climate-induced flow variability (i.e., less precipitation and flows); or (iii) spatially and temporally variable incision, depending on local hydraulic/sedimentary/tectonic conditions.…”

Uncovering the Controls on Fluvial Bedrock Erodibility and Knickpoint Expression: A High‐Resolution Comparison of Bedrock Properties Between Knickpoints and Non‐Knickpoint Reaches

“…Discontinuity spacing is similarly difficult to quantify: the common practice of measuring discontinuity intersections along a scan line (e.g., Allen et al., 2013; Lima et al., 2021; Spotila et al., 2015) can be highly subjective, requiring selection of representative measurement locations in settings with spatially complex networks of intersecting joints, fractures, and/or bedding planes. Therefore, it remains unclear which factors are most important in setting bedrock erodibility, and therefore how to best leverage measurable bedrock properties to adequately represent erodibility when attempting to characterize relationships between erodibility and channel form and calibrate numerical landscape evolution models (Kent et al., 2020). Consequently, there is a need for detailed bedrock channel surveys which carefully quantify both rock strength and discontinuity spacing to make meaningful additional progress toward demystifying fluvial bedrock erodibility.…”

“…This is further complicated by a lack of established methods for systematically quantifying either rock strength or discontinuity spacing in natural settings. Rock strength is often estimated using the Schmidt rebound hammer (Allen et al., 2013; Bursztyn et al., 2015; Kent et al., 2020; Murphy et al., 2018; Shobe et al., 2017; Zondervan, Stokes, et al., 2020; Zondervan, Whittaker, et al., 2020), but this tool is not suitable for very weak rocks or those with closely spaced discontinuities (Goudie, 2006), sometimes requiring significant portions of study areas to be eliminated (Bursztyn et al., 2015; Duvall et al., 2004; Wohl & Achyuthan, 2002). Discontinuity spacing is similarly difficult to quantify: the common practice of measuring discontinuity intersections along a scan line (e.g., Allen et al., 2013; Lima et al., 2021; Spotila et al., 2015) can be highly subjective, requiring selection of representative measurement locations in settings with spatially complex networks of intersecting joints, fractures, and/or bedding planes.…”

“…However, the relative importance of these properties in determining erodibility has not been quantitatively demonstrated, though discontinuity spacing is hypothesized to be dominant via its direct control on plucking, the more efficient erosional mechanism (Whipple et al, 2000(Whipple et al, , 2013. Furthermore, the few field-based studies seeking to investigate the role of erodibility in determining channel form often focus on quantifying either rock strength or discontinuity spacing, but not both (e.g., Bursztyn et al, 2015;Kent et al, 2020;Lima et al, 2021;Zondervan, Whittaker, et al, 2020), which may confound meaningful insights due to reliance on inadequate proxies for erodibility. This is further complicated by a lack of established methods for systematically quantifying either rock strength or discontinuity spacing in natural settings.…”

Knickpoint morphotectonics of the Middle Shire River basin: Implications for the evolution of rift interaction zones

“…Thus, active early‐stage rift zones, such as the humid rift basins along the Western Branch of the East African Rift System, present an excellent opportunity to explore the salient geomorphic structure and landscape evolution peculiar to early‐stage rifting. In such settings, geomorphic indicators such as drainage patterns, channel geometry, river behaviour, knickpoints and slope attitudes provide insights into the interactions between active crustal deformation and landscape evolution (e.g., Castillo et al, 2013; Gallen & Fernández‐Blanco, 2021; Jiang et al, 2016; Kent et al, 2021; Molin & Corti, 2015; Vita‐Finzi, 2012).…”