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

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.

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

Methods; Tables S1–S15; and Figures S1–S4.

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

Methods; Tables S1–S15; and Figures S1–S4.

Parameterization of river incision models requires accounting for environmental heterogeneity: insights from the tropical Andes

Landscape evolution models can be used to assess the impact of rainfall variability on bedrock river incision over millennial timescales. However, isolating the role of rainfall variability remains difficult in natural environments, in part because environmental controls on river incision such as lithological heterogeneity are poorly constrained. In this study, we explore spatial differences in the rate of bedrock river incision in the Ecuadorian Andes using three different stream power models. A pronounced rainfall gradient due to orographic precipitation and high lithological heterogeneity enable us to explore the relative roles of these controls. First, we use an area-based stream power model to scrutinize the role of lithological heterogeneity in river incision rates. We show that lithological heterogeneity is key to predicting the spatial patterns of incision rates. Accounting for lithological heterogeneity reveals a nonlinear relationship between river steepness, a proxy for river incision, and denudation rates derived from cosmogenic radionuclide (CRNs). Second, we explore this nonlinearity using runoff-based and stochastic-threshold stream power models, combined with a hydrological dataset, to calculate spatial and temporal runoff variability. Statistical modeling suggests that the nonlinear relationship between river steepness and denudation rates can be attributed to a spatial runoff gradient and incision thresholds. Our findings have two main implications for the overall interpretation of CRN-derived denudation rates and the use of river incision models: (i) applying sophisticated stream power models to explain denudation rates at the landscape scale is only relevant when accounting for the confounding role of environmental factors such as lithology, and (ii) spatial patterns in runoff due to orographic precipitation in combination with incision thresholds explain part of the nonlinearity between river steepness and CRN-derived denudation rates. Our methodology can be used as a framework to study the coupling between river incision, lithological heterogeneity and climate at regional to continental scales.

How rivers incise to survive periodic inputs of immobile landslide-derived boulders

<p>Bedrock landsliding provides a strong negative feedback on bedrock river incision by causing long-lived burial events and hence hiatuses in downcutting.  Nevertheless, rivers in tectonically active settings carve deep canyons despite being periodically inundated with immobile boulders. How is this possible? In this contribution, we explore the processes through which rivers incise bedrock canyons within the Franciscan mélange in the actively uplifting California Coast Range. The Franciscan mélange is well known for its “melting ice cream topography” in which slow-moving landslides (“earthflows”) festoon the walls of river canyons and deliver car- to house-sized boulders to channels.  </p><p>Analysis of valley widths and river long profiles over ∼19  km of Alameda Creek (185  km<sup>2</sup> drainage area) and Arroyo Hondo (200  km<sup>2</sup> drainage area) in central California shows a very consistent picture in which earthflows that intersect these channels deposit immobile boulders that force tens of meters of gravel aggradation for kilometers upstream, leading to apparently long-lived sediment storage and channel burial at these sites. In contrast, over a ∼30  km section of the Eel River (5547  km<sup>2</sup> drainage area), there are no knickpoints or aggradation upstream of locations where earthflows impinge on its channel. Neither boulder supply nor transport capacity explains this difference. Rather, we find that the dramatically different sensitivity of the two locations to landslide blocking is linked to differences in channel width relative to typical seasonal displacements of landslides. The Eel River is ∼5 times wider than the largest annual seasonal displacement. In contrast, during wet winters, earthflows are capable of crossing and blocking the entire channel width of Arroyo Hondo and Alameda Creek. Hence, by virtue of having wide valley bottoms, larger rivers are more likely to simply flow around the toes of earthflows.  </p><p>For the smaller rivers in our study area that are chronically buried in landslide debris, our field observations provide evidence for two processes that may allow periodic bedrock river incision. Narrow channels in the Franciscan mélange that are buried in debris can incise epigenetic gorges around the margins of boulder jams during periods of earthflow dormancy when boulders are no longer input into channels.  Alternatively, during periods of earthflow dormancy, abrasion (and hence size reduction) of boulders in place from suspended sediment may ultimately render boulders mobile.  </p><p>Without explicit representation of these three processes, modeling the coupling of hillslope and channel evolution in this setting is not possible. </p><p><br><br></p>

Lithology and orographic precipitation control river incision in the tropical Andes

Process-based geomorphic transport laws enable to assess the impact of rainfall variability on bedrock river incision over geological timescales. However, isolating the role of rainfall variability on erosion remains difficult in natural environments in part because the variability of rock strength and its resistance to incision are poorly constrained. Here, we explore spatial differences in the rate of bedrock river incision in the Tropical Andes. The Ecuadorian Andes are characterized by strong rainfall gradients due to orographic precipitation sourced in the Amazon basin. In addition, the tectonic configuration has generated a profound lithological heterogeneity. The relative role of either these controls in modulating river incision on millennial time scales, however, remains unclear. Using 10Be catchment-wide erosion rates, meteorological and hydrological data, as well as data on bedrock erodibility, we provide quantitative constraints on the importance of rainfall variability and lithological variations. Explicit incorporation of rock erodibility in river incision models predicated on the stream power equation enables us to identify a first order control of lithology on river incision rates. Rainfall variability based on a spatially and temporally explicit hydrological dataset and a stochastic-threshold river incision model explain regional differences in river incision that cannot be attributed to topographical and/or lithological variability.

Anthropogenic strath terrace formation caused by reduced sediment retention

Across North America, human activities have been shown to cause river incision into unconsolidated alluvium. Human-caused erosion through bedrock, however, has only been observed in local and isolated outcrops. Here, we test whether splash-dam logging, which decreased in-stream alluvial cover by removing much of the alluvium-trapping wood, caused basin-wide bedrock river incision in a forested mountain catchment in Washington State. We date incision of the youngest of four strath terraces, using dendrochronology and radiocarbon, to between 1893 CE and 1937 CE in the Middle Fork Teanaway River and 1900 CE and 1970 CE in the West Fork Teanaway River, coincident with timber harvesting and splash damming in the basins. Other potential drivers of river incision lack a recognized mechanism to cause T1 incision or are not synchronous with T1 incision. Hence, the close temporal correspondence suggests that reduced sediment retention triggered by splash damming led to the observed 1.1 mm⋅y−1 to 23 mm⋅y−1 of bedrock river incision and reduction of the active floodplain to 20% and 53% of its preincision extent on the Middle and West Forks, respectively. The development of such anthropogenic bedrock terraces may be an emerging, globally widespread physiographic signature of the Anthropocene.

Glacial limitation of tropical mountain height

Absent glacial erosion, mountain range height is limited by the rate of bedrock river incision and is thought to asymptote to a steady-state elevation as erosion and rock uplift rates converge. For glaciated mountains, there is evidence that range height is limited by glacial erosion rates, which vary cyclically with glaciations. The strongest evidence for glacial limitation is at midlatitudes, where range-scale hypsometric maxima (modal elevations) lie within the bounds of Late Pleistocene snow line variation. In the tropics, where mountain glaciation is sparse, range elevation is generally considered to be fluvially limited and glacial limitation is discounted. Here we present topographic evidence to the contrary. By applying both old and new methods of hypsometric analysis to high mountains in the tropics, we show that (a) the majority are subject to glacial erosion linked to a perched base level set by the snow line or equilibrium line altitude (ELA) and (b) many truncate through glacial erosion towards the cold-phase ELA. Evaluation of the hypsometric analyses at two field sites where glacial limitation is seemingly marginal reveals how glaciofluvial processes act in tandem to accelerate erosion near the cold-phase ELA during warm phases and to reduce their preservation potential. We conclude that glacial erosion truncates high tropical mountains on a cyclic basis: zones of glacial erosion expand during cold periods and contract during warm periods as fluvially driven escarpments encroach and destroy evidence of glacial action. The inherent disequilibrium of this glaciofluvial limitation complicates the concept of time-averaged erosional steady state, making it meaningful only on long timescales far exceeding the interval between major glaciations.