River incision into bedrock is an important process in the context of landscape evolution. Climate can affect the river incision process in several ways. Firstly, precipitation is the main source of river discharge that is required to initiate the motion of river sediment and expose the riverbed to erosion. However, the relationship between river incision and river discharge is often non-linear, in that river discharge has to exceed a certain erosional threshold to mobilize bedload sediment and expose the riverbed to erosion. Whether this erosion threshold plays a significant role, or not, depends on the flux and grain size distribution of river sediment. Besides its effect on discharge, the climate’s influence on bedrock weathering also affects river incision, by changing bedrock erodibility as well as the grain size distribution of river sediment. Finally, climate sets discharge variability, which determines how often and by how much river discharge exceeds the erosion threshold. Discharge variability is typically high in arid regions, whereas river discharge is less variable in humid regions. Although it is evident that climate has an effect on river incision, studies that have investigated the effect of climate on 10Be-derived erosion rates – which in a steady state landscape equal river incision rates – have often found ambiguous relationships. This is most likely because other non-climatic factors (e.g., tectonic uplift rates, lithology, biota) interplay to obscure potential climatic trends.
In this PhD thesis, I investigate the role of climate on various aspects of the river incision process in the Coastal Cordillera of central Chile. I focus on regions that are underlain by similar granodioritic lithology, but are exposed to contrasting climate regimes (arid, semi-arid, mediterranean, and humid-temperate). Using this approach, I aimed to reduce the variations in non-climatic factors that may obscure the climatic effect on erosion and river incision processes. I used in situ cosmogenic-10Be in river sediment to quantify erosion and river incision rates. In situ cosmogenic 10Be is produced in quartz grains in the upper few meters of the earth’s surface by high energy cosmic rays. The 10Be concentration reflects the time that grains are exposed to cosmic rays, which is proportional to the residence time of grains in the surface layer (i.e., inversely to the erosion rate). As a result of this, cosmogenic-10Be is frequently measured in river sediment to constrain catchment average erosion rates, which in steady state landscapes should equal river incision rates.
In the first study (Chapter 3), I investigate grain size-dependent 10Be concentrations in river sediment. In most studies, the sand fraction of river sediment is used to measure 10Be-derived erosion rates, however, in catchments where 10Be concentrations vary between different grain size classes, this may result in biased erosion rate estimates. I investigate the controls of precipitation, hillslope angle, lithology and abrasion on grain size-dependent 10Be concentrations in Chile and in other landscapes around the world. I sampled 7 different grain size classes in 4 catchments located in the above-described climate regions in the Chilean Coastal Cordillera. The results reveal that regional precipitation regime affects grain size-dependent 10Be concentrations through its effect on 1) the depth of erosion processes, and 2) the depth of biotic soil mixing, which produces a constant 10Be concentration over depth. To put this in a broader perspective, I compiled 10Be concentrations across different grain sizes sampled at the same sample location for 73 catchments around the world. Based on this global compilation, I conclude that grain size-dependent changes in 10Be concentrations have a high likelihood of occurring in catchments with thin soil layers, where deep-seated erosion processes (e.g., landslides) excavate coarse grains from greater depth, where 10Be concentrations are lower. Typically, such catchment characteristics are found in landscapes that feature steep topography (>25°) and high mean annual precipitation rates (>2000 mm yr-1). I additionally find that the modification of the grain size distribution by fluvial abrasion can result in grain size-dependent 10Be concentrations. This mainly occurs in catchments with easily erodible lithologies and long sediment travel distances (>2300–7000 m, depending on lithology). I conclude that roughly 50% of the previously published 10Be-derived catchment average erosion rates potentially contain a grain size bias, because the catchments feature one or more of the catchment characteristics that can lead to grain size-dependent 10Be concentrations.
In the second study (Chapter 4), I investigated how climatic forcing can affect discharge variability, by studying El Niño Southern Oscillation (ENSO)-induced hydrological extremes along a climate gradient in central Chile (~28-42°S). This study focusses on discharge time series of 183 river catchments, that are located in the high elevation Andes and the low elevation coastal region, and feature different hydrological regimes: snowmelt-dominated (nival) versus rainfall-dominated (pluvial). The river discharge data shows that the hydrological response to ENSO differs strongly along the climate gradient and shows clear contrasts between basins with the nival and pluvial discharge regimes. The semi-arid region experiences the strongest river discharge anomalies during both El Niño (increasing discharge) and La Niña events (decreasing discharge), whereas the hydrological anomalies are the smallest in the humid-temperate region. Furthermore, the magnitude and frequency of extreme discharge events increases in the semi-arid and mediterranean regions during the warm and wet El Niño phase, whereas discharges in the humid-temperate region are most sensitive to rainfall deficits during La Niña events revealed by a higher frequency of low flow conditions. Snow dynamics introduce large contrasts in the hydrological response between basins with the nival and pluvial discharge regimes. First of all, snowmelt dynamics induce a delayed discharge peak. Snowmelt-dominated basins, experience the largest El Niño-induced discharge peak during the snowmelt season in summer, whereas the ENSO-induced climatic anomalies are most extreme during winter and autumn. Moreover, the discharge variability is lower for snowmelt-dominated basins because snowmelt produces non-flashy river discharge over a longer hydrological response time. Finally, basins with the nival-type of discharge regime are not as strongly affected by droughts than pluvial type of basins during La Niña, because snowmelt-generated runoff provides a minimum river discharge level. The results of this study reveal that ENSO-induced climatic and hydrological anomalies contribute strongly to the high discharge variability that has been observed in the semi-arid region. Which implies that ENSO has an important effect on river incision processes in the semi-arid region. Finally, I discuss the implications of the results of this study for water resource management in Chile.
In the next study (Chapter 5), I investigated long-term catchment average erosion rates in catchments along a climate gradient in Chile. I sampled ~10 catchments in three of the four climate regions (semi-arid, mediterranean, and humid-temperate). I specifically selected catchments that feature differences in normalized channel steepness (ksn) between the catchments, which is a topographic metric that reflects tectonic uplift rates in a steady state landscape. The 10Be-derived catchment average erosion rates revealed an increasing trend with ksn. Besides this, however, a secondary influence of climate was evident: the slope of ksn-erosion rate relationships was markedly steeper for the humid-temperate region compared to the semi-arid region. In other words, for a given normalized channel steepness index, the highest erosion rates were observed for the humid-temperate region and the lowest for the semi-arid region. I compiled and recalculated previously published 10Be-derived erosion rates of ~150 catchments in Chile to compare my results to the large-scale erosional dynamics in Chile. While my new results agreed well with published erosion rates from other catchments in the Coastal Cordillera, erosion rates in the Andes are higher, which I posit is due to higher precipitation rates and steeper topography. Where previous studies had difficulties with depicting a consistent climatic signal along the latitudinal gradient, new data analysis suggests, that the erosion rates in Chile reflect a combined tectonic and climatic signal, which agrees with my own data from the Coastal Cordillera. The large degree of scatter in the compiled dataset is likely induced by non-climatic factors (e.g., lithology or biota). I conclude that, in Chile, the erosional efficiency increases with increasing precipitation, which provides empirical evidence for the understanding that arid landscapes have to become steeper than humid landscapes to reach erosion rates that equal tectonic uplift rates in a steady state landscape.
In the final study (Chapter 6), I tested whether erosion thresholds, which are set by the amount and grain size of river sediment, play a significant role in the river incision processes in catchments in the Chilean Coastal Cordillera. I applied the stochastic-threshold stream power model, calibrated with field data, and compared the best fit model results to the 10Be-derived erosion rates and median grain sizes that I measured for each river catchment. The results reveal that erosion thresholds do play a role in the relatively gently sloping catchments of the Chilean Coastal Cordillera. A sensitivity test revealed that the modelled erosion rates of the semi-arid region rapidly decrease under erosion thresholds that are set by a grain size of > 1 cm. River incision still seems to occur in the humid-temperate region for erosion thresholds that are set by considerable grain sizes, but rapidly decrease for grain sizes of >10 cm. I conclude that in gently sloping basins the sensitivity of river incision rates to erosion thresholds strongly depends on river discharge, because the channel steepness is too low to facilitate the mobilization of river sediment. The results suggest that river incision occurs rather infrequently in the semi-arid region, whereas it occurs more continuously in the humid-temperate region. I have planned to further test the sensitivity of the model to input parameters and investigate the magnitude and reoccurrence time of river incision processes in the different climate regions of the Chilean Coastal Cordillera to validate these findings, and to better understand river incision processes in gently sloping landscapes. This is the first study that tests the threshold-behaviour of the process of river incision in gently sloping basins in regions that are exposed to different climates.
To conclude, in this PhD thesis I investigated the effect of climate on several aspects that are relevant in the process of river incision. This study contributes to the general understanding of the effect of climate on landscape evolution in gently sloping mountain ranges that cover roughly ~15% of the Earth's surface.
