Variability in fluvial geomorphic response to anthropogenic disturbance

Abstract

Humans have greatly impacted the processes and intensities of erosion, sediment transport and storage since the introduction of agriculture. In many regions around the world, accelerated floodplain sedimentation can be related to increases in human pressure on the environment. However, the relation between the intensity of anthropogenic disturbance and the magnitude of change in fluvial sediment dynamics is not straightforward and often non-linear. Here, we review a number of case studies from contrasting environmental settings in the European loess belt, the Eastern Mediterranean mountain ranges and the eastern USA. Detailed field-based sediment archive studies and sediment budgets covering time periods ranging from 200 to over 5000 year, as well as the use of pollen and sediment provenance techniques, show that no overarching concept of changes in floodplain sedimentation following anthropogenic disturbance can be established. Slope-channel (dis)connectivity controls the existence of thresholds or tipping points that need to be crossed before significant changes in downstream sediment dynamics are recorded following human impact. This coupling can be related to characteristics of human pressure such as its duration, intensity and spatial patterns, but also to the geomorphic and tectonic setting. Furthermore, internal feedback mechanisms, such as those between erosion and soil thickness, further complicate the story. All these factors controlling the propagation of sediment from eroding hillslopes to river channels vary between regions. Hence, only unique patterns of fluvial geomorphic response can be identified. As a result, unravelling the human impact from current-day sediment archives and predicting the impact of future human disturbances on fluvial sediment dynamics remain a major challenge. This has important implications for interpreting contemporary sediment yields as well as downstream sediment records in large floodplains, deltas and the marine environment, in terms of changes in the drivers of environmental change.

Sediment cascades and the entangled relationship between human impact and natural dynamics at the pre‐pottery Neolithic site of Göbekli Tepe, Anatolia

“…Nonlinear responses of the geomorphological system to climatic and anthropogenic triggers, its system‐inherent complexity and internal feedback mechanisms, and the complex interdependencies among them often complicate the identification of clear cause–effect relationships (Verstraeten et al ., 2017). Further, the climatic and anthropogenic triggers are often interacting; one signal might level or accelerate another signal (cf.…”

Meta-Analysis of Geomorphodynamics in the Western Lower Bakırçay Plain (Aegean Region, Turkey)

“…Soil depletion [16] and a decreased erodibility due to higher stone coverage of soils after a period of erosion during the Iron Age around Sagalassos are a reason why geomorphodynamics might peak before the climax in human activities during the Classic period [12,141]. The depletion of soils on slopes that were sensitive to erosion might have caused a shift of agricultural activities and settlement locations to plain areas around Sagalassos that were less susceptible to erosion [12]. Preliminary data of Knitter and Ludwig [142] on the archaeological site distribution pattern in the Pergamon Micro-Region indicate a change of the preferred locations in the Classic period.…”

“…In addition to a general trend of changes in the importance of human impact on sediment dynamics in the eastern Mediterranean, local and regional studies offer a nuanced view on the topic [8]. This becomes especially important when considering different types of archives along a sediment cascade [8][9][10][11][12].…”

Holocene demographic fluctuations, climate and erosion in the Mediterranean: A meta data-analysis

“…We specifically address these various landscape settings, as it is impossible to define one isolated relationship between erosion histories and sediment stratigraphy for a given type of sediment record. Indeed, many studies have shown that colluvial, alluvial, lacustrine and coastal records all respond differently to human-induced erosion depending on the timing, duration, intensity and spatial extent of human impact (Dusar et al, 2011b;Fryirs, 2013;Houben et al, 2013;Verstraeten et al, 2017;Zolitschka et al, 2003). A critical comparison of how the various sedimentary environments responded to changes in demography, anthropogenic land cover and/or climate change may include therefore valuable information on the complexity of past human-environment interactions.…”