Abstract
Abstract. Deforestation is associated with a decrease in slope stability through the alteration of hydrological and geotechnical conditions. As such, deforestation increases landslide activity over short, decadal timescales. However, over longer timescales (0.1–10 Myr) the location and timing of landsliding is controlled by the interaction between uplift and fluvial incision. Yet, the interaction between (human-induced) deforestation and landscape evolution has hitherto not been explicitly considered. We address this issue in the North Tanganyika–Kivu rift region (East African Rift). In recent decades, the regional population has grown exponentially, and the associated expansion of cultivated and urban land has resulted in widespread deforestation. In the past 11 Myr, active continental rifting and tectonic processes have forged two parallel mountainous rift shoulders that are continuously rejuvenated (i.e., actively incised) through knickpoint retreat, enforcing topographic steepening. In order to link deforestation and rejuvenation to landslide erosion, we compiled an inventory of nearly 8000 recent shallow landslides in © Google Earth imagery from 2000–2019. To accurately calculate landslide erosion rates, we developed a new methodology to remediate inventory biases linked to the spatial and temporal inconsistency of this satellite imagery. Moreover, to account for the impact of rock strength on both landslide occurrence and knickpoint retreat, we limit our analysis to rock types with threshold angles of 24–28∘. Rejuvenated landscapes were defined as the areas draining towards Lake Kivu or Lake Tanganyika and downstream of retreating knickpoints. We find that shallow landslide erosion rates in these rejuvenated landscapes are roughly 40 % higher than in the surrounding relict landscapes. In contrast, we find that slope exerts a stronger control on landslide erosion in relict landscapes. These two results are reconciled by the observation that landslide erosion generally increases with slope gradient and that the relief is on average steeper in rejuvenated landscapes. The weaker effect of slope steepness on landslide erosion rates in the rejuvenated landscapes could be the result of three factors: the absence of earthquake-induced landslide events in our landslide inventory, a thinner regolith mantle, and a drier climate. More frequent extreme rainfall events in the relict landscapes, and the presence of a thicker regolith, may explain a stronger landslide response to deforestation compared to rejuvenated landscapes. Overall, deforestation initiates a landslide peak that lasts approximately 15 years and increases landslide erosion by a factor 2 to 8. Eventually, landslide erosion in deforested land falls back to a level similar to that observed under forest conditions, most likely due to the depletion of the most unstable regolith. Landslides are not only more abundant in rejuvenated landscapes but are also smaller in size, which may again be a consequence of a thinner regolith mantle and/or seismic activity that fractures the bedrock and reduces the minimal critical area for slope failure. With this paper, we highlight the importance of considering the geomorphological context when studying the impact of recent land use changes on landslide activity.
Highlights
On steep terrain, the erosion caused by shallow landslides increases significantly as a result of deforestation (e.g. Montgomery et al, 2000; Mugagga et al, 2012)
When forests are permanently converted to grassland or cropland, the consequences of deforestation for landsliding can last much longer or even be permanent (Sidle et al, 2006). While these general principles are well described, we do not yet fully understand the extent to which the response to deforestation is modulated by tectonic forcing, which typically occurs over timescales of 0.1–10 Myr (e.g. Whipple and Meade, 2006)
While the landslide erosion rate is higher in rejuvenated landscapes due to a steeper relief, the relative effect of slope steepness on landslide erosion appears to be weaker in rejuvenated landscapes: we found that steep (> 35◦) forested slopes display higher shallow landslide erosion rates in relict landscapes than in rejuvenated landscapes (Fig. 12c)
Summary
The erosion caused by shallow landslides (with a maximal depth of a couple of meters) increases significantly as a result of deforestation (e.g. Montgomery et al, 2000; Mugagga et al, 2012). A key distinction can be made here between actively incising, rejuvenating landscapes, in which landslides are a prime slope-limiting mechanism and “old”, so-called relict landscapes, where hillslopes have had a long time to adapt to river incision (Burbank et al, 1996; Larsen and Montgomery, 2012). These two landscape types can be expected to respond differently to deforestation: in rejuvenating landscapes, hillslopes are already continuously adapting to river incision through landsliding (Egholm et al, 2013). Climatic variations can drive differences in landscape response to deforestation (Crozier, 2010), and in the context of lithologically diverse landscapes, the effect of rock strength on both knickpoint retreat and landsliding must be acknowledged (Parker et al, 2016; Baynes et al, 2018; Campforts et al, 2020)
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