Biogeomorphology from Space: Using optical satellite imagery time series for analyzing the dynamic interaction of vegetation and hydromorphology along the Naryn River, Kyrgyzstan

Under natural conditions, the structure and development of river corridors is controlled by an interplay of hydrological, geomorphological and ecological processes. Over the past decade, the concept of biogeomorphology has become increasingly popular to describe and analyze the manifold feedback mechanisms within river systems leading to an increasing number of studies. However, the majority of this work focuses either on conceptual development or on investigations on the scales of single geomorphic units or study reaches. Only very few studies enlarge the spatial scale to entire river corridors or networks despite the fact that recent frameworks emphasize these scales to be relevant for river research and management. A recent trend in remote sensing of terrestrial ecosystem is the use of dense imagery time series to assess trends and disturbances of vegetation development. In this study, we transfer this idea to the analysis of biogeomorphological interactions within fluvial environments on large spatial scales. We take the Naryn River in Kyrgyzstan as an example for demonstrating our satellite time series approach to biogeomorphological analysis of river corridors. The Naryn is still in a natural state on an entire flow length of more than 600 km with full longitudinal and lateral connectivity. Along the central part of the catchment, the Naryn is a highly dynamic braided river system shaped by the annual summer floods of a glacial discharge regime. This makes this river ideal to study large scale biogeomorphological dynamics. In our study, we follow the well-established concept of biogeomorphological succession suggested by Dov Corenblit and his colleagues. We mapped the different succession phases in the field and used the results to derive spectral-temporal indices of the succession phases. These indices are on the one hand used for a supervised classification based on Sentinel-2 imagery. On the other hand, we use Sentinel-2 as well as the longer term Landsat imagery time series to analyze the data for statistical trends and changepoints and evaluate this regarding biogeomorphological succession and disturbance events. The results reveal that dense time series of optical satellite imagery are well suited data sources to derive indicators of biogeomorphological interactions on large spatial scales. Especially when using the recently available Sentinel-2 imagery, such indicators have the potential to analyze biogeomorphological dynamics of entire river corridors or networks in a spatially and temporally continuous way at a reasonable spatial resolution.