ボリビアアマゾン盆地におけるIchilo川の平面形状変遷に関する研究

The Amazon basin is constituted by a large river network, where we can find meandering rivers with the fastest migration rates. Most of the studies in the Amazon basin are concentrated in the large tributaries of the Amazon river that have caught the attention of many researchers. Their planform shape and the changes that they experience depend on many factors such as the climate, human interactions, vegetation, etc. However, small meandering rivers in the southern part of the Amazon basin have not been as much studied as the large tributaries of the Amazon river. These rivers exhibit different characteristics compared to the larger tributaries such as short inundation periods and high fluctuation in the water levels. There is still the need to keep studying these rivers in order to answer important questions about their planform characteristics. This thesis examines the characteristics of a small meandering river in the southern part of Bolivian Amazon: the Ichilo River. The work is based in satellite imagery analysis, field observations and numerical simulations. First, a spatiotemporal analysis is performed using Landsat satellite imagery. Different meandering characteristics such us migration rates, cutoffs, widths, oxbow lakes are qualitatively and quantitatively analyzed in order to understand what are the main controls of meander migration. Water levels collected for a period of 30 years, are statistically analyzed showing patterns with high variability. Planform shapes of the river for along 30 years (1988-2018) show different meandering mechanisms. Cutoffs are plenty observed in the Ichilo, both chute and neck cutoffs, with prevalence of the later. Ten neck cutoffs (1 human-induced) were identified along 150 Km of river reach. This study suggests that neck cutoffs play an important role in the planform development of the Ichilo River. Also, I recognize the effect that El Niño La Niña Southern Oscillation (ENSO) may have played in the migration rates observed in the year 2008 together with another high precipitation event in 2014, after which cutoffs events were observed. As cutoffs were identified as main controls on meander migration a field survey was performed to obtain detailed information in one bend where a cutoff is expected to occur. The topographic surveys in the river were performed by using a combined methodology of Structure from Motion (SfM) photogrammetry using Unmanned Aerial Vehicles (UAVs) and a Multibeam Echosounder (MBES). Topographic data such as DEMs and orthophotos were obtained with high accuracy in a relatively shorter period of time in the location where traditional topographic works would have been more expensive and riskier. The bank profiles show a river with cohesive banks and dense vegetation with bank failures mainly in the outer banks. Also, the bathymetry shows riverbed scouring in two points in the river: downstream the confluence with the Sajta and in the area located near the port of Puerto Villarroel. Now consider the current planform shape of the Ichilo River. What are further changes expected in the river? Here, I present a numerical model that captures important characteristics that have been observed from the results of the spatiotemporal analysis (such as neck cutoffs) and the field works to reproduce the trends of meander migrations in the Ichilo River including the development of cutoffs. The model considers the following characteristics: a) Planform shape evolution after cutoff occurrence, b) Profile change due to cutoff occurrence and c) Inner bank accretion and outer migration as a result of the water level variation. The model is tested in three parts of the Ichilo River, showing its ability to reproduce the trends of meander migration in terms of the planform shape evolution, location of cutoffs and area of abandoned oxbow lakes, being these, one of the first results that show meandering evolution including the development of cutoffs in the Amazon basin. The purposed model can be applied to assess the trend of planimetric changes of meandering rivers at the scale of the Ichilo River due to climate change and/or other perturbations which is important for the communities that inhabit the banks of the Ichilo River