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Abstract The Amazon basin is experiencing severe droughts that are expected to worsen with climate change. Riverine communities are especially vulnerable to these extreme events. This study investigates the experiences of Brazilian Amazonian communities during droughts occurring from 2000-2020. We assess the distribution of settlements at risk of prolonged isolation during extreme low-water periods, along with impacts reported in digital news outlets. Using historic time series of river levels from 90 gauges, we look at how long droughts lasted in regions with reported impacts. Results indicate that the droughts in 2005, 2010, and 2016 were the most severe, with over an additional month of low water levels in those years. Such drought events routinely disrupt inland water transport and isolate local populations, limiting access to essential goods (food, fuel, medicine) and basic services (healthcare, education). Given this new reality, Amazon countries must develop long-term strategies for mitigation, adaptation, and disaster response. 

Small stream dams and their reservoirs are ubiquitous in Brazil but mostly unmapped in existing datasets. Often used for cattle watering, farm-scale hydropower, irrigation, and/or fish aqua-culture, these reservoirs can have immense cumulative impacts on aquatic habitats, down-stream water quality, greenhouse gas emissions, and water balances. By applying deep learning to data from Landsat and Sentinel satellites, we created the first comprehensive datasets on small surface-water reservoirs in Brazil, including annual maps from 1984 to 2023. Then, we synthesized these maps with land-use data, property information, and semi-structured inter-views conducted in Mato Grosso state to elucidate the drivers and impacts of reservoir creation and persistence. First, we used high-resolution Sentinel-1 and -2 satellite data from 2021 to map over 1 million in-stream reservoirs smaller than 1 km2, a massive increase from existing datasets. Using weather data and climate models, we estimated that these reservoirs lose approximately 11.7 km3yr-1 to surface evaporation, a number projected to increase by 3-13% as climate change drives warmer and drier weather. Next, we showed that these reservoirs can be consistently and accurately mapped over time using Landsat 5, 7, and 8 data with only a slight de-crease in performance compared to Sentinel. From 1984 to 2023, the number of reservoirs in Brazil increased dramatically from 265,617 to 1,040,754 and the total surface area increased from 3526 km2 to 8629 km2. Then, we drew on interviews and MapBiomas land-use data to demonstrate that creation typically accompanied clearing of forest and savanna for cattle pasture in the first-waves settlement in the so-called hollow frontier of Mato Grosso. In many cases, these reservoirs were obsolete legacies of the hollow frontier, left over after intensified crop agriculture replaced dispersed cattle ranching. However, others are still critical and flexible sources of water, particularly for small-scale dairy, beef, fruit, and vegetable producers. Finally, we analyzed how the overarching discourse of resource abundance in eastern Mato Grosso obscures daily experiences of water scarcity, showing that the abundances of land, capital, and infrastructure contribute to and intersect with water inequality to favor large, export-oriented crop farms over small-scale producers. Applying an interdisciplinary approach to synthesize deep learning and remote sensing with political ecologies of water and agriculture, we highlight the important role of these reservoirs in agricultural expansion and intensification in Brazil and especially Mato Grosso state. Although they have widespread negative environmental impacts, reservoirs are also a critical form of water infrastructure for many. As climate change and the adoption of irrigation alter agricultural water use across Brazil, our results can inform water management decisions that balance the positive and negative social, environmental, and economic effects of stream damming.