Naturalmente complexa, nitidamente fragmentada: sensitividade da paisagem na bacia hidrográfica do rio Puraquequara, Manaus – Amazonas / Brasil

Abstract

This work analyzes landscape sensitivity in the Puraquequara River Watershed (BHRP), in the eastern sector of Manaus, as a meaningful case for understanding the interaction between natural processes and human pressures in the urban Amazon. The choice of study area is justified not only by its location within an urban expansion zone, but by the combination of morphostructural diversity, strong hydrological seasonality, high water-sediment connectivity, and the coexistence of landscape units with contrasting dynamics, tested by the flood-and-low-water pulse of the Amazonian hydrological regime. The BHRP stands out among the watersheds surrounding Manaus for bringing together, along a single gradient, headwaters embedded in protected primary forest, internal morphostructural diversity, and a heterogeneous front of cultivated areas, urbanization, industrialization, and infrastructure in the middle and lower course. Understanding this watershed requires acknowledging that the landscape results neither from natural processes alone, nor solely from human actions. Over time, physical forces and social decisions became intertwined, shaping a system marked by different rhythms and intensities of transformation. Over the past four decades, this intertwining has accelerated: the urban expansion of Manaus, intensified since the mid-1980s by demographic growth associated with the Zona Franca de Manaus free trade zone, reorganized flows, altered connections, and modified the way the watershed responds to disturbances. Starting from the hypothesis that different sensitivity levels result from the interaction between soil resistance, the intensity and spatial distribution of human interventions, and fluvial seasonality, the research articulates three analytical axes: relief morphometry, evaluation of soil physical properties (mechanical resistance to penetration, infiltration rates, and bulk density) and multitemporal interpretation of land use and land cover between 1984 and 2024. Based on this framework, sensitivity maps were produced for two complementary scenarios: one grounded in natural factors and another incorporating human intervention. This approach allows distinguishing the watershed's natural geomorphological response potential from the effects produced by the intensification of human pressures. The results indicate that the watershed is organized as a mosaic of units with contrasting behaviors. Lower-slope surfaces, divergent landforms, and continuous vegetation cover tend to buffer disturbances, yielding slower and more gradual responses. In contrast, steep slopes, convergent forms, and sectors of high hydrosedimentary connectivity operate close to functional thresholds, such that localized changes can trigger rapid and intense responses. Soil physical properties reinforce this differentiation, revealing coherent patterns according to morphostructural organization and predominant land uses. The multitemporal reading shows that the landscape has become more heterogeneous and less capable of buffering impacts over four decades, especially in the middle and lower course of the right bank, where urban expansion, opening of branch roads, road infrastructure, and large-scale developments have reconfigured the spatial distribution of sensitivity. The comparison between scenarios demonstrates that human interventions do not create sensitivity from scratch: they displace, amplify, and reorganize it across space. It is concluded that the BHRP cannot be read either as a simple urban watershed under pressure, or as a residual natural space, but as a hybrid system whose singularity lies in the coexistence between headwaters sheltered by protected areas and the heterogeneous advance of human pressures along the channel. The analysis reveals that landscape units are organized into two types: fixed units, which persist regardless of the hydrological cycle, and ephemeral units, which emerge and disappear with the seasonal flood-and-low-water pulse, a distinction essential for understanding the system's different sensitivity thresholds and for guiding environmental planning and management in Amazonian areas under accelerated urbanization.