The Amazonian river-floodplain systems face unprecedented threats from the construction of hydroelectric power plants aimed at meeting Brazil's energy demands. However, evidence suggests that the long-term economic, social, and environmental damages outweigh the hydroelectricity advantages. The Volta Grande do Xingu was dammed and its water diverted to energy production, resulting in habitat loss due to the altered hydrological regimes. Flooded environments are vital for ecosystem services, including food production, biogeochemical cycles, and climate regulation. The construction and operation of hydropower plants disrupt these services, affecting Indigenous populations and ribeirinhos communities. Environmental Impact Assessments often underestimate these impacts, and the responsibility for monitoring falls on the companies themselves, leading to conflicts of interest and inadequate reporting. This triggered an Independent Territorial Environmental Monitoring (MATI-VGX) initiative, led by Indigenous and ribeirinhos researchers in partnership with academics. MATI-VGX's observations reveal changes in flooded ecosystems, including increased seedling density and leaf-cutter ant colonies in the understory, changes in plant species composition, disruption of the phenological cycle, and vegetation mortality. Moreover, reduced fruit dispersion disrupts the aquatic food web and threatens Indigenous and ribeirinhos food sources. These changes indicate ecosystem decay and disruption of ecological processes. Importantly, these impacts are noticeable after a short period of water diversion, highlighting the urgency of mitigation efforts. A research agenda must quantify tree mortality, understand physiological mechanisms, and assess the cascade effects on wildlife and food security. Ultimately, restoring natural hydrological patterns is essential for preserving Amazonian biodiversity and the livelihoods of Indigenous and ribeirinhos communities.