LAND COVER CHANGE IN PERU'S FLUVIAL MARGINAL STRIP THROUGH MULTITEMPORAL SENTINEL-2 ANALYSIS (2018-2025): A SATELLITE-BASED INDICATOR OF ENCROACHMENT RISK

Peru's Autoridad Nacional del Agua (ANA) has approved, through administrative resolution, the delimitation of 3,826 fluvial marginal-strip segments nationally, documented through 378,438 georeferenced control points. This study evaluates, for the first time at the national scale, whether a satellite signal consistent with encroachment is present, comparing two Sentinel-2 spectral indices, the Normalized Difference Vegetation Index (NDVI) and the Normalized Difference Built-up Index (NDBI), between a baseline (2018-2019) and a recent period (2023-2025), using freely available imagery through Microsoft Planetary Computer. Of 937 segments with valid satellite coverage (24.5% of the 3,826 approved segments), 234 (25.0%) showed a signal consistent with encroachment, concentrated in Amazonian departments (San Martin, 76.2%; Madre de Dios, 75.9%; Huanuco, 66.7%). Departments with large, already consolidated urban corridors, such as Arequipa, Lima, and Ayacucho, showed low encroachment signal (3.1%-7.7%), possibly because the index saturates in already densely built-up areas.