Synergistic process interactions and morphological change in a river reach subject to multiple disturbances, the Laja River, Chile

This work studies the multiple sources of impacts and disturbances in the Laja River (Central Chile) and evaluates the changes in water and sediment flows and planimetric geomorphic changes. The disturbances sources correspond to hydroelectric plants, water withdrawals for irrigation and the sustained decrease trend in rainfall in the basin. The changes in the plan-view shape of the river reach were quantified using remote sensing techniques, through a supervised classification of Landsat 5 TM and 8 OLI satellite images, identified Water (W), Islands and Riverine Vegetation (IRV) and Bars and Banks Without Vegetation (BBWV), obtained a Kappa index>0,83 for a period of 15 years (2006-2021). Compared with historical records, the period of analysis shows a decrease in annual rainfall by 17.5%. In addition, water withdrawals for irrigation have contributed to a 64% decrease in monthly stream discharge during the dry season. As a consequence of the decrease in annual rainfall and water withdrawals for irrigation, the sediment transport capacity has also decreased by 10.5%. The changes in morphological driving variables (stream flows and sediment transport regimes) have manifested themselves in morphological changes, where it was possible to establish that a change in the channel form occurred in the last 15 years, going from a river with a single channel to a braided one. An important vegetation establishment has accompanied this morphological change on both riverbanks and the central bars. The colonizing vegetation corresponds to fast-growing non-native species (Salix spp., Populus spp. and Alnus spp). A stabilization of the channel form is expected, consolidating itself as a braided section with alternating vegetated bars.