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

<p>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.</p> <p>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).</p> <p>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%.</p> <p>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 (<em>Salix</em> spp., <em>Populus</em> spp. and <em>Alnus spp</em>). A stabilization of the channel form is expected, consolidating itself as a braided section with alternating vegetated bars.</p>