Cambios de la línea costera en el delta del río Balsas, Pacífico mexicano, entre los años 1943-2009

Since the second half of the twentieth century, the undertaking of a number of hydraulic and hydroelectric projects has contributed to the disruption of the balance of water and sediment flows in river basins, leading to accelerated processes of progradation and regression in deltaic and other coastal landforms. The retention of sediments in reservoirs has not only resulted in coastal regression, but has severely impacted natural and socio-productive ecosystems as well. In the case of the deltaic plains of the Balsas river in Mexico, the construction of El Infiernillo and La Villita hydroelectric plants in 1964 and 1973, respectively, led to a marked regression of the delta as well as several morphodynamic changes in riverine and coastal areas. The Balsas river basin is located between 17° and 20° North and 97° 30’ and 103° 15’ West, occupying the border between the states of Guerrero and Michoacan along the Mexican Pacific coast. The delta, itself an expression of fluvial sedimentation at the lower course, as the river leaves the mountainous terrain of the southern Sierra Madre, consists of a vast plain representing one third of the total watercourse from north to south, and is divided into two distributary arms leading all the way into the Pacific Ocean, where clear signs of progradation such as cross stratification and valley fills have been detected on the gentle slopes offshore, far away from the coast. This study analyzes the morphodynamic changes taking place at the Balsas river delta front over 66 years between 1943 and 2009, using documentary evidence from official maps (1964, 1980, 1990, 1998, 2001 and 2003), as well as photographic restitution of the coastline using various aerial photographic materials (1943, 1958-59, 1963, 1974, 1976-1977, 1981-1982, 1985, 1996, 2006 and 2009). In the western sector, between 1943 and 1958, prior to the construction of the engineering projects, a significant trend was observed in the progradation of the delta front of the order of +1,103 m, whereas at the apex of the delta the coastline advanced towards the sea at a rate of +73.53 m/year. The most significant changes in the regression of the delta, with maximum regressive rates of -20.5 m/year, occurred during the stage of controlled water flows between 1963 and 1974, due to the closure of reservoirs, after a long phase of natural progradation from 1943 to 1958. In the western part of the delta, the average progradation rate reached -8.8 m/year (1958-2009), in contrast to the progradation rate of +27.47 m/year prior to the construction of the projects. In the eastern part of the delta, on the other hand, regression prevailed during all the periods analyzed, especially between 1963 and 1974, when even the delta coastline lacked effective protection. The maximum linear regression at the coast reached -391.83 m during that period. Later, in 1979, jetties and breakwaters started to be built to reduce the erosive impact of waves. These coastal protections resulted in a sustained reduction of erosion (average rate of -1.45 m/year) until 2009. When compared to the changes in the western sector, the regressive phase in the eastern part of the delta is notorious. The most significant extreme value occurs in the period 1963 to 1974, with a loss of beach surface of 86.68 ha, indicating an average rate of coastal regression of -10.12 m/year. It is expected that the process of coastal regression at the delta keep a low rate in coming years, given the routine maintenance to coastal protections and other infrastructure at the Lazaro Cardenas industrial port. Though they generally help improve the environmental and socio-economic conditions of upstream areas, dams may cause multiple imbalances and may adversely impact the environment towards the river mouth, leading to problems such as land losses, damages to the flora and fauna, decline of the water table, subsidence of alluvial and deltaic plains by differential sediment compaction, and the intrusion of saline water in areas adjacent to the coast, mainly during high tide, resulting in soil salinization and loss of fertility. These impacts are already identified at the Balsas River delta plain.