Resumo

CHAVEZ LOPEZ, Saúl. Anthropic effects on the geomorphology and morphodynamics of the coastal zone of La Paz lagoon, Baja California Sur, Mexico. Invest. Geog [online]. 2020, n.103, e60084.  Epub 09-Mar-2021. ISSN 2448-7279.  https://doi.org/10.14350/rig.60084.

The aridity and geographic isolation of the region currently occupied by the State of Baja California Sur have determined its status as a sparsely populated region for most of its history, which has allowed the ecosystems therein to remain pristine or little impacted. Nevertheless, the State has not been immune to the global trend of vigorous population growth in coastal regions. This trend became evident in La Paz city since the 1990s (population as of 1995: 158,800; as of 2015: 253,021 inhabitants), caused by a tourism-driven economy that has led to accelerated, unplanned urban sprawling (urban area as of 1995: 44.7 km2; as of 2017: 85.7 km2). The objective of this study was to identify and analyze anthropic effects on the geomorphology and morphodynamics of the coastal zone, starting with the geomorphological reconstruction of the likely original conditions, followed by an analysis of the population growth and economic activities leading to urban expansion, to finally elucidating their effects on and the evolutionary response of the coastal zone. We used linear regression methods for analyzing geomorphological and morphodynamic changes from historical documents, particularly those that are, or can be, georeferenced such as maps, aerial photographs, and satellite images. This allows utilizing both methods over long-term, large (kilometer) spatial scales. Linear regression methods used all the shoreline location data points available to identify changes over time and estimate evolutionary rates. One distinctive advantage of linear regressions is the possibility of using coastlines as delineated from aerial photographs or historical maps, since beach profiles encompassing long-term, large (kilometer) spatial scales are usually not available.

The study area comprises two distinct geomorphological units - the alluvial plain and the coastal zone - subject to increasing anthropic activity as evidenced by urban expansion over time. This affects the geomorphological environment, causing major changes in the physical environment, such as fragmentation in parcels and streets, deforestation, and land leveling, as well as the diversion and filling of streambeds. The direct effects of anthropogenic activities on the coastal zone lead to the alteration and partial or total loss of coastal ecosystems such as coastal plains, marshes, mangrove swamps, beaches, and dunes, caused by the construction of tourism infrastructure including hotels, marinas, docks, golf courses, and buildings ranging from residential areas to car junkyards. At the same time, the effects of human activities on the alluvial plain indirectly impact the coastal zone through the reduction of the effective runoff area caused by unabated urban expansion. This, in turn, impairs the sedimentary balance that gave rise to alluvial coastal environments, in which the intensity of the dynamic agents (such as wind, tides, and waves) acting on the coastal zone remained constant. These agents have been acting with the same intensity but, in the absence of sedimentary material, conditions have changed sufficiently to affect the morphodynamics of the coastal fringe of the La Paz lagoon. The evolutionary trend of the La Paz lagoon shows that it has experienced an erosive process at a rate of 0.00227 km2 over 44 years (from 1973 to 2017, or 0.3 m/ year). Paradoxically, the poor or nil urban growth planning is not only causing the loss of the resources that support tourism activities, but is also exacerbating the vulnerability to flooding events in L a Paz city and surrounding suburbs. Therefore, it is crucial to properly weigh the benefits of continuing building hotels, marinas, golf courses, residential areas, and other types of infrastructure, versus those of conserving marshes, mangrove swamps, beaches, and dunes, whose proper use and management would contribute to the sustainable economic and environmental development of the region.

Palavras-chave : geomorphology; morphodynamics; urban area; remodeling; erosion.

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