Geolimnology of the lentic bodies of the drainage basin of Arroyo La Vigilancia, Chascomús, Buenos Aires Province, Argentina.

Abstract

The southwest area of Chascomús region is characterized by important superficial aquatic resources. These integrate the drainage area of laguna Esquivel (577 km2 ), a tributary of the Salado River basin. This paper deals with the western end of this basin, represented by the drainage area of Arroyo La Vigilancia (99.3 km2 ), a water collector of the Cildáñez (0.97 km2 ), Espadaña de Pérez (2.9 km2 ), Chica (0.75 km2 ) and San José de la Leña (2.38 km2 ) ?lagunas?. The region climate is temperate humid, with average annual temperature of 15.6 o C, with mild winters and hot summers. Average rainfall in Chascomús is 920 mm/ year (period 1888/2003), evenly distributed throughout the year. The region is a loess plain of gentle relief with a very slight slope to southeast (average slope 0.04 %). It possesses very scarce low-altitude heights, blanketed by primary Holocene loess deposits. The average altitude of the region is 14.5 m above sea level. The drainage of the area is naturally endorheic and faulty; this fact determines the stagnation of the waters, which are at present drained by artificial canals. The tributaries of the lentic environments of the system are represented mainly by artificial canals and the outflow waters is made only by canals. The 3.5 km outflow stream of laguna San José de la Leña is a canal superimposed on the primitive Arroyo La Vigilancia and constitutes the main effluent stream of the region. The canals of these lagunas have had a negative effect on these water bodies, accelerating their degradation. The lake basins are located in gentle depressions of aeolic origin, surrounded by a deflation-accumulation relief. At the present time there has been a remarkable leveling due to sheet flow toward the lake beds. This phenomenon confers a concave-convex aspect to the landscape. The convex sandy silty relief surrounds the lake basins and the concave relief, constituted by muddy materials, is the collector of water surpluses. This morphology generates hypodermic drainage from the highest sectors to the depressions. From the geomorphological point of view, these ?lagunas? and their drainage basin constitute elements belonging to the geomorphologic unit of the ?laguna Esquivel basin?. These elements, defined by their parameters, include positive and negative erosion landforms and those of anthropic origin. The positive landforms are scarce and correspond to very gentle rises 2 m high, while the negative ones are represented by lentic basins, gentle depressions (?bajos?) and small water courses. The anthropogenic landforms are canals, bridges and roads. The canals contribute with water and sediments to the lake system. The roads affect runoff as hydraulic barriers or flow paths. The geomorphologic features, the morphometric parameters and the relationship with groundwater of these lacustrine bodies allowed their classification as follows: lagunas Cildáñez and San José de la Leña are permanent lakes and lagunas Chica and Espadaña de Pérez are temporary lakes; both groups are degraded by hydrophytes, with a tendency to become a swamp. The physical and chemical water characteristics of these ?lagunas? were defined, indicating their suitability for aquatic life, recreation and irrigation. Three stratigraphic units outcrop in the region; the oldest one is Buenos Aires Formation of Late Pleistocene age, which constitutes the regional substrate. It is overlain in disconformity by La Postrera III Formation of Late Holocene age, where the present soils are developed. The third unit is represented by recent alluvium coastal sediments, originated by the destruction of deflation/accumulation relief of the region. The infilling sediments of the lakes were studied by auger samples and their textural, statistical, mineralogical and paleontological parameters were assessed. Two well defined lithostratigraphic units were recognized in lagunas Cildáñez, Chica and Espadaña de Pérez and four in San José de la Leña. The upper unit is made up of pelitic sediments assigned to the present freshwater body; they are underlain by palustrine muddy sediments of the Luján Formation, Río Salado Member of the Early Holocene. In San José de la Leña, the third unit is composed of greenish sandy silts from a brackish water environment with mixohaline and freshwater remains, correlated with the Luján Formation, Lobos Member of the Upper Late Pleistocene. The fourth unit is made up of pale brownish sandy silts with freshwater remains, assigned to the Luján Formation, La Chumbiada Member of the Late Pleistocene. The origin of the lacustrine basins would be related in laguna San José de la Leña to the aeolian events of late Pleistocene of Bonaerense age, while the origin of the other water bodies would be ascribed to an ultimate Pleistocene/Early Holocene aeolian episode which excavated basins and deposited the loess of La Postrera II Formation. The invading macrophyte populations, the hydrological regime and the water dynamics of the four water bodies were also briefly characterized.