In the piedmont and alluvial plain areas that make up the study area, 5 main sub-environments were identified (Fig. 6): Distal Andean Piedmont (DAP), Desaguadero River Valley and associated lake systems (DRV), Western Piedmont of the Serranías Occidentales (PSO), San Juan River spill zone (SRD) and Dune fields of the old Bermejo River alluvial plain (DBF). The units of mountains corresponding to the Serranías Occidentales (SO) range present in the study area, are identified in the map in Figure 6 but are not described in this contribution.
A notable general feature of the area of interest is the great difference between the western sector (DAP) and eastern sector (PSO) with respect to the Desaguadero River Valley (DRV), both in drainage density, vegetation, and in the dominant geoforms and/or geomorphological associations.
The marginal subunits of the Desaguadero River Valley and the associated lake systems are first briefly described in order to be further described later for the Desaguadero River system.
Distal Andean Piedmont (DAP)
This subunit is part of an extensive morphological unit that Polanski (1952) called “Gran Llanura de la Travesía” and which forms a wide sedimentation basin along the orographic blocks parallel to the Precordillera, the Cordillera Frontal and the Macizo de San Rafael, in the west, and the Sierras de San Luis (Sierras Pampeanas Occidentales), in the east (Abraham and Prieto 1981).
In the analyzed area the most outstanding geomorphological features are the dune fields, the sand beds and the deflation and temporary waterlogging surfaces, all of them associated with the old spills of the Tunuyán and Mendoza rivers (Ojeda et al. 2013). The latter suffered the displacement of their main course associated with avulsion and/or tectonism processes (Ortíz et al.1977, Martinez et al. 2008) until they reached their current position.
The relief is characterized by a very gentle slope to the east, variable between 0.02 and 0.05 %, with internal slopes that generally do not exceed 3 m in height, associated with the presence of lunettes and longitudinal dunes. Surrounding the dunes and the temporary deflation and waterlogging surfaces, there are also medanous accumulations (sandy geoforms with no defined shape) and thin sandy layers that, in general, do not exceed 1m in thickness. Except for localized sectors, the dune geoforms are highly immobilized due to the fixation of the deposit by plant colonization.
The temporary deflation and waterlogging surfaces form units with a flat bottom and a slight slope against the regional slope, developed on reddish sandy siltstones. They are completely devoid of vegetation so they are often informally referred to as "peladales" and are easily distinguishable in satellite images due to their light shades. González Díaz and Fauqué (1993) pointed out that these areas are locally referred to as “ramblones”, whereas, due to their occasional flooding by rainwater or river overflows, they are also known as “bañados” or “barreales” (Costa et al. 2001).
Western Piedmont of the Serranías Occidentales (PSO)
It comprises a foothill strip with variable widths between 5 and 20 km, located at the foot of the elevations of the Serranías Occidentales (González Díaz 1981) whose elevations include, from south to north, the Cerrillada de las Cabras (768 m.a.s.l.), the Sierra de El Gigante (1140.8 m.a.s.l.), the Sierra de las Quijadas (1069.8 m.a.s.l.) and the sierras of Guayaguas (830 m.a.s.l.), Cantantal (892.7 m.a.s.l.) and Alto Pencoso (670 m.a.s.l.) (Fig. 6).
This piedmont unit is made up of different aggradation levels that, from its middle and distal sectors, form a gentle piedmont descent with slopes of 4% in the proximal sectors and 0.24% in the distal ones. These are overlain by large alluvial fans, such as the one on Cerro Cantantal (43 km2), Sierra de las Quijadas (21 km2), Agua Amarga (104 km2), Sierra de El Gigante (209 km2), Represa del Carmen (107 km2) and Arroyo Jarilla (97.6 km2).
Other prominent geoforms include streams associated with current surface runoff, which can develop wide flood plains, with values between 200 and 1200 m wide and encasement smaller than 5 m. These ephemeral courses discharge the sediments transported after the great rains as lobes in the distal foothill zone and in some sectors invade the environment of the old dried lakes associated with the Desaguadero River.
Unlike what occurs on the right bank of the Desaguadero River, aeolian geoforms are very scarce, except in those sectors where recent runoff from foothill streams and alluvial systems that remain dry most of the time, are remobilized by wind action. In the distal sectors the fluvio-aeolic rework of the foothill deposits themselves develops sandy layers with thicknesses of 60 cm to 1 m.
Dune Fields of the old Bermejo River alluvial plain (DBF)
The Bermejo River valley has been defined as an extensive sandy intermountain desert (Suvires 1984) where the Bermejo river converges with the San Juan River in La Tranquita, 7 km north of the town of La Tranca (Gez 1938). Its waters are turbid, slightly reddish and hardly mixed with the clear waters of the confluent river. In the sector covered by this study, there are large areas of deflation and temporary waterlogging, limited on the leeward side by sandy accumulations of the lunette type and with partially saline floors. In the leeward sectors there are also longitudinal dunes, product of the remobilization of the sands of the old flooding plain.
The geomorphological pattern is similar to that described for the distal Andean piedmont sub-environment, where lunettes and longitudinal dunes indicate winds from the E-SE quadrant. The course of the Bermejo River can be followed in satellite images up to about 30 km north of the spills of the San Juan River, then it is lost due to the sandy remobilizations of the old alluvial plain.
San Juan River spill Zone (SRD)
It comprises the final reaches of the San Juan River on the tripartite boundary of Mendoza, San Juan and San Luis provinces. Due to the decrease in river energy because of the low slope, morphologies of spills and ephemeral lake bodies are developed, locally referred to as Lagunas del Rosario, which integrate the Guanacache Lake system. Vitale (1940) also referred to them as Bañados de San Miguel and La Tranca.
In its south-southeast limits, the transition from non-incised to incised channels can be observed. The latter are taken as the boundary of the Desaguadero River basin. Satellite images records indicate recent activity in this subunit until 2017, although with a clear decrease in the surface area involved compared to that of 1998.
Desaguadero river valley and associated lake systems (DRV)
González Díaz (1981) characterized the environment of the Desaguadero River as a “ponded river” constituted by extensive and shallow lakes to the north and with a lower section with salt marshes and circumstantial ponds.
Due to the practically null contributions of flows from the San Juan River, currently the Desaguadero River can be classified as a non-permanent course, with a local feeding by seasonal rains that in the last 5 years averaged a value of 250 mm/year, according to the information provided by La Tranca meteorology station (REM 2021). Likewise, data from the Km 47.3 station (SNIH 2021), located more than 150 km from the study area in the mountain area on the San Juan River, clearly show how at the beginning of the twentieth century, maximum flows of 745,000 m3/s were recorded, to then decrease to maximum values of approximately 400,000 m3/s and finally at 100-200 m3/s towards the end of the century, with a last outstanding record of 150 m3/s in the year 2008. Finally, the capacity data of the N° 1219 "El Encón" Station (SNIH 2021), located about 20 km upstream of the current terminal spills of the San Juan River, indicate values that since 2015 oscillate between only 4 and 8 m3/s (Fig. 7).
In addition to the decrease in ice-snow melting flows from the cordillera, there are water restrictions associated with the construction of 4 dams (Ullum, Los Caracoles, Punta Negra and Cuesta del Viento) along the San Juan and Jachal rivers, on San Juan province, of which Punta Negra and Los Caracoles are for consumptive use (González 2016).
Based on the detailed hydrological mapping using Google Earth © images, the headwater of the Desaguadero River can be located at the convergence point of a series of incised-channel tributaries which generate a subdentritic to subrectangular network, located between 1 and 2 km south-southeast of the terminal sector of the Guanacache complex system of channels and ephemeral lakes.
From the morphological point of view, the Desaguadero River corresponds to a mostly rectilinear course for the 142 km section included in the study area, with a sinuosity index of 1.28 and a very low longitudinal slope with frequent values between 0.03 and 0.06%. These characteristics are attributed to the main control exercised by regional tectonic structures and their location parallel to the direction of the main landscape geoforms.
The current channel has a variable width of 36±15 m, commonly between 2 and 5 m above the levels of its former floodplain, including the floor of the former temporary ponds, although in some sectors it can reach 6 to 8 m deep (Fig. 8). The channel bed is represented by thin thicknesses of sandy-loamy sediments, which partly expose sedimentary rocks referred to the Desaguadero River Member (Chiesa et al. 2015) of the Arco del Desaguadero Formation (Rodriguez and Barton 1993).
The incision of the main current channel of the Desaguadero River in its former floodplain was pointed out by Gez (1938) who indicated its main receding headwaters at Salto de Acevedo, about 50 km south of Laguna Silverio (L3.1), approximately, in the Laguna El Tapón (L8) (Table 1, Fig. 10a). Vitali (1940) points out that the drying up of the ponds had already been foreseen in 1883; and in 1928 as date of the incision, they had observed a 2 m drop in the riverbed, in the place where a metal bridge was being built over the Desaguadero River. Rodríguez (1966) and Chiesa et al. (2010) also suggested a relatively recent incision of the current channel; the latter are based on radiocarbon dating and the evolution of the Bebedero paleolake, the local base level of the entire river system (González 1981).
Currently, the headwater associated with the current incised channels is located near La Tranca locality (San Juan), about 80 km north of the location mentioned by Gez (1938). According to the local residents, this position was reached after erosive processes temporarily associated with the huge earthquake of Caucete (San Juan) in 1977. This allows us to approximate an average backward erosion speed of the main slope of 2 km / year.
From the intersection of the Desaguadero River with the Arroyo Jarilla to the north, there are notoriously horizontal surfaces, constituted by silt-sandy sediments, by sectors covered with saline precipitates, corresponding to the floors of the old ephemeral lakes.
The ephemeral lakes are characterized by soils like El Ramblón series (Peña Zubiate y d’ Hiriart 2009), with little genetic development (A1-ACk-Ck), of dark brown coloration (7.5YR3/4 to 4/4 according to Munsell Color System), sandy loam texture with abundant calcium carbonate, and belonging to the Order of Entisols.
The origin of the ephemeral lakes of the Desaguadero River is interpreted as a combination of factors such as important flood flows during the ice melting season, the low longitudinal gradient of the riverbed and its floodplain and an impermeable bed (González Díaz 1981; Ojeda et al. 2013). During its active period, its character was of dominant clastic sedimentation and associated with foothill periglacial river systems. Even after the important incision of its main riverbed, the seasonal flood flows inundated the ancient plains and the hydro-aeolian depressions excavated. The last major event was recorded in the summer of 1987-1988 (Ojeda et al. 2013).
The combination of erosion-wind accumulation processes with the formation of lunettes in the leeward margins, developed the flood depressions and together with the periodic floods, they shaped their morphological characteristics and traits.
The floods of Desaguadero River developed systems of chained ephemeral laked, which occupied large but shallow areas. According to Gez (1938), with the exception of Las Quijadas and Silverio ephemeral lakes, all other low lands that are covered by floods are shallow embalmed where large masses of cattail grow.
The dimensions of the bodies decrease noticeably from the headwaters of the river to the south and also have a variable degree of internal geomorphological complexity. In some areas there were differentiations in a multiple set of smaller lakes, called here Ephemeral Lake Complexes (ELC), while in others, the internal complexity is much lower or there is a single body categorized as Ephemeral Lake (LE) (Table 1). In both cases, the lakes or complexes of ephemeral lakes are limited to the west by lunettes of variable dimensions, whose maximum heights are reached to the south, such as those associated with Gualén-El Tapón lake, with maximum slopes of up to 14.7 m (Fig. 9).
In the north of the study area, near La Tranca the so-called "Guanacache Lakes Complex" (L2) begins, shared by the provinces of Mendoza and San Juan and associated with the dynamics of the San Juan river. From the sources of Desaguadero River to approximately 50 km downstream and with a maximum width of 16 km, L3, a complex of ephemeral lakes develops, that contains two main lakes registered as such in historical maps called Silverio Lake (L3.1) and El Porvenir Lake (L3.2).
Continuing to the south there are smaller lake complexes such as L4 and L5, without specific names so far. L6 corresponds to El Ramblón, which is characterized by a very saline floor, which some authors call “Gran Bajo Salitroso” (Peña Zubiate y d’ Hiriart 2009). To the east and southeast of the above is El Quebrachito (L7), one of the best defined ELC in terms of its internal structure and which will be used as an example to analyze its evolutionary development. Then, El Gualén - El Tapón (L8), Corral de Tierra (L9) and South Azud Lake (L10). The latter three have an elongated structure in a north-south direction, large lunettes on their eastern edges, and a relatively simple internal structure.
Table 1
Ephemeral lakes of the of Desaguadero River (CLE= Ephemeral Lake Complexes; LE= Ephemeral Lake)
|
Name
|
Other names
|
Surface. (ha)
|
L1
|
ELC Guanacache Lakes
|
Rosario Lake (Province. Of Mendoza and San Juan)
|
No data
|
L2
|
ELC Guanacache Lakes
|
Basin of lakes and reservoirs - Rincón Lake
|
35393.0
|
L3
|
ELC Las Quijadas Lakes
|
Silverio Lake (L3.1)
El Porvenir Lake (L3. 2)
|
57035.2
|
L4
|
ELC Arroyito
|
-
|
7787.9
|
L5
|
ELC unnamed
|
-
|
1423.3
|
L6
|
LE El Ramblón
|
La Masa Cruz and Los Chosmes Lakes
|
5672.2
|
L7
|
ELC El Quebrachito
|
Quebrachito Lakes
|
5815.0
|
L8
|
LE Gualén - El Tapón
|
Santiago or Salto Lake
|
5655.2
|
L9
|
LE Corral de Tierra
|
-
|
464.0
|
L10
|
LE South Azud Lake
|
-
|
1962.7
|