Monte Desert

Monte is a Spanish landscape and vegetation term as well as the name of a biogeographic region. English-language research commonly uses “Monte Desert,” while Argentine national mapping separates Monte de Sierras y Bolsones from Monte de Llanuras y Mesetas. This page combines those adjoining ecoregions because together they describe the familiar Monte dryland; it does not use “Monte” for every arid part of western Argentina.

The area depends on the boundary system. The 2009 review's geomorphic map totals 466,975 km². Current National Parks summaries give approximately 11.7 million hectares for Sierras y Bolsones and 35.4 million hectares for Llanuras y Mesetas—about 471,000 km² combined after converting at 100 hectares per square kilometre.^[1][2][3] The roughly 4,000 km² difference reflects classifications and broad ecotones, not a measured expansion of the desert.

The traditional biome description begins at about 24°35′S in Salta, occupies inner Andean basins in Catamarca and La Rioja, crosses the Precordillera and Sierras Pampeanas setting, and continues through the basins of San Juan, Mendoza, and San Luis. It then spreads through western La Pampa and eastern Neuquén, central Río Negro, and northeastern Chubut, reaching the Atlantic near its southeastern end.^[3] National Parks uses a somewhat different northern limit—Jujuy to northern Mendoza—and carries the southern plains-and-plateaus ecoregion to the far south of Buenos Aires as well as the Río Negro and Chubut coasts.^[1][2]

The western margin is usually the clearest because Andean and pre-Andean relief produces an elevation and climate break. The 2009 synthesis places that upper boundary near 1,500–1,700 m in western Mendoza and 2,800–3,000 m in La Rioja, while noting that the limit varies with latitude and criterion.^[3] Eastward, Monte shrubland grades into dry Chaco and Espinal; southeastward it intermingles with the Patagonian steppe. Those are ecotones—transition belts—not surveyed borders.

In the northern Monte, north–south ranges enclose valleys and bolsons, broad internally drained or partly drained depressions receiving sediment from adjacent high ground. Rock-cut pediments and depositional alluvial fans meet on basin floors. Central sectors contain extensive Quaternary plains made of river, lake, and wind-worked sediment. Southward, gravel levels, low relief, broad depressions, volcanic terrain, and stepped plateaus become more prominent; Miocene marine sediment is exposed locally in erosion fronts and Atlantic cliffs.^[3]

The review's mapped landform budget makes the balance concrete. Gravel plains on different levels cover 87,630 km² (18.7%); alluvial–aeolian plains 56,445 km² (12.1%); alluvial plains 53,179 km² (11.4%); and bolson landforms 39,287 km² (8.4%). Active dunes and dune fields occupy only 2,088 km² (0.5%).^[3] These figures belong to that paper's 466,975 km² polygon and should not be transferred to a differently drawn official ecoregion map.

The Monte is too extensive to have one rock type or moment of formation. Northern mountains expose metamorphosed basement and varied igneous, volcanic, and sedimentary units; uplift associated with the Andean system created strong relief and sediment traps. Weathering and runoff then moved rock from slopes into fans, plains, and enclosed basins. The central sandy plain contains Quaternary fluvial, lacustrine, and aeolian deposits, while parts of the south carry Pliocene–Pleistocene gravels above older sedimentary units.^[3]

Wind reworks rather than explains the whole landscape. The distribution of alluvial and aeolian deposits records shifts between wetter and drier phases during the Quaternary, and active sand occurs in separated fields such as Cafayate, Campo del Arenal, Médanos Grandes, and northeastern Río Negro.^[3] It is therefore more accurate to describe a long-lived tectonic and sedimentary framework repeatedly modified by floods, deflation, dune movement, and erosion than to assign a single age to “the desert.”

The Monte is not one closed basin. The 2009 inventory intersects 22 watersheds, and in most cases neither the headwaters nor the outlet lies inside the biome.^[3] Argentina's national basin framework distinguishes exorheic drainage that reaches the sea, endorheic drainage that ends in a lake, lagoon, or salar, and arreic terrain where water evaporates or infiltrates before forming a connected channel network; all are relevant to the Monte's physical setting.^[5]

The San Juan provides a measured example of an allochthonous river—water generated beyond the dryland it crosses. Its approximately 38,462 km² catchment, mapped in a national basin sheet using 2002 information, extends from high Andean headwaters beyond the Monte to the Desaguadero system. The river begins at Las Juntas where the Castaño and Los Patos meet, flows mainly west to east through the Precordillera, and has a snowmelt regime with spring–summer high water. Torrential summer channels entering it from arid slopes build alluvial fans.^[6] The catchment area is not an area measurement of the Monte itself.

Other large corridors include the Mendoza, Tunuyán, Colorado, Negro, and Chubut systems. Between them, short storm-fed channels lose flow on fans or terminate in closed lows. Many playas and salinas remain dry seasonally or for years; when exposed, their fine beds can supply windblown dust.^[3] Water distribution therefore depends as much on distant mountain snow, substrate, and basin geometry as on rain falling over the desert floor.

Aridity results from a persistent precipitation deficit, regional circulation, and topographic barriers rather than latitude alone. In a peer-reviewed analysis using 1961–1990 gridded climate data, annual precipitation was 100–350 mm over most of the Monte and the ratio of precipitation to potential evapotranspiration was 0.05–0.5. The ratio expresses water supply relative to atmospheric drying demand; values below 1 indicate a deficit.^[4] A separate geographic review found a wider less-than-100-to-450-mm range when wetter margins and strong relief effects were included.^[3]

North of about 35°S, Atlantic-sourced moisture and monsoon-like summer circulation dominate; spring and summer supply about 70–75% of annual precipitation. The high Andes and Precordillera disrupt that flow, making the southern La Rioja–northern Mendoza sector and the Uspallata–Calingasta–Iglesia valleys especially dry. South of about 40°S, lower Andean elevations allow greater Pacific influence and westerly winds prevail year-round, so precipitation is distributed more evenly through the year.^[4] A single rainfall total or seasonal label cannot represent the full 20-degree latitude span.

Long dry intervals are punctuated by short geomorphic events. Convective storms activate mountain gullies and fan channels; snowmelt raises selected Andean rivers; floodwater spreads sediment across low-gradient surfaces or ends in playas. Dry lake beds then expose silt and salts to deflation, while dunes migrate or are stabilized locally. These processes operate at different rates, so a quiet gravel surface, an active channel, and a dust-producing salar can lie within the same regional climate.

The Monte belongs in the Desert Hub because water deficit structures the region, but its defining cross-section is from mountain basin to Atlantic plateau—not from dune crest to dune crest. Its official two-part name, mixed drainage, and mapped landform proportions are the most reliable guide to what the feature actually contains.

1. Administración de Parques Nacionales, Argentina, “Monte de Sierras y Bolsones” (accessed 30 August 2026). Source for the official northern ecoregion name, approximate 11.7-million-hectare area, Jujuy-to-northern-Mendoza extent, intermontane valleys, bolsons, mountain slopes, and up-to-200-mm annual precipitation summary.
2. Administración de Parques Nacionales, Argentina, “Monte de Llanuras y Mesetas” (accessed 30 August 2026). Source for the official southern ecoregion name, approximate 35.4-million-hectare area, Mendoza-to-Atlantic orientation, southern Buenos Aires–Río Negro–Chubut coastal extent, and plains-and-stepped-plateaus description.
3. Abraham, E. et al., “Overview of the geography of the Monte Desert biome (Argentina)”, Journal of Arid Environments 73, 144–153 (2009), doi:10.1016/j.jaridenv.2008.04.008. Source for the traditional 24°35′–44°20′S frame, boundary and elevation qualifications, 466,975 km² geomorphic inventory and landform proportions, geology, Quaternary surface processes, 22-watershed count, and dry-water-body behavior.
4. Labraga, J. C. & Villalba, R., “Climate in the Monte Desert: Past trends, present conditions, and future projections”, Journal of Arid Environments 73, 154–163 (2009), doi:10.1016/j.jaridenv.2008.03.016. Source for the 1961–1990 gridded 100–350 mm range over most of the Monte, precipitation-to-potential-evapotranspiration ratio, circulation controls, seasonal precipitation shares, and north–south change in moisture source.
5. Secretaría de Obras Públicas, Argentina, “Mapa de Cuencas Hidrográficas de la República Argentina” (accessed 30 August 2026). Source for the national definitions of exorheic, endorheic, and arreic drainage and the distinction between a basin and an ecoregion.
6. Subsecretaría de Recursos Hídricos, Argentina, Cuenca del Río San Juan, basin sheet 54 (underlying area and map information dated 2002; accessed 30 August 2026). Source for the approximately 38,462 km² catchment, Castaño–Los Patos confluence at Las Juntas, west-to-east Precordillera course, snowmelt regime, Desaguadero connection, and fan-building summer torrents.
7. Acebes, P. et al., “Abiotic gradients drive floristic composition and structure of plant communities in the Monte Desert”, Revista Chilena de Historia Natural 83, 395–407 (2010), doi:10.4067/S0716-078X2010000300007. Source for the published traditional 62°54′–69°50′W longitudinal frame; the paper attributes that boundary to Morello's 1958 phytogeographic treatment.