Laguna del Negro Francisco
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| Laguna del Negro Francisco | |
|---|---|
 | |
  Laguna del Negro Francisco | |
| Location | Atacama Region |
| Coordinates | 27°28′S 69°14′W / 27.467°S 69.233°W[1] |
| Catchment area | 930 square kilometres (360 sq mi) |
| Basin countries | Chile |
| Surface area | 20.7 square kilometres (8.0 sq mi) |
| Average depth | 1 metre (3 ft 3 in) |
| Surface elevation | 4,125 metres (13,533 ft) |
Laguna del Negro Francisco is a lake in the Atacama Region of Chile and the southernmost closed lake in the country. It is situated 200 kilometres (120 mi) northeast of the city of Copiapó. The lake is about 10 kilometres (6.2 mi) long and 4 kilometres (2.5 mi) wide with a surface area of about 20.7 square kilometres (8.0 sq mi) and a depth of about 1 metre (3 ft 3 in). A peninsula, probably formed by a moraine and subsequently modified by wind-driven accumulation of sand, separates the lake into a north-northwesterly and a south-southeasterly half with different colour and water composition.
The lake is of tectonic origin and lies within a basin bordered by mountain ranges to the east and the west and two volcanoes north and south. It formed when the Astaburuaga River was redirected into the lake basin from the east, and this river is also its main source of water. Water levels have fluctuated over the last 6,000 years and the lake is currently in a period of low water level. In 1996 the lake was classified as a Ramsar site and it currently lies within the Nevado Tres Cruces National Park. In the past there were plans to redirect water flowing into the lake to the Copiapó River; presently a mining company holds water rights to the Astaburuaga River.
Hydrology and biology
Laguna del Negro Francisco lies 200 kilometres (120 mi) northeast of the city of Copiapó in the Atacama Region of Chile,[2] and can be reached by south-bound roads that diverge from the Copiapó-Tinogasta road close to the Salar de Maricunga.[3] A National Forest Corporation building is close to the southwestern shore of the lake.[4] The area has been called a "scenic beauty".[5]
The lake lies at an elevation of 4,125 metres (13,533 ft),[1] is 10 by 4 kilometres (6.2 mi × 2.5 mi) wide[6] and has a surface area of 20.7 square kilometres (8.0 sq mi), but a depth of only about 1 metre (3 ft 3 in);[1] the Ministry of Public Works estimated a mean depth of 7 metres (23 ft) in 1941.[7] The water surface can fluctuate between 17–31 square kilometres (6.6–12.0 sq mi) from year to year.[8]
The lake is subdivided into a northern or northwestern and a southern or southeastern part by a[9][1] 20 metres (66 ft) high peninsula, on which wave action has led to the formation of sand spits.[10] The peninsula was formed either by wind-driven accumulation of sand as a sandbank or as a moraine left by a former glaciation.[9][1] Under present-day water levels, the sandbank separates the lake into two parts with different water composition: The southern part has fresher water and thus has a different colour than the saline northern part, as wetlands formed in the southern part.[9][11]
The land around the lake is formed by alluvial fans[9] and in the eastern sector by alluvial deposits such as gravels, sands and silts.[1] A number of mountains surround the lake, such as the 6,080 metres (19,950 ft) high Copiapó and the 5,880 metres (19,290 ft) Jotabeche.[7]
The Astaburuaga River enters the lake on its eastern shore;[12] it originates on the mountains east of Laguna del Negro Francisco where it is nourished by snowmelt.[8] Other tributaries are the Quebrada Azufre in the northwest, Rio de la Sal in the northeast and Rio La Gallina in the southwest;[4] the tributaries of Laguna del Negro Francisco feature wetlands[6] and marsh vegetation has developed at the mouth of the Astaburuaga.[12]
The lake is polymictic,[1] meaning that the water in the lake is usually layered but mixes over several times in the year.[13] Aragonite, calcite, dolomite, gypsum and halite precipitate out of the water.[14] In sediments remnants of characeae, diatoms, ostracods and Ruppia have been found;[15] today only one ostracod species persists in the lake.[16] Flamingos exist in the area and the lake is an important site for birds.[17]
The watershed of Laguna del Negro Francisco covers a surface area of 930 square kilometres (360 sq mi).[1] The Astaburuaga River is the principal inflow; other creeks that enter the lake and groundwater play only a minor role.[15][2] The mean discharge of the Astaburuaga is about 0.888 cubic metres per second (31.4 cu ft/s),[18] but it varies strongly over time, including at different hours of the day as the water of the river freezes and remelts.[19] In addition, some water from the Astaburuaga River flows into the Cíenaga Redonda basin rather than Laguna del Negro Francisco;[18] presently, an alluvial cone separates the Astaburuaga River in the Valle Ancho gorge from the Salar de Maricunga/[20] Cíenaga Redonda watershed.[18]
The lake has no outlet[1] and is the southernmost closed basin in Chile;[21] however during former lake level highstands the lake overflowed at 4,236 metres (13,898 ft) into the Salar de Maricunga.[15] The lake was once thought to be the source of the Copiapó River.[6]
Geology
Geologically, the lake is located in a 200 square kilometres (77 sq mi)[2] north-south trending depression between the Andes in the east and the Cordillera Domeyko in the west;[22] subduction of the Nazca Plate beneath South America is responsible for the formation of these mountain ranges.[23] On these sides normal faults border the depression which is closed to the north and the south by the Quaternary volcanoes Copiapó and Jotabeche, respectively. Oligocene to Pliocene andesites of the Negro Francisco Formation and Quaternary sediments cover the depression, the latter especially in the east.[1] The depression extends past these volcanoes to the Salar de Pedernales and the Salar de Maricunga;[22]
Climate and life
There is little information on the climate of the lake, but average temperatures are 1–2 °C (34–36 °F) and precipitation mostly falls during winter from cold fronts and cutoff lows, with a probably smaller amount of summer precipitation;[15] the total amount of precipitation is about 250 millimetres per year (9.8 in/year).[a] Evaporation is about six times larger than precipitation.[22] The region is windy[9] and is considered to be a cold desert.[24]
Vegetation is scarce at these altitudes[25] and is classified as steppe vegetation. Animals found in the region include guanacos, pumas, vicuñas and vizcachas, while birds include the Andean goose, three flamingo species[26] Andean gull, Baird's sandpiper, crested duck and horned coot; in total there are about 17 bird species.[27] The area is a protected area[2] as part of the Nevado Tres Cruces National Park[8] and was declared a Ramsar site in 1996, a site of international importance to waterbirds.[28]
