This weekend I read an interesting article  on an aquatic monster at Lake Tar, which I had not heard about before and which may throw some light on an incident reported by Clemente Onelli during his 1922 Plesiosaur expedition. Lets take a look at this "sighting":
This lake (49° 15' S; 72° 0' W) is located on the Patagonian steppe, in the Argentine province of Santa Cruz. It drains west into Lake San Martin and from there into the Pacific Ocean. It is set at 218 m above sea level (714 ft. ) and it has a surface area of 53 km2 (20 sq.mi.). Its name, in the native Tehuelche language, means "dirty" due to the murky color of its waters. 
The article, from which I will quote extensively as I have not been able to find the original publication.
Argentine historian Manuel Llarás Samitier wrote this story in the now defunct magazine "Argentina Austral" about 50 years ago [*]:
A mysterious halo floated over the region of Lake Tar since before the days of the colonization of the wide mountainous region formed by the Santa Cruz lakes. It is known that the natives entered the neighborhood reluctantly and they did not hide this feeling. They said that they did not like to hunt in the fields close to those lakes because in their waters monstrous animals lived [...] the blurred versions mentioned a very large animal and the local Indians compared it to a big bellied horse that had a long neck without a mane that used to come out [of the lake] to graze on the grassy shores on the nights on which the moon shined" 
Llarás Samitier wrote about a very strange incident which happened in 1913, which, is coincidential with an alleged sighting mentioned by Clement Onelli during the plesiosaur expedition of 1922. As usual, it seems that Onelli, mixed up the names and forgot the place. Though he got the date right.
The incident is the following (Llar&aacut;as Samitier's story is in italics and taken from ):
The foreman of a sheep ranch close to lake tar asked two of his men, who were from the Argentine province of Corrientes (a sub-tropical province in northeastern Argentina, along the Paraná and Uruguay Rivers) to go with a local native Indian guide to search for some lost animals.
They followed the tracks and finally arrived at Lake Tar. As it was a cold and windy afternoon, and the weather was getting worse, they camped behind some shrubs about 100 m (300 ft.) from the lake's shore.
Before dawn, a strange noise woke them up:
despite the whistling of the wind they clearly heard as if one or several animals were splashing in the mud by the [lake's] shore. The moon was shining in the sky so they did not take long to see two gigantic shadows that passed in front of them, and they observed that as they walked, they entertained themselves by biting at the grass, lifting and turning their long necks [...] some times the monsters would lift their heads simultaneously, as if to capture some strange noise, and then they would keep on grazing calmly."
One of the men, named Florencio Almada wanted to get closer to see the better, but his companions did not let him do so. The native said that they were the famous giant horses of Lake Tar.
Almada was not satisfied, and at dawn, when the noises had quieted down, he went to look at the tracks by the shore, and found, well printed in the mud, the foot steps of animals that he later described as similar to those of gigantic yacarés.
Yacaré (Caiman yacare); is a species of caiman -crocodile- native to South America, and which is also found in Corrientes, the place where Almada was born and bred.
He had hunted them in the past and he now wanted to hunt the "monsters" with the help of his companions, they did not do so.
Back at the ranch, Almada became obsessed with the beasts and the other ranch hands did not believe his story. This was worsened by the silence of his two companions, who did not want to talk about the sighting. As the situation worsened, the foreman gave him permission to go back to the lake to hunt his monsters. He left for the lake:
with a pair of lassos, boleadoras [a local weapon], knife and revolver [...] before leaving he threatened his colleagues : if he could not bring the monster back alive, he would have the pleasure of inviting those who had mocked him, to eat the grilled ribs of a giant crocodile
After a couple of days, the foreman became worried when Almada did not return, so he went out to search for him with the Indian and another man:
When the arrived, they found nothing [...] night fell without any news. Before dawn they lit a bonfire to show their position to the missing man. Nothing. With the first lights of sunrise they saw some caranchos [carrion eating birds] flying over the lake's shore. When they got closer they saw Almada's horse floating in the water. The animal seemed to have been dragged more than 100 m [300 ft.] in the mud. On its saddle they found a piece of lasso. But of Almada, not even a miserable "Moon" brand Alpargata [cheap kind of shoe]. They also found the revolver with the six used bullets, evidence that he had shot before using the lasso. 
It does not surprise me to notice that the animals are not carnivores, but herbivores, something that I have mentioned before (when addressing the sustainability -i.e. survival- issue): the lake monsters are grass eaters. By the way, Tar was a place where the natives camped, as it had good pasture for their horses .
Futhermore, a similar event was reported in 1957 at Lake Viedma which is 42 km (27 mi.) south of Lake Tar. Viedma flows east towards the Atlantic Ocean and is separated from Tar by the continental divide which rises to about 750 m (2.460 ft.) above sea level.
I have not been able to find the full article, but Google books has allowed me to identify the original source: Argentina Austral, v. 38, n.os 429-434 - 1967. As the following image shows:
Source: Online, Google Books.
I believe that the article by Samitier is "Realidad y leyenda del mylodon" and it is on page 68 of the above mentioned magazine. Should anyone come across it, please let me know. Thank you.
 Mendoza, Roberto. 2009. Buscando al Plesiosaurio hecho en la Patagonia. Nucleo. Producción y Desarrollo Sustentable. Año 1, N° 6. June 2009. pp. 37+
 Gaea. (1946). Geografía de la República Argentina. Vol. 7, No. 2. By Sociedad Argentina de Estudios Geográficos Gaea. Coni. Ed. pp. 588.
 Moreno, F., (2007). Exploración de la Patagonia Sur II: el lago Argentino y los Andes meridionales. 1877. B. Aires: Continente.
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©