The Plesiosar Lagoon is located in the Epuyén River Valley, Chubut. And according to several sources, it is set on the western shore of the Epuyén River. It is barely larger than a pond and there is a bog between it and the river. It is closed in on the west by Mount Pirque. The lagoon is close to where the Pedregoso Creek joins the Epuyén River.
In the map above, I marked it with a red circle.
Today I read an article  about the area's geology and in it found the following:
Lagoon de las Mercedes (also named Chulta, Shulten, or of the Plesiosaur).
I have already posted about the Plesiosaur at Epuyen so I will just summarize the story below:
An American gold prospector and otter hunter named Martin Sheffield wrote a letter in 1922 to the Director of Buenos Aires' Zoo, Clemente Onelli. He wrote that he had seen an enormous animal with the head of a swan and a crocodile body in "the lagoon where I have set up my hunting place".
Sheffield had met Onelli when he worked for him on the Argentine-Chilean Border Committee in the early 1900s.
Onelli told the press and caused a media uproar, an expedition was quickly organized and sent. After scouring the area it found nothing.
Sheffield's family's home was apparently built close to the Plesiosaur Lagoon (red circle above), but, could Sheffield's hunting shack be located at Chulta Lagoon?
Lake (or Lagoon) Chulta or Las Mercedes
This Las Mercedes or (de las Mercedes according to another source) Lagoon laguna de las Mercedes is located close to the village of Epuyén: from there you should take National Highway 40 north towards El Hoyo, and take a right turn on the junction with Provincial Highway 70 taking the climb towards El Maiten. The road is set on the slope of Mount coihue and after 8.5 km (5 mi.) you should take the road that branches to the left. 3 km (2 mi.) ahead is the Las Mercedes or Chulta Lagoon. Which is private property.
It is a small lake of the "Precordillera" at 580 m (1,900 ft) above sea level and it has a surface area of 80 ha (197 acres). It drains into the Las Minas creek and from there through the Epuyén River, Lake Puelo and Puelo River into the Pacific Ocean.
Local historian Casamiquela called it “Lake Chalta”  and supposed that its name came from the Tehuelche word Chaltén whose meaning is not quite clear (some say it means "mountain or hill that smokes" in an erroneous allusion to its alleged volcanic origin or, perhaps due to the clouds that snag against its summit). It is a sacred place because the demigod hero of the Tehuelche, Elal was taken there by a swan that set him on the mountain's peak. 
It was mentioned for the first time by the Spanish explorer Antonio de Viedma in his diary (1782), who spotted the mountain after discovering the lake that now bears his name. Viedma said it was called Chaltel because that was the name the natives gave it :
Chaltén is an impressive peak 3,405 m (11,164 ft.) high, located in the Southwestern corner of Santa Cruz province in the Los Glaciares national park, on the border between Chile and Argentina. It is also known as Fitz Roy. It is very far from Epuyén and Chubut province, which makes me doubt Casamiquela's comment regarding the lake's name.
The official tourism websites on Epuyén on the other hand say that the Plesiosaur Lake is another one (which I had enclosed in a red circle in my map), and they treat it as a different entity to Lake Chulta or Las Mercedes.
Which is the real and genuine lagoon?
Where did the mix up come from? A mystery indeed. I will continue my research on this matter.
 Miro, Roberto, (1967). Geología glaciaria y pre-glaciaria del Valle de Epuyén Revista de la Asociación Geológica Argentina. Tomo XXII, No. 3, Jul. Sept. 1967. pp. 183.
 Casamiquela, Rodolfo. (1987). Toponimia indígena del Chubut. Gobierno de la Provincia del Chubut, pp. 52
 Runa : archivo para las ciencias del hombre, Vol. 3. Ed. El Instituto, 1950. pp 193 y 198
 Pedro de Angelis [Ed.] (1837). Colección de obras y documentos relativos a la historia antigua y .... Vol. 6. pp. 56.
 Sitio web del Gobierno de la provincia del Chubut.Laguna del Plesiosaurio y Epuyén
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©