Early Patagonian cartography was sketchy, ill informed and cloaked in secrecy. After Magellan’s voyage of discovery (1520), several Spanish expeditions sailed past the Patagonian coast along the new route to the East Indies Spice Islands, some capsized or were disbanded by the “roaring 40s” at the Strait of Magellan, a few managed to sail through the Strait. English privateers later followed by the Dutch in the late 1500s early 1600s navigated the Patagonian coast en-route to loot the rich Peruvian and Mexican Spanish possessions.
The outcome of these expeditions was published as journals and also maps. These maps depicted some geographical features that have since vanished, mainly islands and rivers. The Bahia Sin Fondo River is one of them.
It would not be until the late XVIIIth and early XIXth centuries that serious scientific explorers would visit Patagonia (Bouganville, Cook, Fitz-Roy and Darwin) and with them, a more clear picture would take shape.
Bahia Sin Fondo (which in English means “Botomless Bay”) was discovered by Magellan’s expedition on February 24, 1520, the day that Saint Matthias is celebrated (who was chosen to replace Judas as one of the twelve Apostles). Being very Catholic, the Spaniards named the bay after him. Francisco Albo, pilot of Magellan's ship, the “Victoria”, (the first ship to sail around the world) recorded it in his diary (Spanish text is below):
On the 24 of that month […] we went straight into a very big bay, which we named Bay of Saint Matia [sic], because we found it on his day; and we went well into it and we could not find bottom till we went all the way into it, and we found 80 [Spanish] fathoms [approx. 133 m or 435 ft.]… .
Nearly three hundred years later, English Captain P. Parker King published his account:
St. George's Gulf, called in the old charts ‘Bahia sin Fondo,' or Deep-Sea Gulf, was formerly considered to be a deep sinuosity of the coast, into which a river emptied its waters after winding through a large tract of country; for, until the Descubierta and Atrevida's [*] voyage of discovery, very vague accounts had been given of this, or indeed of any other part of the coast. The Gulf, upon that examination, was found to possess no river or creek in any part excepting on the north side, where there are several deep bays and coves, which are, and have been frequented by our sealing vessels. 
The two ships mentioned above (Descubierta and Atrevida) were part of Alessandro Malaspina's scientific expedition, commissioned by the Spanish Crown and undertaken between 1787 and 1794. Note that King calls the bay “St. George’s Gulf”
The missing river
So, in 1794 Malaspina proved that there was no river flowing into San Matís Gulf,
The myth persisted until one hundred years ago, the Encyclopedia Britannica in its 1911 edition, stated the following (bold font is mine):
To the south of the Rio Negro [river] the Patagonian plateau is intersected by the depressions of the Gualicho and Maquinchau, which in former times directed the waters of two great rivers (now disappeared) to the gulf of San Matias, the first-named depression draining the network of the Collon-Cura and the second the Nahuel-Huapi lake system. In 42° S. there is a third broad transverse depression, apparently the bed of another great river, now perished, which carried to the Atlantic the waters of a portion of the eastern slope of the Andes, between 41° and 42° 30' S […]Among the depressions by which the plateau is intersected transversely, the principal are the Gualichu, south of the Rio Negro, the Maquinchau and Balcheta (through which previously flowed the waters of lake Nahuel-Huapi, which now feed the river Limay); 
The above echoes what I wrote in my previous post (Vanishing Rivers), quoting Clemente Onelli regarding the native myth on a river the formerly drained Lake Nahuel Huapi through Makinchao into the gulf of San Matís.
The Gulf itself, golfo de San Matías is located between 40°47' S and 42°13' S on Argentina's Atlantic coast, with an area of approximately 18 000 km2 (6,950 sq. mi.), is the second largest in Argentina. Its Bottomless name is well earned, as its maximum depth is over 200 m (655 ft.) and approximately 55% of the gulf is over 100 m deep (322 ft.).
The Knights Templar in Patagonia
While reading and doing my research for this post, I came across several maps published by different websites that support the notion that the Knights of the Order of the Temple sailed across the Atlantic to Patagonia bringing the Holy Grail with them (perhaps some medieval dragon swam behind them and that is the origin of Nahuelito).
Jokes aside, the Knights Templar (Order of the Temple) was a Middle Age organization that arose around 1130 and was disbanded by Pope Clement V in 1312. Most of its members had been imprisoned, tortured and burned alive in 1307.
Those who believe in Patagonian Templars contend that a hill (as far as I can see, a natural hill, that is, not man-made) by the coast at San Matías, known as “El Fuerte” (The Fort) was actually a Templar castle and that as it appears in several “old” maps described as an “ancient” fortress, it must be one (perhaps it was a Spanish outpost, but I have not found any evidence to support that theory).
The interesting thing is the “ancient” map part, which I copy below, and have taken from these esoteric sites (In case you want to know, I do not believe in their theory, as far as I am concerned the Knights of the Temple never sailed to Patagonia).
The two maps shown above depict rivers flowing int Golfo San Matías (St. Mathew’s Gulf), they also show Cerro "El Fuerte" (The Fort Hill). The top one is said to date to 1780. I don't know who drew the bottom map.
In one map, the river flows into the sea just south of the "fort", in the other it flows straight into the San Antonio Inlet, and is named curu leuvu. These are Mapuche native words and mean: curu = black and leuvu = river. Coinciding with the Spanish name of the river (which in the map is drawn above -i.e. to the north- of curu leuvu as R. Negro (or Rio Negro - Black River).
Below is another map by de Moussy dated 1865. Check it out online it is zoomable (see  below). It has the caption "Ancien F. Abandoné" or "ancient abandoned F[ort]" and the symbol of a fort. This later map lacks a river flowing into the Gulf.
Another map, dated 1838 and published in England,  also shows a "fort" at San Matís.
The following text  published in 1867, mentions the "fort" as a hill that resembles a fortification. It also mentions "Escondido" (hidden) creek, which in my opinion may be a relict river bed belonging to the "missing river".
Regarding the "fort" (i.e. natural hill) and the other features mentioned in this post, the following map which I prepared based on Google Earth material will give you a clear idea of the location of the rivers and the gulf. (Fuerte is located at 41°06'S, 65°10'W).
Besides these, there is another map, published which shows Valcheta River flowing into the Gulf at San Antonio Este (it runs from the upper left corner parallel to a range of hills and then bends towards the Gulf).
This map was drawn around 1879 just after the military Campaign (Campaña al Desierto) which concluded the long war with the natives of the Pampas and northern Patagonia. It was prepared by Argentine Colonel Manuel Olascoaga in 1881. 
Valcheta, nowadays is a small stream that drains into a closed basin and has no link to the sea. It is quite close to San Antonio (75 km - 47 mi.) and perhaps did drain into my "Elpalafquen River" on its way into the Atlantic.
Should I come across more maps, I will post them here.
 Fernández de Navarrete, M., (1837). Coleccion de los viages y descubrimientos que hicieron por mar los españoles desde fines del siglo XV: con varios documentos ineditos concernientes a la historia de la marina castellana y de los establecimientos españoles en Indias. VoI. iv. pp. 229. Madrid: Imprenta Nacional
 King, P. P. (1839). Narrative of the surveying voyages of His Majesty's Ships Adventure and Beagle between the years 1826 and 1836, describing their examination of the southern shores of South America, and the Beagle's circumnavigation of the globe. Proceedings of the first expedition, 1826-30, under the command of Captain P. Parker King, R.N., F.R.S. London: Henry Colburn. Page 581
 the eleventh edition of the Encyclopaedia Britannica
 Fundación DelphosThe Holy Grail’s arrival in Argentina Online (English language) also click here
 Olascoaga, M. (1974) Estudio Topográfico de la Pampa y Rio Negro (1881). Buenos Aires: Eudeba. Map.
 Findlay, A., (1867). A sailing directory for the Ethiopic or South Atlantic ocean, including the coasts of South America and Africa. R. H. Laurie. pp.423.
 Carte de la Patagonie et des archipels de la Terre de Feu, des Malouines et des cotes occidentales jusqu'au Golfe de Reloncavi. Par le Dr. V. Martin de Moussy 1865. Grave par L. Kautz, r. Bonaparte 82 - Paris. Paris, Imp. Lemercier, r. de Seine 57. (Paris Librairie de Firmin Didot Freres, Fils et Cie., 1873)
 South America sheet V. Patagonia. (with) Isle of Georgia. (with) The South Shetlands… Chapman and Hall. London (1838)
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©