If the Patagonian lakes are home to lake “monsters” and mysterious creatures, we can learn more about these cryptids if we study the lakes themselves and their origin.
Patagonian lakes, both in Argentina and Chile are mostly located along the Andes mountain ranges which run in a north-south direction and are cut on an east-west direction by many valleys, some of which are filled with deep freshwater lakes. A few exceptions are lakes set in the steppe (i.e. Lake Colhue Huapi – which is fed by water from the Andean snow melt) some of which lake drainage and are fed by the scarce rainfall.
These Andean lakes were carved by the glaciers during the Ice Ages.
Ice Ages and the formation of lakes
Sixteen different “Ice Ages” have been recorded in the southern tipo of Patagonian mainland during the last 2 million years, and at least six in Tierra del Fuego Island during the last million years.
The most severe of these was the “GGP” or Gran Glaciación Patagónica (Great Patagonian Glaciation), about 1 million years ago. During this event, the ice sheets reached the Atlantic Ocean at the area of the Strait of Magellan. Later glacier advances extended for several hundred kilometers but did not reach the sea (i.e up to 300 km in Tierra del Fuego). 
All of Patagonia is strewn with rounded boulders and pebbles created by the glaciers and their melt waters.
The last glacial event began about 25,000 years ago and most of southern Patagonia was covered with glaciers or a frozen tundra with permafrost. Only on certain warmer spots along the sea shore did the Nothofagus forest survive.
Below are some maps showing the areas that were covered by the ice sheets:
The striped area was covered by glaciers. The dotted red line is the border between Chile and Argentina.
In gray the icefields, in black, current glaciers. Arrows show the direction of the ice sheet flow.
Ice starts to melt: paleolakes
After deglaciation, the forests retreated together with the glaciers, their source of water, to occupy their current habitat in the Andes, leaving the now arid Patagonian plateau to shrubs and dwarfish bushes (a dry steppe).
During the ice ages, the glaciers and their moraines formed large natural dams that contained what are known as paleolakes. These large bodies of water had very high water levels and drained towards the Atlantic Ocean. The wide and deep valleys that cross the Patagonian steppe from west to east witness the might of the rivers that drained these lakes. Now most of these valleys hold small rivers (i.e. rivers Chico, Coyle, Gallegos) and some even run dry before reaching the coast (i.e. Deseado River).
During the late Pleistocene epoch (between 13,200 and 7,800 years ago), climate grew warmer and the glaciers began melting. This changed not only the weather and local ecology, but also affected the landscape as the paleolakes fragmented into smaller ones and their water level dropped, in some cases below the eastern terminal moraines, blocking their outflow into the Atlantic Ocean, and causing them to empty westwards into the Pacific Ocean.
These paleo lakes were, from north to south, the following 
Elpalafquen 41° S. Was a vast lake system that drained through the Limay River into the Atlantic (which still drains many lakes of the eastern slope of the Andes such as Nahuel Huapi, Espejo, Correntoso, Gutiérrez, Moreno, Traful, etc.) it broke up about 13,200 years ago and some lakes began draining into the South Pacific through the Manso River (such as Mascardi, Steffen, Fonck, Guglielmo, Césares, Hess, etc.).
Caldenius 47-48° S, included current lakes Azara, Belgrano, Mogote, Nansen, Volcán and Burmeister, all of which are now part of the Argentine Perito Moreno National Park. It drained into the Atlantic until ten thousand years ago when it also began to drain through a channel cut through the Andes. Its level was over 100 m (300 ft.) higher than the current water level of Lake Belgrano.
Fuegian paleolake 54-55° S, began melting 7,800 years ago. It comprised lakes Fagnano, Yehuin and Chepelmut as well as the Esperanza bog. Nowadays only Esperanza drains towards the Atlantic through the del Fuego River. The other three lakes drain into the Strait of Magellan.
All that remains of these imposing glaciers that once covered a very large surface of Patagonia are two continental ice sheets covering an area of 21,000 km2 (8,110 sq. mi.). These are the third biggest extensions of continental ice after Antarctica and Greenland. Located roughly 73°W and between 47° and 51°30’ S they are fed by the abundant winter snowfall and maintained by frost most of the year; however, they are now imperiled by climate change as can be seen in the following photographs of Upsala Glacier (Southern Icefield), comparing the same view in 1928 and 2004:
Upsala Glacier (49°50' S, 73°17'W) has a surface area of 870 km2 (340 sq.mi.) and a length of about 60 km (37 mi.), making it the third largest Southern Hemisphere glacier outside Antarctica,and South America's longest glacier. It is shrinking rapidly. See Fig. 53  which shows a map with the retreating front of this glacier (1968-1995).
See my photograph of the glacier in my post Here.
Northern Patagonian Icefield (47°00’S, 73°39’W) is about 120 km long (74 mi.) and 40-60 km wide (25 – 37 mi.) and covers an area of 4,200 km2 (1,640 sq. mi.), capping the Andes.
Southern Patagonian Icefield (between 48°50’S and 51°30’S is about 40 km wide and covers a larger area (13,200 km2, about 5,150 sq.mi.). This is the largest mass of ice outside of the Antarctic.
The remaining glaciers are distributed along the tall Andean peaks in smaller icefields, the larges of them is on Tierra del Fuego Island and has a surface area of about 2,500 km2 (975 sq.mi.) it straddles Darwin Cordillera and Mount Sarmiento.
Impact on cryptids
From the above we can see that there were no lakes before the Ice Ages (2 million years ago max.). They must have been excavated in a gradual process, by which each successive glaciation dug deeper trough shaped valleys which during the warmer inter-glacial periods filled with water. And were repeatedly filled up again with ice sheets hundreds of meters thick during each new glacial period.
This must have exterminated any animals attempting to colonize these lakes until the last deglaciation some 10,000 years ago.
The current lakes are the remnants of gigantic paleolakes that broke up and split, shedding their water into both oceans. Their fauna prior to the introduction of salmonids in the 1900s was the local endemic Patagonian species, which hitch-hiked their way from lake to lake perhaps on the feet or feathers of the local water fowl.
This excludes the existence of any large "non-fish" freshwater creatures such as plesiosaurs and so on.
Large tropical mammals could have moved into the now arid Patagonia, but if they were aquatic creatures accustomed to the warmer and more humid conditions prevailing to the north of Patagonia, we must identify how they crossed the dry barren areas that now separate the Patagonian Andean forests from these northern tropical areas.
That is what we will do in our next post Here.
 Coronato, A., Borromei, A., and Rabassa, J. Paleoclimas y Paleoescenarios en la Patagonia Austral y en Tierra del Fuego durante el Cuaternario.
 Del Valle, R., Tatur, A., and Rinaldi, C., Cambios en lagos y circulación fluvial vinculados al calentamiento climático del Pleistoceno Tardío-Holoceno Temprano en Patagonia e Isla 25 de Mayo, Islas Shetland del Sur, Antártida. Revista de la Asociación Geológica Argentina 62 (4): 618- 626 (2007)
 USGS. Historic Fluctuations of Outlet Glaciers from the Patagonian Ice Fields. Fig. 54.
Glasser, N., Harrison, S., Winchester, V., and Aniya, M. Late Pleistocene and Holocene palaeoclimate and glacier fluctuations in Patagonia . Global and Planetary Change, Volume 43, Issues 1-2, August 2004, Pages 79-101.
Copyright 2009 by Austin Whittall ©