Lake Yoa

Infobox lake
lake_name = Lake Yoa
image_lake =
caption_lake =
image_bathymetry =
caption_bathymetry =
location = Ounianga Kebir, Sahara
coords = coord|19.0|N|20.5|E|region:TD_type:waterbody|display=inline,title
type =
inflow = "underground aquifer"
outflow = "evaporation, subsurface drainage"
catchment =
basin_countries = Chad
length =
width =
area = 3 km²
depth =
max-depth = 25 m
volume =
residence_time =
shore =
elevation =
islands =
cities =
Lake Yoa is one of a series of lakes in the Ounianga Serir basin of northeastern Chad. These lakes are remnants of a much larger lake which occupied this basin during the African Humid period which lasted from approximately 15,000 to 5,500 years before present. There are currently ten lakes in the basin with a total surface area of approximately 20 square kilometers.

These lakes form an hydrological system that is unique in the earth’s deserts (Kropelin 2007). Ordinarily when water is exposed at the surface in highly arid environments it becomes saline due to a high rate of evaporation. In this case, even though the rate of evaporation from Lake Yoa is equivalent to a staggering 6 meters a year, the total lake depth is 25m, unique physical factors combine to keep all of the lakes, except the central Lake Teli, fresh. First, water accumulated in an underground aquifer during the wet millennia is supplied to the lakes. Second, wind blown sand separates the basin into ten lakes with Lake Teli occupying a lower position than the lakes around it. Third, thick mats of reeds cover the surface of the fresh water lakes where they slow evaporation, but are absent from the saline waters of Lake Teli. As a result, greater evaporation takes place from the surface of Lake Teli, keeping its water level low. This allows water from the adjacent lakes to flow through the permeable dune barriers into Lake Teli keeping their waters fresh.

Lake Yoa recently became of interest in the study of global climate trends when a team headed by Stefan Kropelin, of the University of Cologne, recovered a core of sediment from the bottom of the lake. Because Lake Yoa has been in existence continuously since the Humid Period its waters have protected the sediments accumulated at the bottom from erosion and dispersal. After analysis of the pollen preserved in the core, Kropelin and his colleagues concluded that the shift from forest to desert in the area of Lake Yoa occurred gradually, with intervening periods of scrubland and grassland before the onset of full desert conditions. This conclusion contrasts with work done by Peter deMenocal of Columbia University and colleagues, who in 2000 drilled a core of ocean sediment off the western coast of Mauritania. Because of the dust levels in this core deMenocal and his co-authors concluded that desert conditions came on quickly, over a period of a few centuries.

The discrepancies between these two accounts are not surprising given the differences in the data examined. The ocean core represents what is essentially a mass survey of the entire northern portion of the African land mass while the Lake Yoa data would survey more specifically conditions in the areas to the south and west of it, which supplied material to the prevailing trade winds of Holocene time before they crossed Lake Yoa. It is possible both that, northern Africa became quickly and generally drier, and that, in specific areas the process of desertification moved through a series of more gradual ecological transitions.

References

* [http://www.nytimes.com/2008/05/09/science/09sahara.html?ref=science Shift from Savannah to Sahara was Gradual, Chang, Kenneth, New York Times, May 9, 2008]
* [http://www.icms.com.au/inqua2007/abstract/1342.htm The Saharan lakes of Ounianga Serir (NE Chad): a unique hydrogeological system, Kropelin, Stefan, XVII INQUA Congress, 2007]