Newsletter and Technical Publications
Lakes and Reservoirs vol. 2
The Watershed: Water from the Mountains into
Endorheic Lakes: Waterbodies That Don't Flow to the Sea
The vast majority of the precipitation falling onto the land surface will
eventually find its way back to the oceans via rivers, lakes or wetlands, or
else be evaporated or transpired back into the atmosphere. There is one class of
water-bodies, however, for which this journey is terminated prior to its surface
water reaching the oceans. Such water-bodies exist in closed or endorheic
watersheds, which contain rivers or lakes that do not drain to the oceans. This
interruption of surface water flow results from a balance between inputs
(precipitation + surface flows) and outputs (evaporation and seepage). Lakes in
endorheic watersheds are often called “terminal ” or sink lakes.
Although most of the millions of freshwater rivers and lakes drain to the
ocean, endorheic water-bodies represent a special class of water-body like the
Aral and Caspian Seas. Most are located far inland from the sea. Their
watersheds often are contained within a mountain range or other natural geologic
feature that has severed their direct hydrologic connection to the ocean.
Because their inflowing waters subsequently flow into dry watercourses or are
evaporated, minerals and other inflow erosion products concentrate within these
water-bodies. With a continuing mineral input, some water-bodies typically
become saline compared to water-bodies that drain to the oceans. Because
evaporation plus seepage are the major water outflow pathway, endorheic
water-bodies also tend to be more sensitive to pollutant inputs than
water-bodies that drain to the oceans.
Endorheic water-bodies exist in belts between the northern and southern
margins of desert zones in both the Northern and Southern Hemisphere. Although
endorheic water-bodies usually contain water of lower quality than that of
water- bodies draining to the oceans, they often are of great importance to
their watershed inhabitants.
|Photo 46: Size of the Aral Sea before 1985.
|Photo 47: Size of the Aral Sea in 1992 after water diversions for
irrigation had begun.
Surprisingly, endorheic water-bodies include some of the largest lakes in the
world. One major endorheic lake, for example, is the Aral Sea (Photos 46 and
47), in south-central Asia. Considered by many to be an “inland sea ”, it
actually is a large terminal lake. Its two major inflow rivers, Amu Darya and
Syr Darya, previously maintained the lake within acceptable boundaries of water
quantity and quality for many beneficial human water uses. A thriving commercial
fishery of economic importance to the watershed inhabitants also existed.
However, the two river inflows were more or less completely diverted beginning
in the middle of the last century for agricultural irrigation.
The diversion of its tributary inflows completely changed the character of
the Aral Sea and its watershed as well as the socioeconomic status of the
watershed inhabitants. Prior to diversion of its major tributaries, the lake had
a surface area of about 62,000 square kilometres and a water volume of 970 cubic
kilometres. Once large enough to be visible from spacecraft circling the Earth,
the lake has since shrunk to about one-third of its original size, and its water
has become more saline than ocean water. A once-thriving commercial fishery has
been completely destroyed, and the Aral Sea watershed is now beset with
excessive fertilizer and pesticide use, as well as salinized soils, which are
believed to be responsible for significant human health impacts. What makes this
situation noteworthy is that this devastation of the Aral Sea ecosystem occurred
essentially within the space of a single human generation, an extremely short
period for the extent of the ecosystem degradation that has occurred. It
provides a sobering example of unsustainable socioeconomic development of a
watershed, and the serious environmental, ecosystem, human health and social
consequences that can arise from such actions.
|Photo 48: View of the Caspian Sea from space.
Other major endorheic water-bodies include the Caspian Sea (Photo 48), of
eastern Europe and Pyramid Lake in the western United States. The Caspian Sea
has the largest surface area of any lake in the world (more than 436,000 square
kilometres), and was originally connected hydrologically to the Atlantic Ocean
to the west and the Indo-Pacific Ocean to the east. With its hydrologic
connections subsequently terminated over geologic time by mountain formation,
etc., a vast isolated inland water basin was formed. The Caspian Sea remains the
largest saline water-body in the world cut off from the ocean. Its major river
inflow is the Volga River. Thus, the flow and quality of the Volga River, as
influenced by its watershed population, land use and human activities, can also
significantly affect both the water-level and quality of the Caspian Sea.
Pyramid Lake is a large lake located in the desert region of the western United
States, containing relatively salty water. It is also very sensitive to the
quantity and quality of its inflowing tributaries.