Introduction to Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 287

1,000,000

100,000

10,000

1,000

100

10

1 1

0.01 0.1 10 100 1,000 10,000 100,000

Reservoir size (km 2 )

FIGURE 9.16 Comparison of the number of reservoirs worldwide vs. their size. (Based on data from
Downing, J.A., Prairie, Y.T., Cole, J.J., Duarte, C.M., Tranvik, L., Striegl, R., McDowell, W.H., et al.,
Limnology and Oceanography 51, 2388-2397, 2006.)

Much of the total inland water is contained within a relatively few large lakes. The surface
areas, the maximum depths, and the volumes of lakes over 1000 km 2 in surface area are provided
in Table 9.2. Of these, the Caspian Sea, a large salt lake, is by far the largest. The Black Sea is an
inland waterbody connected to the Atlantic Ocean. Downing et al. (2006), based on a global model,
estimated that there were approximately 304 million lakes in the world, with a total area of 4.2 mil-
lion km 2 and dominated in area by millions of waterbodies smaller than 1 km 2 .
In North America, the Laurentian Great Lakes contain the largest continuous mass of freshwater
on earth (Wetzel 2001) with a combined volume of 24,620 km 3 making up approximately 20% of
the total volume of all freshwater lakes, and a surface area representing approximately 10% of that
of all inland waters combined (Table 9.1).
There are many reservoirs on planet Earth. Based on a global model, Downing et al. (2006)
estimated that there are more than 500,000 reservoirs, with an average surface area of 0.5 km 2
(Figure 9.16), covering approximately 0.26 million km 2 . Of the world's reservoirs, as compiled by
Downing et al. (2006), 24 exceed 1000 km 2 in area. For example, the Three Gorges Dam in China
slightly exceeds 1000 km 2 in size, compared to Lake Volta, formed by the Akosombo Dam in the
West African nation of Ghana, which covers an area greater than 8500 km 2 .
In the United States as well, small reservoirs (and ponds) dominate, with their numbers dif-
icult to estimate. Using geographic information systems (GIS), Smith et al. (2002) estimated that
the area of farm ponds in the United States was approximately 21,000 km 2 . For larger reservoirs,
information is available from the NID by the U.S. Army Corps of Engineers. The inventory includes
all signiicant hazard potential classiication dams and low hazard or undetermined potential clas-
siication dams which equal or exceed 25 ft. in height and which exceed 15 acre-ft. in storage
(1 acre-ft. = volume represented by 1 acre in area 1 ft. deep), or equal or exceed 50 acre-ft. in stor-
age and exceed 6 ft. in height. The 2009 NID databases included 82,842 dams, of which the major-
ity (73,423) are earthen dams. Of the dams in the NID, 66% were built prior to 1970, and only 2290
have been built since 2000.

REFERENCES
Bowen, R. 1982. Surface Water . Allied Science, London.
CGER. 1996. Freshwater Ecosystems: Revitalizing Educational Programs in Limnology . Commission on
Geosciences, Environment, and Resources, National Academy Press, Washington, DC.