Inland Salt Lakes

There are areas on all continents where topography and geological structures allow the accumulation of water with very restricted or no outflow to the oceans. Solutes from inflowing water are retained as the water itself evaporates and saline lakes are formed with various salinities (Table 10.1). Completely closed catchment areas, with no outflow, are termed endorheic, but this does not generally mean hydraulic isolation. In most cases, in addition to rainfall and river inputs, water both enters and leaves inland lakes as flow of groundwater, so that their salt compositions do not match the salts in rivers and streams flowing into them. This is well demonstrated by the largest of all salt lakes, the Caspian Sea, in which ionic ratios Na⁺/Mg²⁺, Ca²⁺/Mg²⁺ and https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315148038/efb223d3-97c8-455d-9c45-0c8e2b289d76/content/c010_equ_0001.tif"/> ​ ​ Cl − ​ / SO 4 2 − differ from the ratios in the river input by factors of 2.33, 0.14 and 0.44, respectively. An interesting example of a lake that is apparently endorheic, but in which the water is fresh, is Lake Chad. In a dry area of high evaporation, bounded by four countries (Chad, Cameroun, Niger and Nigeria) all with irrigation ambitions, water is lost by groundwater flow into neighbouring wetlands, with a small river inflow sufficing to maintain a fresh lake. As the data in Table 9.3 show, there is also a compositional difference between the oceans and their river input, but in that case, the major reason is that salt is introduced by hydrothermal circulation of mid-ocean ridges.