<P> The climate of the delta is described as arid . It receives only 250--500 mm (9.8--19.7 in) of rain in a normal year . Average temperatures for the delta region range from 21 - 30 ° C (70 - 85 ° F) in July, and 10 - 21 ° C (50 - 70 ° F) in January . During the summer, the delta experiences intense monsoonal winds from the southwest, causing parts of the delta to be covered by sea - water . When this water retreats, it leaves behind salts in the delta's soil . During the winter the winds in the delta come from the northeast . </P> <P> The summer monsoonal winds also contribute to high wave energy levels . In fact, the delta is subjected to the highest wave action of any river delta in the world . (The amount of wave energy the Indus delta receives in a single day is greater than that received by the Mississippi River Delta in a year .) Throughout history the delta has survived this wave action because of the large discharge of fresh water to counter the erosional impact of waves . This large amount of wave energy, coupled with lack of silt flowing in from the Indus river (as mentioned above), has resulted in the formation of sand beaches . </P> <P> The Indus river started to form some 50 million years ago . 25 million years ago, the Indus Plain was lifted . </P> <P> Mangrove forests are one of the most abundant ecological communities of the delta, but they have declined over the years . According to Haq et al., they covered an area of 600,000 hectares (1,500,000 acres) until the 1980s and could be found along the entire 240 km coastline, in 40% of the tidal zone, and in 10% of the delta fan . Memon reports that the area of mangrove forests was 263,000 hectares in 1977 and 158,500 ha (392,000 acres) in 1990 . Both agree that they were sixth - largest mangrove forests in the world . Previously there had been eight species of mangroves, although only four now remain: Aegiceras corniculatum, Avicennia marina, Ceriops tagal, and Rhizophora mucronata . Among these Avicennia marina constitutes 99% of the mangrove population . As of 2007, the mangroves of the Indus delta are the largest desert climate mangrove forests in the world . </P>

What are the causes of dying delta in sindh