Tonlé sap means “large Freshwater River” but it is more commonly known as the “Great Lake” and is a combined lake and river system of major significance to Cambodia. It was designated a UNESCO biosphere site in 1997.
The Tonlé sap is linked to the sea via the Tonlé sap River, which converges with the massive Mekong River in Phnom Penh.
According to Wikipedia for most of the year the lake is relatively small—around one metre deep and with an area of 1,000 sq. miles. When water is pushed up from the Mekong into the lake, it increases its area to approximately 6,200 sq miles, with a depth of nine metres. This expansion floods the nearby fields and forests, providing a great breeding ground for fish.
Along with seasonal expansion and shrinking of the river, the Tonlé sap is also unusual for its biannual flow reversal. The river reaches maximum flow in August and September, when the Mekong River swells massively as it collects melted water from the Himalayas as well as heavy monsoon rains in its upstream drainage basin that spans five countries. Converging with the Tonlé sap in the Phnom Pen, the volume surge reverses much of the flow back up into the Tonlé sap floodplain. At the start of the dry season, the Mekong River water levels drop, returning the Tonlé sap River flow to its usual seaward direction, exposing the muddy lake plain. May to October is the wet season in Cambodia, bringing 75% of its rainfall and the dry season is from October to April, which explains why the humidity was so high and why this particular upstream traverse was the penultimate for the season.
The recent TV programme that I mentioned in my early postings suggests that the Tonlé sap is threatened by a number of hydroelectric dams that are being constructed on the upper Mekong River in Northern Cambodia, Laos and China. Some 14 major dams are under construction or planned for completion by 2020. The impact on fish stock in the Mekong and Tonlé sap is likely to be huge.
See: http://en.wikipedia.org/wiki/Hydropower_in_the_Mekong_River_Basin
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