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Lake Tana, source of the Blue Nile
5 November 2004
This three-image combination of Envisat radar images
highlights the change from autumn to spring around
Lake Tana in northern Ethiopia.
With a surface area of 2156 square kilometres but an
average depth of only 14 metres due to high levels of
sediment. Lake Tana is the largest single lake in
Ethiopia and forms the main reservoir for the Blue
Nile. The Lake is located at an altitude of 1788
metres on the north central plateau of Amhara.
Lake Tana is used for fishing, farming and
transportation, and has a thriving tourist industry
centred on the more than 30 islands on the Lake, home
to numerous Ethiopian Orthodox Church monasteries
dating back more than five centuries.
The Blue Nile runs from Lake Tana's southeast corner,
flowing south over a lava dam to form the Tisisat
Falls - the name in Ethiopian means 'the water that
smokes' - then flowing northwest to merge with the
White Nile and form the full-fledged Nile itself. The
Blue Nile contributes two thirds of all the Nile
discharge, along with most of the sediment carried
along 4750 kilometres through to Egypt and the
Mediterranean.
To the northeast of the lake is Simen Mountains
National Park, a UNESCO World Heritage Site. These
jagged mountain peaks and deep gorges are home to a
number of rare animals including the Gelada baboon and
Walia ibex, a type of goat.
Radar images measure surface roughness rather than
reflected light, so the smooth Lake waters are as
distinctive as the harsh landscape around it. The
colour in the image comes from the fact that this is
actually a combination of three Envisat Advanced
Synthetic Aperture Radar (ASAR) images, and is used to
highlight changes occurring between acquisitions.
A colour is assigned to each date of acquisition: red
for 3 September 2003, green for 27 November 2003 and
blue for 4 March 2004. The images were acquired in
wide swath mode, covering an overall span of 400 km.
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