Western Sahara once had a large river network

If in August 2011 the discovery of an underground ‘river’ in Brazil at a depth of 4 km and stretching for about 6,000 km made news (The Hindu , August 31, 2011), scientists have now discovered evidence of a large river system that once existed in Western Sahara.

Based on remote sensing data, the existence of a 520 km-long vast ancient river network has been established under the parched sands of the Mauritania coast. The river is “shallowly buried” at present under wind-borne sediments, says a paper published a few days ago in the journal Nature Communications . Water may last have flowed through the channels some 5,000 years ago.

Recently, a submarine channel system — the Cape Timiris Canyon — was discovered on the Western Sahara margin off the Mauritania coast. Generally, large-scale submarine channels occur off major river mouths. Hence, the Cape Timiris Canyon must have been linked to a major river system some time in the recent past.

Earlier studies have argued for the presence of a large river system in Western Sahara. Now, combined with geomorphological and sedimentary data already available, the French team “suggests that a major river system was indeed reactivated during some of the humid periods of the last 245,000 years,” and this river system could have contributed sediments to the Tropical Atlantic margins during that time.

The authors utilised the data provided by the Japanese PALSAR radar that uses L-band frequency to discover the ancient river system. The L-band radar has the potential to see through even metres of loose sand material and can identify geomorphological features that are now buried under wind-borne sediments. However, the radar could not peer through thick sand dunes.

But whatever has been reconstructed perfectly matches with an earlier digital projection model of a large river system. In fact, a fifth of the drainage network reconstructed matches perfectly with the digital projection model.

Satellite images reveal that the course of the ancient river is perfectly aligned with ancient valleys identified in the Arguin Basin and proximal tributaries of the Cape Timiris Canyon system.

The river would have got its water from heavy rainfall in the Western Saharan region during the early Holocene African humid periods. “At that time, equatorial lakes reached their highest level and the present-day Saharan desert was the location of extensive vegetation, animal life and human settlements,” the authors write. The presence of sediments transported by river were deposited in the Arguin Basin during the mid-Holocene (between 11,000 and 6,500 years ago) suggesting that the Tamanrasett River was active during that period.

According to the authors, the presence of the river system during the recent humid periods provides the “missing link between the development of lakes over Algeria, and Mauritania, fluvial evidence in Algeria and the riverine signals recorded in the Arguin Basin.” The present study supports the hypothesis that there was a major drop in wind-borne sediments to the Northeastern Tropical Atlantic Western Ocean during the time the river network was active.

“This finding provides yet another evidence of the extent of the Saharan wetting during AHPs [African humid periods] and should thus provide valuable constraints for numerical simulations of West African climate throughout the late Quaternary,” they write.

Published in The Hindu on November 16, 2015