Geoscience Reference
In-Depth Information
10
Desert rivers
Hence in regions of small rainfall, surface degradation is usually limited
by the slow rate of disintegration; while in regions of great rainfall it is
limited by the rate of transportation.
G.K. Gilbert (1843-1918)
Report on the Geology of the Henry Mountains
(1877, p. 105)
10.1 Introduction
During the first seven months of 1999, the arid Alashan Plateau of Inner Mongolia in
northern China was still in the grip of one of the worst droughts in human memory.
The entire region had endured eight years of below-average precipitation, and the local
Mongolian herdsmen were becoming increasingly worried on behalf of their camels,
sheep and goats. The drought effectively ended at midday on 5 August 1999, when
a sudden thunderstorm in the mountains of the western Alashan unleashed more
than 300 mm of rain over the next thirty hours (Williams,
2000
a). This would be
sufficient to replenish water levels in the piedmont wells and to sustain plant growth
for at least three more years, according to the elderly Mongolian farmer and his
wife in whose home we had taken refuge. We had been crossing the wide, boulder-
strewn bed of an ephemeral stream channel which had barely flowed for several
years but soon became a raging torrent carrying football-sized granite boulders along
its bed.
At about the same time, more than half a world away, unusually heavy rains fell
onto the parched clay plains of the central Sudan, flooding the hollows between the
dunes immediately east of the White Nile, triggering sheet-floods from the low upland
ridge separating the valleys of the lower Blue and White Nile rivers (Williams and
Nottage,
2006
) and causing breaching and overflow from the main canal feeding
into the most enduring and successful large irrigation scheme in Africa - the Gezira
Irrigation Scheme in central Sudan.
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