Tracing the Source of the Elephant And Hippopotamus Ivory from ...

fed by the Sobat. The Nile is constrained to these sources, as north of the Atbara there is
no inflow. However, the relative contributions of these sources varies: “whereas the flow
rate of the White Nile is well regulated by lakes and swamps, resulting in a relatively
constant proportion (16%) of the average discharge of the Nile, the Blue Nile and the
Atbara discharge rates differ extremely as a result of the seasonal rainfall pattern”
(Gerstenberger et al. 1997: 349-350).
The strontium isotope ratio of the White Nile is 0.7114, a high value typical of
silicate weathering of cratonic rocks including the Sabaloka granulites, migmatities and
granites. The Blue Nile flows a shorter distance in a floodplain than the White Nile-
across metasedimentary gneiss, oceanic volcanosediments and ophiolitic rocks. As to be
expected from this type of the geology, the 87 Sr/ 86 Sr of the Blue Nile is a relatively low
0.7056. The Aswan High Dam reservoir represents a major modern alteration to the
river, but for the purposes of this thesis has only slight consequences as it serves to level
out the seasonal fluctuations of the strontium isotope values caused by the variable
discharge rates of the Blue Nile. The effective outflow from the Aswan Dam is 0.7062
(Gerstenberger et al. 1997: 352-353). As Figure 56 demonstrates, the 87 Sr/ 86 Sr values
remain fairly constant with distance downstream from Aswan, although at a slightly
higher strontium isotope ratio of approximately 0.7070, probably due to evaporation
along the course of the river.
Taking another look at the principal lakes and rivers of the Upper Nile, Talbot et
al. (2000: 343-345) note a Sr ratio of 0.7113 for Lake Victoria and 0.7073 for Lake
Edward, whereas Lake Albert, which receives water from both lakes, has an intermediate
value of 0.7102. The Bahr el Jebel river downstream from the lakes has a higher Sr ratio
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