This is a typical problem of negative externalities: the Ethiopians aren’t factoring in the welfare of Kenyan Turkana residents in the decision to build the dam. There’s actually some research showing that this is a common problem. From a recent World Bank paper by¬†Sheila Olmstead and Hilary Sigman:
This paper examines whether countries consider the welfare of other nations when they make water development decisions. The paper estimates econometric models of¬†the location of major dams around the world as a function of the degree of international sharing of rivers. The analysis finds that dams are more prevalent in areas of river basins upstream of foreign countries, supporting the view that countries free ride in exploiting water resources. There is weak evidence that international water management institutions reduce the extent of such free-riding.
By their very nature dams generate inequality in the flow of water between upstream and downstream areas. It is easier to pay the cost of hurting downstream communities when they are are in a different country (hey, they don’t vote for you). Ergo, countries are more likely to build dams when the costs are external.
It would be interesting to see what mitigates these effects – it is possible that Kenya’s relative indifference is due to lack of political power on the part of the northern tribes. Are dams with substantial cross-border costs less likely in areas where the proximate ethnic group is quite powerful?