The Damm was constructed about 20 years ago if not before but it never affected the flow, that's not the issue, what ppl never put in consider is that the river doesn't come straight to the Somali border the the river flows over 500 km In the Somali region dds, and the once conflict prone region has been seeing peace and developments and many started farming which in some ways means that they are taking thier fair share of the water recourses and The more those folks farm the worse it gets for the one's that recieve the flow last, places like lower shabelle are pretty much done, and by the way thinks are looking, even in the rainy season the flow of the river will probably just cut of in the border region.
The biggest mistake Somalia ever made was not having an water sharing agreement with the highlanders, now it's pretty much game over in less then a few years