The dam hasn't even built yet
Melka Wakena Dam was built in 1989. Ethiopia has had full control of the Shabelle for almost 30 years, but most people do not know this. They can literally press a button and stop its flow. Ethiopia is probably holding back more water than it needs to spin the turbines of the hydro powerplant at Melka Wakena. They are most likely trying to fill the reservoir in case the drought lasts longer than expected. It is usually passive which is why it has gone unnoticed until recently.
FAO hinted at this just yesterday, citing this and water diversion in DDSI as possible reasons. They skirt around the dam issue and just blame irrigation in DDSI, but I suspect the dam is more to blame.
http://reliefweb.int/map/somalia/somalia-drought-conditions-jan-2017River levels along the Shabelle River in Ethiopia are also currently below normal. Water levels have decreased significantly in the Melka Wakena Hydroelectric Power Station in Ethiopia located in the upper part of the Shabelle river. There are reported cases of water diversion from the river for irrigation purposes in Gode and Khellafo areas (some 50 kilometers from the Somali border). With no rains expected in the coming week, the situation is likely to deteriorate further with reduction of the river flow inside Somalia.
The low flows along the Shabelle River both in Somalia and Ethiopia are due to a hydrological drought within the basin, specially in the upper part.
Melka Wakena reservoir
Melka Wakena Dam itself
Melka Wakena substation for power distribution to more than 100K Ethiopians