With the centralized supervision of the Ajuran, farms in Afgooye, Bardhere and other areas in the Jubba and Shabelle valleys increased their productivity. A system of irrigation ditches known locally as "Kelliyo" fed directly from the Shebelle River and Jubba rivers into the plantations where sorghum, maize, beans, grain and cotton were grown during the "gu" (Spring in Somali) and "xagaa" (Summer in Somali) seasons of the Somali calendar. This irrigation system was supported by numerous dikes and dams. To determine the average size of a farm, a land measurement system was also invented with "moos", "taraab" and "guldeed" being the terms used. The urban centers of Mogadishu, Merca, Barawa, Kismayo and Hobyo and other respective ports became profitable trade outlets for commodities originating from the interior of the State. The Somali farming communities of the hinterland from Jubba and Shebella valleys brought their crops to the Somali coastal cities, where they were sold to local merchants who maintained a lucrative foreign commerce with ships sailing to and coming from Arabia, Persia, India, Venice, Egypt, Portugal, and as far away as Java and China. The source of the Shebelle River is cultivated by the Arsi Oromo, Sidamo and mostly Somalis, respectively. It is surrounded by a sacred enclosure wooded with juniper trees, which as of 1951 was under the protection of a Muslim member of the Arsi. In 1989, with the help of Soviet engineers, the Melka Wakena dam was built on the upper reaches of the Shebelle River in the Bale Mountains. Producing 153 megawatts, this dam is Ethiopia’s largest hydroelectric generator. The recent history of the Shabelle is marked by frequent destructive flash floods. The Shabelle is said to have flooded every other year prior to the 1960s; that decade had only two devastating floods, the "hidigsayley" in 1965, and the "soogudud" in 1966. In the 1970s, the most devastating flood was the "kabahay" of 1978. In 1996, floods devastated three woredas in Ethiopia. On 23 October 1999, the river unexpectedly flooded in the middle of the night, destroying homes and crops in 14 out of the 117 kebeles in Kelafo woreda, as well as 29 of the 46 kebeles in neighboring Mustahil woreda.