One of Kariba Dam's floodgates has been opened by Zimbabwean and Zambian water authorities prompting flood alerts along the Zambezi River.
Hundreds turned up to the Kariba dam to watch the spectacle
It was opened to prevent the dam wall from being damaged as a result of heavy rainfall this season.
Thousands of people in Mozambique have been evacuated from areas further down the Zambezi River valley already struggling to cope with flood waters.
Zambia and Zimbabwe's armies are ready to intervene, says Reuters new agency.
"Defence forces from both countries are ready to respond to any eventuality and are monitoring the situation," Zimbabwean energy official Munyaradzi Munodawafa is quoted by Reuters as saying.
The opening of the floodgate attracted hundreds of local and foreign tourists who came to witness the rare spectacle, which last took place in 2002, the Zambia National Broadcasting Corporation reports.
Torrential rains in Zambia and Zimbabwe have swollen the Zambezi river to well above the flood limits.
Valleys in Malawi and Mozambique are bearing the brunt as the waters flow down to the Indian Ocean.