Karimenu II Dam in Kiambu County has reached full capacity in the wake of the ongoing heavy rains and is currently spilling over the excess water.
In an update on Saturday, the Athi Water Works said the current water volume is 23 billion litres of water.
The authority said the first spilling occurred at 8am Saturday after the water capacity hit the pilling level of 1,855 feet above sea level.
The spillover happened a day earlier than anticipated going by an update issued on Thursday when the water was 89.9 per cent full, holding 20 billion litres of water.
"The dam is expected to spill over by this Sunday. The spillway design has flood control, and drainage works providing safety to downstream areas against failure by overtopping during the El Nino," Athi Water Works said in the update on its X handle.
But in its update Saturday morning, the authority said spilling started early hours of the morning.
"Karimenu II dam starts spilling. We have prepared an emergency response plan that highlights all possible failure modes of the dam and their related emergency responses."
The dam serves Gatundu, Ruiru, Juja, Githurai, Tatu City and parts of Nairobi providing more than 70 million litres per day.