The Sh7.8 billion Thiba Dam in Gichugu Constituency, Kirinyaga County, has now been filled to full capacity.
The dam at Rukenya finally attained its full capacity of 15.6 cubic metres in the exercise that started in May.
The surplus overflows through the spillway and back to river Thiba.
"It has taken three and a half months since the process of filling the dam started," said Mr Daniel Nzonzo, the head of corporate communications at the National Irrigation Authority (NIA).
The spillway is a safety evacuation channel where overflows drain into the river channel.
Mr Nzonzo said the river hinterland had a 180 cubic metres catchment during the rainy season, most of which will flow through the spillway.
The water being released from the dam back to the river through the spillway is four cubic metres per second.
"There is no risk of spill over to the surrounding areas or the dam breaking or causing flooding. The walls are strong to withhold the water," he said.
It was good news to local rice farmers as the dam will benefit them, through irrigation, during dry seasons.
"The gates of the dam will be opened to allow water to flow to the rice paddies at the Mwea Rice Scheme during dry seasons to ensure continued production," said Nzonzo.
The dam is 40 metres high, and one kilometre wide - sitting on 542 acres. Constructed by a German contractor, Strabag, it had initially been projected to cost Sh8.2 billion.
"With an additional 10,000 acres from the current 25,000 acres, we will have 35,000 acres in the rice scheme and farm in two seasons annually. This will raise production by about 86,000 tonnes from 140,000 tonnes," Nzonzo said at the site on Monday.
“Rice is the third most consumed grain in Kenya after wheat and maize, hence the need to increase production."
NIA said it is also expanding other rice-growing zones such as Hola in the Tana River in the ongoing efforts to make Kenya's food secure.
“The approach the authority is taking will include expanding its rice growing zones, including Hola and Bura in Tana River County, West Kano, Ahero, Bunyala, and Migori by more than 10,000 acres to reduce demand for imported rice,” Nzonzo said.
Project engineer Stephen Mutinda said 24-hour security will be provided at the dam to avoid cases of drowning and vandalism.