KenGen has warned of imminent flooding in Tana River's lowland areas after Kiambere Dam overflowed on Tuesday.
The dam, which is part of the Tana's Seven Forks, rose by 1.6 metres since Monday evening.
The electricity generating company says the water rose above the maximum design level of 700 metres above seal level.
"This has consequently spilled excess water...we are kindly requesting communities living near the banks of Tana River downstream of Kiambere to be alerted of impending increase in river flows."
Julius Odumbe, operations manager Eastern Hydro, said this in a April 24 letter to Garissa county commissioner.
He said the overflow has been caused by the heavy rains that are pounding most parts of Mount Kenya, Aberdare catchments, as well as local run off.
The heavy rains, which has wreaked havoc in most parts of the country, has seen Kamburu, Gitaru, and Kindaruma dams also rise to spilling levels.
Kiambere is the reservoir at the downstream end of the Seven Forks.