"We have set up a centre near the scene for families to report missing members to enable us to reunite them", said Nakuru Governor Lee Kinyanjui.
He said the incident occurred on Wednesday evening, causing extensive damage to homes, schools and businesses.
The water has caused huge destruction of both life and property.
Kenya Red Cross said on Twitter that it had rescued 39 people so far.
Local leaders are now seeking to find out whether the farmer was licensed to erect those dams, amid concerns on the condition of the remaining two which are also said to be full, our correspondent says.
Wanjiku said at least one looked like it too was ready to burst.
KRC has already made an appeal for $5 million following the heavy rains that have wreaked havoc across the country with 11 counties been most hit by the latest spate of floods.
People watch as flash floods rage past a part of the main highway to Kenya's border town with Tanzania on March 15, 2018 in Isinya, around 36 miles southeast of capital Nairobi.
Among villages affected are those in the farmlands of Nyakinyua, Endao, and Arutani where hundreds of people were displaced.
He said the county government will donate foodstuffs and other items to the displaced families on Thursday.
As the rains worsen, the need for humanitarian assistance to victims becomes more pressing. Caught between different extremes, millions of Kenyans are in dire straits.
Kenya, like other countries in East Africa, has experienced heavy rain over the past two months. Military helicopters have been deployed to assist with relief efforts. On Wednesday, the United Nations office for the co-ordination of humanitarian affairs reported a death toll of 118, up from 100 on May 4.