Homes were swept away and roads were cut off after the dam burst in Kamuchiri village in western Kenya.
"Forty-two dead, it's a conservative estimate. There are still more in the mud," Nakuru county governor Susan Kihika told AFP news agency.
More than 100 people have been killed in floods that have devastated parts of Kenya in the last month.
Rescue teams are digging through the mud in search of survivors in the village near the small town of Mai Mahiu, about 60km (37 miles) from the capital Nairobi, local media reports say.
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