From the back cover:
After a hundred pioneer years the colony on Hestia seemed to be nearing its end. Its holdings on that green and fertile planet were still limited to a single river in one valley. Everywhere else hostile fauna hedged them in.
A dam could break the deadlock, and they needed an engineer to construct it for them. Sam Merrit was the man who came in answer to their S.O.S.--and he wanted out as soon as he had landed. But once down on Hestia, it was live or die with the colony.
So Sam stayed--to discover certain anomalies that the hardscrabble colonists had thrust from their minds.