Hestia — C.J. Cherryh

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.