Those Dam Goblins!

From publisher blurb: Several months ago human settlers discovered a marsh covered valley which they knew would provide fertile crop land if only the marsh could be drained. The settlers dammed the river and may have damned themselves! The diverted river has awoken the cliffs themselves. Strange rumblings in the earth and smoke from the cliff tops are sure signs of trouble. The residents of Fertile Valley need some heroes, can you save Fertile Valley before it’s too late?