A drought. The Maasoretic critics, in their prosaic realism, were unable to see how a "sword" could be "upon the waters;" hence they altered khereb into khoreb. But the sword is merely a symbol of the Divine vengeance, and may be interpreted differently according to the exigencies of the context.
Render, Sword upon the waters. They are mad upon their idols; rather, through Terrors they befool themselves. "Terrors" is a synonym for the gods of the heathen, which inspired a feeling of awe rather than affection, unlike Jehovah as he revealed himself through the authors of the psalms and prophecies.