Definition
River
A river is an accidental channel of white space running down several lines of a paragraph, caused by word spaces stacking vertically. Rivers appear in justified text with wide spacing.
Rivers are easiest to see by turning the page upside down or squinting, which removes the words and leaves the pattern. They are a symptom rather than a fault in themselves: the cause is a measure too narrow for the type size, justification without hyphenation, or a font set too loose. Fix the cause — a slightly wider measure, hyphenation on, or a size down — rather than nudging individual spaces.
Related tools & guides
See it handled in your own manuscript
Ceeryo sets your .docx as a book — justified and hyphenated text with widows, orphans and rivers controlled, drop caps, running heads — and shows you every spread before you pay anything.