In typography, rivers are visually unattractive gaps appearing to run down a paragraph of text, due to an accidental alignment of spaces. They can occur regardless of the spacing settings, but are most noticeable with wide inter-word spaces caused by full text justification or monospaced fonts.
Rivers occur due to a combination of whether the type appears broad or skinny, the values assigned to the widths of various characters, and the degree of control over character spacing and word spacing. Broader typefaces are more prone to exhibit rivers, as are the less sophisticated typesetting applications that offer little control over spacing.
Typographers can test for rivers by turning a proof sheet upside down to examine the text. From this perspective, the eye is less likely to recognize words and the type can be viewed more readily as an overall pattern.
A related but less frequently used term is lake, which refers to a cluster of adjacent or intertwined rivers that create a lighter area within a block of type. Typesetters today are less likely to make adjustments to conceal rivers and lakes than they would using the more traditional methods.