In typography, rivers, or rivers of white, are gaps appearing to run down a paragraph of text, due to a coincidental 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.
Typographers try to minimize or eliminate the river effect. They can test for rivers by turning a proof sheet upside down (top to bottom) 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. Other related terms are ‘lakes’ and ‘holes,’ which refer to a cluster of adjacent or intertwined rivers that create a lighter area within a block of type.
The Daily Omnivore 
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