In typography, rivers (or rivers of white) are gaps in typesetting which appear to run through a paragraph of text due to a coincidental alignment of spaces. Rivers 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 are less noticeable with proportional fonts, due to narrow spacing. Another cause of rivers is the close repetition of a long word or similar words at regular intervals, such as "maximization" with "minimization" or "optimization".
Let me take you back to punched card days, when people would design patterns and shapes using different characters, a single line at a time (one card, one line of 80 characters).
When the cards were just printed (to a nice, noisy, dot-matrix), you'd see the image they designed.