River

Nobody puts a river in a paragraph. It arrives when the word spaces on three or four consecutive lines happen to fall at nearly the same place, and the eye, which is very good at joining gaps into shapes, reads them as a channel running down the column instead of as separate spaces. Once you have seen one you cannot unsee it, which is the whole difficulty: the reader does not notice a river so much as get quietly slowed down by it, and the person who set the type notices nothing at all because they are reading the words.

Justified text is where rivers breed. Justification works by stretching the word spaces until each line reaches both margins, so at a narrow measure the spaces get wide, and wide spaces have more chance of lining up and more visual weight when they do. Ragged text can produce one too, but it has to be unlucky rather than systematic, because its spaces are all the same width. The two tools that close a channel are more break opportunities and a wider column: hyphenation gives the line breaker somewhere else to go, so no single line has to absorb the whole stretch, and a rag instead of justification takes the stretching away entirely.

Seeing them is a trained skill with a low-tech trick behind it. Printers turned the proof upside down, or held it at arm’s length, or squinted, all of which do the same thing: they destroy the words as words and leave the page as a pattern of grey and white. Blurring a screenshot works the same way. A related fault worth the same look is a lake, a single gap wide enough to punch a hole in the grey rather than to run down it.

On the web the odds are worse than in print, because browsers break lines greedily, one line at a time, rather than optimising the paragraph as a whole the way a typesetting system does. There is no reflow pass to go back and fix a bad run, and the column width is whatever the reader’s window happens to be, so a paragraph can be clean at one size and rivered at the next. That is the honest argument against text-align: justify on the web: not that justification is wrong, but that the machinery which made it work in print is not there.