In the olden days a good comp would strive to avoid rivers: those white streams which meander down too many type pages. Here’s one from the Library of America’s Harriet Beecher Stowe volume.
Not that this is a particularly bad case. (There’s a more dramatic example at Wikipedia.) You can make out a river with a side branch in the last paragraph of this page from Uncle Tom’s Cabin. I’ve outlined it in red — on a photocopy, not in the book! Rivers result from a coincidence of word spaces one below the other carrying on for several lines. They can be got rid of by making a correction in one of the early lines: the reflow resulting from such a change will probably work to eliminate the rest of the river. The word spacing in the first line of the paragraph is quite wide: it’s just easier to space out the line like this than it is retrospectively to tighten things up so that the word “from” can be taken back into line 1. Doing that would rearrange the word spaces all down the paragraph, and while it’s possible that another rivulet might appear, the chances are that taking back that one word would eliminate the whole problem. If there’s not enough room to transfer “from” to the first line, you set about attempting the same cure on the second line; “set” should surely be possible to pull back. Sometimes your move will get you into hyphenation hypertrophy: as you are only allowed to have three hyphenated lines in sequence the solution you select may become dauntingly complicated. A distracted comp could be tempted to edit the copy to get around the difficulty. “Aint sh’a peart young un?” will probably never be noticed! After all the author’s not going to be proofreading. In newspaper and jobbing work this way out was not uncommon. One way or another rivers can be eliminated, but as you can see it can be quite time-consuming, thus expensive, so of course more often than not the river is tolerated.
Unjustified setting (ragged right) like the pages in this blog presents less of a risk of rivers. With the constant word space permitted by the removal of the need to fill every line to the same measure, it becomes less likely, though not I suppose impossible, for a river to evolve.
While this sort of thing used to worry skilled craftsmen, we have to admit that as problems go it’s pretty minor. Still it is a fact that I notice rivers when I’m reading: and they do say that anything which distracts the reader from the author’s message should if possible be avoided.