The caption of one of the article's images says "Czech proofreader's marks for a river". Does this mean that points were deducted for a student unintentionally creating a river? The article does not elaborate

There is a section which talks about the (perceived) negatives of rivers. In short: it makes the text harder to read.

> Typographers try to minimize or eliminate the river effect. In Finer Points in the Spacing & Arrangement of Type, Canadian typographer Geoffrey Dowding explains as follows.

> A carefully composed text page appears as an orderly series of strips of black separated by horizontal channels of white space. Conversely, in a slovenly setting the tendency is for the page to appear as a grey and muddled pattern of isolated spats, this effect being caused by the over-widely separated words. The normal, easy, left-to-right movement of the eye is slowed down simply because of this separation; further, the short letters and serifs are unable to discharge an important function—that of keeping the eye on "the line". The eye also tends to be confused by a feeling of vertical emphasis, that is, an up & down movement, induced by the relative isolation of the words & consequent insistence of the ascending and descending letters. This movement is further emphasized by those "rivers" of white which are the inseparable & ugly accompaniment of all carelessly set text matter.