Publishing term--can someone explain it to me?

I was reading about a poorly edited and published book, described thusly:

The text had so many rivers?

Is there anyone with editorial experience who can explain that phrase? Google isn’t helping me here.

A river is an old school typography term. It’s when the (white) spaces between words line up line after line after line, so it appears that there is a thin “river” of white space snaking up and down the page. It is considered bad aesthetics to have many or noticable rivers on a page of type.


A river squiggles its.  way about this block of text
A river squiggles its way.   about this block of text
A river squiggles its way in.  this block of text
A river squiggles its way about.   this block.  of text
A river squiggles its way about a.  block of text
A river squiggles its way about.  this block of text
A river squiggles its wayward.  path through this block of text
A river squiggles its way in.   this block of text
A river squiggles its way.   about this block of text
A river squiggles away.  through this block of text

A river is a pattern of blank spaces in successive lines that draws the eye, and distracts a reader from the content of the page.

Perfect answers, both! Thank you.

Helpful, and damned speedy, too.