I can't see rivers in the example. Here's a screenshot.
If you tweak the css (text-align: justify), 1-4 word sentences do stand out in a bad way.
Personally, I find the double-spaced version more readable.
In order to demonstrate the problem reliably, they would have had to use an image or PDF file. But the fact that your browser fixed it (as did mine) indicates that it's not actually a problem, since it can be solved automatically in software.