That's a pretty useful tutorial and I will definitely make use of that in my current project.
Additionally I would suggest an alternative method for the last step.

Instead of grouping the shapes you could also turn them into paths (Path>Object to Path) and then press Ctrl++ to turn them into one path. This would of course take away the option to change the course of the river with nodes ... but it's more useful for my technique where I treat rivers and lakes like inverted coastlines.