Here are some samples showing the "Rivulet Technique":
This is a close-up from part of the above painting:
My students are ARThouse are currently exploring this technique. It is tricky to do and relies upon utilizing the correct earth colors, such as raw or burnt umber. Actually I wrote an article about the technique for "The watercolor Page" of American Artist magazine back in 1986! For old times sake, here is the original article, click on a thumbnail, and then click the "Image From" text near the bottom to be able to zoom in to read them.

