I mixed up a nice china blue from Pthalo blue and Dioxazine purple, and picked up a superbly beat-up and damaged small flat brush and started scribbling in the dark shapes. The bristles on one side are bent back 360º and they were great for scratching on trees, branches and other rough shapes, while the other side was still straight enough to sort of draw lines and put the paint on flatly. I did use a different tint on the river, a mix of Pthalo and Cobalt blue. I was going to color it in fully, but decided to stop with a value study.
I'm surprised how traditionally "sketchy" it looks, and yet how fresh and "real", as if I'd done it right there. It seems like a great example of brush-stroke texture as detail, and I'm liking how many different looks I got depending on the value and thick-or-thinness of the paint and how much time I spent on that bit. No real painter would ever throw a brush away, unless maybe the handle breaks off, and clearly this is why. What a gold mine!