The art of watercoloring employs a distinctly separate technique from other painting mediums. The keys to good watercolour execution are a subtle mixing of paint colours, the proper dilution of the paint, choosing the right watercolour paper weight and texture for your work and, most importantly, a free and confident wielding of the watercolour brush. Most watercolour works are meant to be impressionistic, characterised by capturing light in a watery image.
Prepare your watercolour paper. Tape the edges of the paper to a smooth wooden board that is large enough to accommodate the paper and provide room for resting your hand and arm.
Sketch the line of the river or other features with a very fine but soft-leaded mechanical pencil. This line should be very faint, perhaps visible only to your eye, unless you like your pencil drawings to show in your final work. If you don't want to use a pencil, use a very small watercolour brush and sketch in your features using heavily diluted raw umber watercolour paint.
Draw in any features that might appear in the midst of the river, such as rocks, boats, vegetation, buildings or animals. Cover these areas with a watercolour blocking agent to preserve these areas while you paint the river as a whole. Cover the banks of the river with this blocking agent as well.
Test your river colours on scrap paper. Few rivers are truly blue in colour. Most rivers are tea-coloured, mossy green, muddy brown or a deep greyed blue. Experiment with layers of colours: darker in the centre of the river, greener towards the banks and tea or brown along the shores.
Practice letting the paint colour flow out of your brush on scrap paper first. Don't "paint" the river or "scrub" the colours onto the paper. Instead, let the "water" in watercoloring do the work---let if flow. Try working "wet on wet" style: Pre-moisten the paper and then add wet paint with the brush. Try working "dry on wet" too: Pre-moisten the paper and touch the brush holding a dryer paint to this wet area.
Apply the watercolour in smooth, flowing brush strokes in the direction you imagine the river is running. Move or "jag" your brush to indicate shifts in the mainstream. "Jag" your brush stroke around any rocks or objects in the river, just as water would flow around objects in real life. Stop painting when the look of the river is right for you. Don't overwork your piece. Let the paint dry completely.
Remove the blocking agent protecting other parts of the painting by pulling it up with a soft gum eraser. Watercolour in these new features.