There are a few ways to make realistic-looking rivers and streams in WorldPainter.

There are multiple automated approaches:

1. The paid River Script, that uses realistic pathfinding to carve out a flowing river. A very good solution for medium-sized rivers that flow downhill. Rivers will alter terrain to find a path.
2. The free Puddler script will also add small streams, rivers, and ponds to your maps. An easy way for small streams flowing downhill while creating ponds. Not suited for very steep terrain.
3. The free ValleyCraft script, which helps with carving out flat rivers and smoothly blending them into existing terrain. Works for all river sizes but requires manually marking the rivers and does not allow sloped rivers. Good for very large scale and ocean-level/shippable rivers.

Make sure to read the guides provided for all scripts.

Alternatively, you can manually make a custom ground cover layer or download one here to draw rivers yourself. If you select this method you will need to smooth out the terrain and ensure it always flows down manually.