There are only 3 ways of doing it at present, and which one you use depends on the scale of your map. In a city map you can draw the river as a smooth polygon and vary it as much as you like, and in an overland map you can draw the river as one line then chop it into sections that you can then give progressively narrower line widths as you go into the mountains, using Change Properties on each line section.

The third way is a macro written by Joachim de Ravenbel, but I've only heard about that one and not tried it. I think it does the same job as cutting the river into sections and making them gradually narrower.

You may have seen trees from SS4 - Dungeons of Schley?

https://www.profantasy.com/products/ss4.asp#&panel1-4

@JimP

If you export a spin series you can then import the whole series as layers to a single image in GIMP. GIMP allows you to export a GIF image, and offers you the option of animation at that stage.

It's how I do them.

These ones.

Are they meant to be that colour, or is the colour just an accident?

When I first looked at the drawing I thought they were meant to be grey like the other lines, but maybe I was wrong?

The drawing tool macro for aligning fields can be found in the Ferraris Style, and in the more recent Darklands City style.

I'll pass on the rest.