Fratalize and edit as many times as you feel you need. When you have enough branches, you can apply more fractalizing at once by increasing the "Subdivisions" parameter.
Next select all the paths and Stroke to Path (Ctrl+Alt+C) and then with them still selected, choose the Tweak tool (Shift+F2) and start playing wit the rivers. Use the Grow mode to widen a stretch of river, Shrink to narrow it, and Push to shift it sideways. You'll have to adjust your Force, Size, and the zoom level to get the right effect and it might require some practice. Some of the tributaries might become detached in the process, but this can be fixed later.
You should end up with curvy, rivers with varying width. Some of the tributaries may get detached in the process but this is easy to fix.
To fix any detached tributaries, use the Edit Paths tool to drag the loose nodes into place. You can also make other fine adjustments to the paths if you feel it necessary.
Congratulations, you now have a river system.