7 - Spread the Scrub items next to each other where you want your swamp/marsh to be

6 - Create SYMBOLS TREES GRASS sheet, and place it above SYMBOLS BACK sheet. Select this sheet to work with and choose Scrub icon (grass).

VERIFY if CC3 keeps the SYMBOLS TREES GRASS sheet or if it simply switches - it happens a lot to me - to the SYMBOLS TREES Sheet. In case of change, Select SYMBOLS TREES GRASS again

5 - draw some secondary rivers, note that they are in their default size (river wide), if you are more precious, adapt each one to a different size. Certainly this greater work would have resulted in a more realistic effect.

4 - Refresh and new Big river

3 - Click Change properties in the left corner of your screen. Select Main River, left mouse button and Do it. It opens a new screen and, in this river, I chose the size 1 (this map is out of scale and is for demonstration purposes only).