No, you'll have to move/delete the tree symbols manually. If you where doing it the other way around, you could have used the exclusion commands to place the forest around the river.
Yeah I have been thinking that I need to do all rivers first as per Mr. Sweeny's tutorials, but I searched high and low to see if there were some type of tool or effects I could use.
There is actually a command that may or may not help you a bit. The SYMDELNEAR command (command-line only command) deletes symbols near another entity (such as a river). However, it works poorly with trees drawn with the forest tool, because the way the forest tool uses symbols containing large group of threes, and how these are laid out. These large chunks are very helpful performance-wise for your map, but make it more difficult to remove individual trees from the forest after making it.
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If you where doing it the other way around, you could have used the exclusion commands to place the forest around the river.
Thank you for your time.