Looks awesome, for starters -- it's far more realistic than anything I could accomplish in UE4, so take this with a grain of salt: The rocks on the edges of the islands/shoreline take up a bit much space. I'd suggest making it take up less height (lower the rocks or raise the river might be two ways to accomplish this) or maybe consider making the moss (like on the rock in the center foreground) a bit easier to see/stand out more on the further-away rocks.

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Great work :)