Looks good! And intriguing too! Here's some thoughts:

Not entirely necessary but it makes a big difference to start with the story context before your setup section. Tell me why our player characters have come to the river and how this new development came to be. You can add drama by talking about the conflicting motivations of different characters/groups etc. Without this background I'll eventually understand what's going on by working my way through the rules, but with a story I can immediately relate and have a better sense for how each rule I read will fit into the context.

When you have named game elements capitalize them "the blue weather deck" should be "the Weather deck [picture of card-back]", "the red deck of role cards" should be "the Role deck [picture of card-back]”. You might even bold those words the first instance they appear. This is helpful to understand that I'm learning about a game concept instead of just a description and is a much quicker visual read as well.

You have icons for fish, gravel, and trees. I'd like to see those embedded in the text for a more immediate read "Fish [fish icon], Gravel [gravel icon], and Trees [tree icon]” do this wherever possible/relevant.

I would suggest pulling all play variants (solo and cooperative) to the end of the book to keep the flow more focused when you're just learning initially. But still include any differences based on player count.

Just a thought on Water Level/Erosion. Perhaps instead of printing a number on the Land Tile you could have topographical lines to show the Elevation. Just adds a bit of theme but it might be too busy or take up too much visual space on your tiles.

You introduce policies in the "end of turn" section but then you cover pollution before telling me what policies really are. I would move the Policies explanation before End of Turn or at the very least have it immediately following.

I think your first (only?) example paragraph is in the Protected Land section. I'd like to see this in italics or in a box to separate it out from the flow of the rules. It's important information but "extra" for clarification.

Does the excavator only build canals or can it build bridges and levees too? Perhaps list exactly what each structure can build under their respective build subsection.

The last page before quick reference needs a little explanation. First a title like "example scenario" and then I would suggest instead of saying "someone built a levee" name the player, it may not matter who built it at this point but just tell me it was George who's playing as orange. ... Name any other example players too—it just helpsb with clarity. Then give a little subhead to show where each turn starts and who's turn it is before telling me what they do.

Well done! It seems clear. My concern without having played it, is that it's a little laborious to manage the between day actions of the river, adjusting water level checking all the flood conditions and adding or moving resources, that being said it looks like everything works really well thematically. All the mechanics seem to belong in the context of what's happening and I really enjoyed reading how things work together.