Great use for a conceptual diagram! The mirror base is perfect and you seem to have most of the symbols. Here are a few suggestions you might want to consider.
The dark brown background of the earth base makes the text hard to read. I would go with a much lighter brown - light tan even.
Try reducing the text. It is very small in font size and most people don't take time to read that much text. In fact, if this is to be a sign or poster, the rule is that the text should be legible 10' away. You may not be able to do that, but get as close as you can. This will enable you to make the symbols embedded in the text much larger too.
Based on the lead headline "..things you can do..", make the sub-headers (pet waste) into 'active' titles, i.e. "1. clean up pet waste".
You might want to add some continuous buffer vegetation along the river on the "clean" side of the diagram.
And lastly, make sure that all the symbols in your diagram are also embedded in your text. If they are not, then perhaps they are simply "decorating" your diagram and taking up wasted space.
Good job, Rachel!