diff --git a/README.md b/README.md
index 8ea2cf5..f68d029 100644
--- a/README.md
+++ b/README.md
@@ -89,6 +89,8 @@ The `clips_file` (the list of clip filenames to annotate) is always read from th
 
 ## Usage
 
+> **River annotation reference:** If you are annotating river footage, consult the [river annotation guide](https://docs.google.com/document/d/1iPN9JxiDtb60kC0yjO8tTM0XEfDTGM5ysw33WY-BEDQ/edit?usp=sharing) for guidance on how to draw masks correctly.
+
 ```sh
 uv run python -m clip_annotator
 # or, if you have the venv activated: