diff --git a/src/bathytools/actions/dig_rivers.py b/src/bathytools/actions/dig_rivers.py
index 6c4b829..8c76b21 100644
--- a/src/bathytools/actions/dig_rivers.py
+++ b/src/bathytools/actions/dig_rivers.py
@@ -686,7 +686,9 @@ def __call__(self, bathymetry: xr.DataArray) -> xr.DataArray:
             allow_broadcast=True,
         )
 
-        # Check if a side is "open", i.e., if it has at least one water cell
+        # Precompute a boolean array of cells that are already underwater
+        # (negative bathymetry). Used later to skip water cells when tracking
+        # river dig cells for intersection detection.
         b_array = bathymetry.elevation.transpose(
             "latitude", "longitude"
         ).values