diff --git a/server/src/atlas/drainage.rs b/server/src/atlas/drainage.rs
index 56c141cd9..8aa57bab8 100644
--- a/server/src/atlas/drainage.rs
+++ b/server/src/atlas/drainage.rs
@@ -1053,7 +1053,7 @@ mod tests {
         // adj[1]={2,3} (row2-row3 touch), adj[3]={1}.
         let (w, h) = (8usize, 6usize);
         let mut labels = vec![1i32; w * h];
-        labels[1 * w + 2] = 2; // basin 2: single-cell hole in basin 1
+        labels[w + 2] = 2; // basin 2: single-cell hole in basin 1 (row 1, col 2)
         for r in 3..6 {
             for c in 0..w {
                 labels[r * w + c] = 3; // basin 3: rows 3-5
@@ -1108,7 +1108,7 @@ mod tests {
         // production behavior.
         let (w, h) = (10usize, 8usize);
         let mut labels = vec![1i32; w * h]; // basin 1: rows 0-2
-        labels[1 * w + 2] = 2; // basin 2: hole in basin 1
+        labels[w + 2] = 2; // basin 2: hole in basin 1 (row 1, col 2)
         for r in 3..5 {
             for c in 0..w {
                 labels[r * w + c] = 4; // basin 4: bridge, rows 3-4