diff --git a/src/simulator/crates/mc-save/tests/round_trip.rs b/src/simulator/crates/mc-save/tests/round_trip.rs
index 5828333d..d28df075 100644
--- a/src/simulator/crates/mc-save/tests/round_trip.rs
+++ b/src/simulator/crates/mc-save/tests/round_trip.rs
@@ -190,12 +190,30 @@ fn hydrology_resolve_mutations_round_trip_byte_equal() {
     let bytes = save(&sf).expect("save after re-solve");
     let loaded = load(&bytes).expect("load after re-solve");
 
-    let original_json = serde_json::to_string(&sf.grid).expect("ser original");
-    let loaded_json = serde_json::to_string(&loaded.grid).expect("ser loaded");
+    // Tiles are an ordered Vec - byte-equal compare covers the five mutated
+    // hydrology fields + river_edges on every tile.
+    let original_tiles = serde_json::to_string(&sf.grid.tiles).expect("ser original tiles");
+    let loaded_tiles = serde_json::to_string(&loaded.grid.tiles).expect("ser loaded tiles");
     assert_eq!(
-        original_json, loaded_json,
-        "post-re-solve grid (mutated river_edges + lakes + riparian + \
-         edge_features) must survive save->load byte-equal"
+        original_tiles, loaded_tiles,
+        "post-re-solve tiles (mutated river_edges + lakes + riparian) must \
+         survive save->load byte-equal"
+    );
+    // edge_features is a HashMap (nondeterministic iteration order), so
+    // compare as sorted pair sets.
+    let sort_edges = |g: &GridState| {
+        let mut pairs: Vec<_> = g
+            .edge_features
+            .iter()
+            .map(|(k, v)| (format!("{k:?}"), *v))
+            .collect();
+        pairs.sort_by(|a, b| a.0.cmp(&b.0));
+        pairs
+    };
+    assert_eq!(
+        sort_edges(&sf.grid),
+        sort_edges(&loaded.grid),
+        "post-re-solve edge_features (river flags) must survive save->load"
     );
     // Spot-check the dam-attributable mutations specifically.
     let (lc, lr, _) = delta.added_lake_cells[0];