diff --git a/src/simulator/api-gdext/src/lib.rs b/src/simulator/api-gdext/src/lib.rs
index 161c9e1c..2066cb21 100644
--- a/src/simulator/api-gdext/src/lib.rs
+++ b/src/simulator/api-gdext/src/lib.rs
@@ -1290,11 +1290,14 @@ impl GdCity {
 
     /// Process food growth. Pass tile yields as JSON array:
     /// `[{col, row, food, production, gold, culture, science}, ...]`
+    /// `growth_modifier` is the happiness-tier scalar from
+    /// `GdHappiness.calculate().growth_modifier` (1.25/1.0/0.5/0.0). Pass
+    /// `1.0` from contexts that don't model happiness (proof scenes, tests).
     /// Returns population delta: +1 growth, -1 starvation, 0 neither.
     #[func]
-    fn process_growth(&mut self, tile_yields_json: GString) -> i64 {
+    fn process_growth(&mut self, tile_yields_json: GString, growth_modifier: f64) -> i64 {
         let ty = Self::parse_tile_yields(&tile_yields_json.to_string());
-        self.inner.process_growth(&ty) as i64
+        self.inner.process_growth(&ty, growth_modifier) as i64
     }
 
     /// Process culture accumulation. Returns true if border expansion is ready.
diff --git a/src/simulator/crates/mc-city/src/city.rs b/src/simulator/crates/mc-city/src/city.rs
index 278445df..6b994b04 100644
--- a/src/simulator/crates/mc-city/src/city.rs
+++ b/src/simulator/crates/mc-city/src/city.rs
@@ -450,9 +450,15 @@ impl City {
     // ── Per-turn processing ────────────────────────────────────────
 
     /// Process food accumulation and population growth/starvation.
+    ///
+    /// `growth_modifier` scales the food surplus before it accumulates into
+    /// `food_stored` — sourced from `mc_happiness::get_growth_modifier`
+    /// (`1.25` Happy / Golden Age, `1.00` Content, `0.50` Unhappy, `0.00`
+    /// Revolt). Callers that don't want the modifier (proof scenes, unit
+    /// tests, Rust-only entry points) pass `1.0` for full-rate growth.
     /// Returns the population delta (+1 for growth, -1 for starvation, 0 otherwise).
-    pub fn process_growth(&mut self, tile_yields: &[TileYield]) -> i32 {
-        let surplus = self.get_food_surplus(tile_yields);
+    pub fn process_growth(&mut self, tile_yields: &[TileYield], growth_modifier: f64) -> i32 {
+        let surplus = self.get_food_surplus(tile_yields) * growth_modifier;
         self.food_stored += surplus;
 
         if self.food_stored >= self.growth_threshold() {
@@ -866,15 +872,42 @@ mod tests {
         // Surplus per turn: (4 + 5) - 1.0*1 = 8.0
         // Threshold at pop 1: 15.0
         // Turn 1: food_stored = 8.0
-        assert_eq!(city.process_growth(&ty), 0);
+        assert_eq!(city.process_growth(&ty, 1.0), 0);
         assert!((city.food_stored - 8.0).abs() < 1e-9);
         // Turn 2: food_stored = 16.0 >= 15 → grow. Carryover = 0.5*15 = 7.5
         // Surplus-over = 16.0 - 15.0 = 1.0. New stored = 7.5 + 1.0 = 8.5
-        assert_eq!(city.process_growth(&ty), 1);
+        assert_eq!(city.process_growth(&ty, 1.0), 1);
         assert_eq!(city.population, 2);
         assert!((city.food_stored - 8.5).abs() < 1e-9);
     }
 
+    #[test]
+    fn process_growth_modifier_scales_surplus() {
+        // Happy / Golden Age modifier (1.25) accumulates 25% more food per
+        // turn than Content (1.0). Mirrors mc_happiness::get_growth_modifier.
+        let mut content = City::found("Anvil", (5, 5), true, 1, None);
+        let mut happy = City::found("Anvil", (5, 5), true, 1, None);
+        let ty: Vec<TileYield> = vec![];
+        // Surplus per turn at pop 1: 4 (city center) - 1.0*1 = 3.0
+        content.process_growth(&ty, 1.0);
+        happy.process_growth(&ty, 1.25);
+        assert!((content.food_stored - 3.0).abs() < 1e-9);
+        assert!((happy.food_stored - 3.75).abs() < 1e-9);
+
+        // Revolt modifier (0.0) freezes accumulation entirely.
+        let mut revolt = City::found("Anvil", (5, 5), true, 1, None);
+        revolt.process_growth(&ty, 0.0);
+        assert!(revolt.food_stored.abs() < 1e-9);
+
+        // Unhappy modifier (0.5) negative surplus still drives starvation —
+        // a half-rate deficit is still a deficit. pop 5, no worked tiles:
+        // surplus = 4 - 5 = -1, scaled = -0.5 → still triggers starve.
+        let mut unhappy = City::found("Anvil", (5, 5), true, 1, None);
+        unhappy.population = 5;
+        assert_eq!(unhappy.process_growth(&ty, 0.5), -1);
+        assert_eq!(unhappy.population, 4);
+    }
+
     #[test]
     fn process_growth_starvation() {
         let mut city = City::found("Ironhold", (5, 5), true, 1, None);
@@ -883,7 +916,7 @@ mod tests {
         // Consumption = 1.0*5 = 5.0. Surplus = 4 - 5.0 = -1.0
         // food_stored: 0 + (-1.0) = -1.0 < 0, pop > 1 → starve
         let ty: Vec<TileYield> = vec![];
-        assert_eq!(city.process_growth(&ty), -1);
+        assert_eq!(city.process_growth(&ty, 1.0), -1);
         assert_eq!(city.population, 4);
         assert_eq!(city.food_stored, 0.0);
     }
@@ -895,7 +928,7 @@ mod tests {
         // Even with negative surplus, pop can't drop below 1
         let ty: Vec<TileYield> = vec![];
         // Surplus = 3 - 2 = 1 (positive), so growth not starvation at pop 1
-        assert_eq!(city.process_growth(&ty), 0);
+        assert_eq!(city.process_growth(&ty, 1.0), 0);
         assert_eq!(city.population, 1);
     }
 
diff --git a/src/simulator/crates/mc-mapgen/src/lib.rs b/src/simulator/crates/mc-mapgen/src/lib.rs
index 31f38978..edac1dea 100644
--- a/src/simulator/crates/mc-mapgen/src/lib.rs
+++ b/src/simulator/crates/mc-mapgen/src/lib.rs
@@ -149,6 +149,13 @@ impl MapGenerator {
         let mut moisture_map: HashMap<(i32, i32), f32> = HashMap::with_capacity((w * h) as usize);
         compute_moisture(&mut gen_tiles, &mut moisture_map, &mut rng);
 
+        // Stage 7.5: Rivers — flow downhill from high-moisture / high-elevation
+        // sources toward the sea. Marks `river_edges` symmetrically on both adjacent
+        // tiles. Runs after moisture (so we know which tiles are wet) and after
+        // tectonic relief (so elevation is final), but before terrain patches so
+        // riverside biome flavour can hook in later.
+        generate_rivers(&mut gen_tiles, &mut rng);
+
         // Stage 8: Terrain patches
         assign_terrain_patches(&mut gen_tiles, &mut rng);
 
@@ -535,6 +542,144 @@ fn compute_moisture(
     }
 }
 
+/// Source-elevation threshold (post-`normalize_elevation`, range `[0, 1]`).
+/// 0.55 is "upper-middle land" — above sea level by a comfortable margin
+/// but well below mountain peaks. Tiles below this never become river sources.
+const RIVER_SOURCE_ELEVATION_MIN: f32 = 0.55;
+
+/// Source-moisture threshold (post-`compute_moisture`, range `[0, 1]`).
+/// 0.50 is the median; combined with the elevation threshold this picks
+/// uplands with above-average moisture as headwaters.
+const RIVER_SOURCE_MOISTURE_MIN: f32 = 0.50;
+
+/// Maximum tiles a single river will traverse before terminating.
+/// Caps runaway flow on flat terrain. A 40×24 duel map has at most 24 tile
+/// hops along its long axis; 30 is generous enough to reach the sea from
+/// any source while still bounding cost.
+const RIVER_MAX_LENGTH: usize = 30;
+
+/// Fraction of eligible (high-elevation, high-moisture, non-water) tiles
+/// that become river sources. ~5% gives ~3–8 rivers on a duel map and
+/// scales linearly with map area.
+const RIVER_SOURCE_FRACTION: f32 = 0.05;
+
+/// Reverse a hex direction index. Direction `dir ∈ [0..6)` corresponds to
+/// `AXIAL_DIRECTIONS[dir]`; the reverse is offset by 3 (E↔W, NE↔SW, NW↔SE).
+#[inline]
+fn reverse_dir(dir: usize) -> i32 {
+    ((dir + 3) % 6) as i32
+}
+
+/// Generate rivers by flowing downhill from high-moisture / high-elevation
+/// sources to the sea. Marks `river_edges` on **both** adjacent tiles using
+/// reciprocal direction indices so river presence is symmetric from either
+/// side of the boundary.
+///
+/// Determinism: PRNG is consumed in a stable order (sorted candidate pool,
+/// `rng.randi_range` indexing). Repeated calls with the same seed produce
+/// identical river edges.
+fn generate_rivers(gen_tiles: &mut HashMap<(i32, i32), GenTile>, rng: &mut Pcg32) {
+    // Gather eligible source candidates — sorted for deterministic RNG consumption.
+    let mut candidates: Vec<(i32, i32)> = gen_tiles
+        .values()
+        .filter(|gt| {
+            gt.biome_id != "ocean"
+                && gt.biome_id != "coast"
+                && gt.elevation >= RIVER_SOURCE_ELEVATION_MIN
+                && gt.moisture >= RIVER_SOURCE_MOISTURE_MIN
+        })
+        .map(|gt| (gt.q, gt.r))
+        .collect();
+    candidates.sort_unstable();
+
+    if candidates.is_empty() {
+        return;
+    }
+
+    let target = ((candidates.len() as f32 * RIVER_SOURCE_FRACTION).round() as usize).max(1);
+
+    // Fisher-Yates style draw without replacement — preserves determinism while
+    // ensuring no source is picked twice.
+    let mut sources: Vec<(i32, i32)> = Vec::with_capacity(target);
+    while sources.len() < target && !candidates.is_empty() {
+        let idx = rng.randi_range(0, candidates.len() as i32 - 1) as usize;
+        sources.push(candidates.remove(idx));
+    }
+
+    // Trace each river from source toward the sea.
+    for (start_q, start_r) in sources {
+        let mut q = start_q;
+        let mut r = start_r;
+        let mut visited: HashSet<(i32, i32)> = HashSet::new();
+        visited.insert((q, r));
+
+        for _step in 0..RIVER_MAX_LENGTH {
+            let cur_elev = match gen_tiles.get(&(q, r)) {
+                Some(t) => t.elevation,
+                None => break,
+            };
+
+            // Pick the unvisited neighbour with the lowest elevation strictly
+            // below the current tile. Ties broken by lowest direction index
+            // (deterministic under sorted iteration).
+            let mut best_dir: Option<usize> = None;
+            let mut best_elev = cur_elev;
+
+            for (dir, &(dq, dr)) in hex::AXIAL_DIRECTIONS.iter().enumerate() {
+                let nq = q + dq;
+                let nr = r + dr;
+                if visited.contains(&(nq, nr)) {
+                    continue;
+                }
+                if let Some(nt) = gen_tiles.get(&(nq, nr)) {
+                    if nt.elevation < best_elev {
+                        best_elev = nt.elevation;
+                        best_dir = Some(dir);
+                    }
+                }
+            }
+
+            let dir = match best_dir {
+                Some(d) => d,
+                None => break, // Local minimum — no downhill option.
+            };
+
+            let (dq, dr) = hex::AXIAL_DIRECTIONS[dir];
+            let nq = q + dq;
+            let nr = r + dr;
+
+            // Mark the edge symmetrically: this tile records the outgoing
+            // direction; the neighbour records the reverse.
+            let dir_i32 = dir as i32;
+            let rev = reverse_dir(dir);
+            if let Some(gt) = gen_tiles.get_mut(&(q, r)) {
+                if !gt.river_edges.contains(&dir_i32) {
+                    gt.river_edges.push(dir_i32);
+                }
+            }
+            if let Some(nt) = gen_tiles.get_mut(&(nq, nr)) {
+                if !nt.river_edges.contains(&rev) {
+                    nt.river_edges.push(rev);
+                }
+            }
+
+            // Stop once the river hits open water — the river has reached the
+            // sea (or a coastal lake-mouth equivalent) and disperses there.
+            let neighbour_biome = gen_tiles
+                .get(&(nq, nr))
+                .map(|t| t.biome_id.clone())
+                .unwrap_or_default();
+            if neighbour_biome == "ocean" || neighbour_biome == "coast" {
+                break;
+            }
+
+            visited.insert((nq, nr));
+            q = nq;
+            r = nr;
+        }
+    }
+}
+
 fn assign_terrain_patches(
     gen_tiles: &mut HashMap<(i32, i32), GenTile>,
     rng: &mut Pcg32,
@@ -664,6 +809,7 @@ fn to_grid_state(gen_tiles: &HashMap<(i32, i32), GenTile>, w: i32, h: i32) -> Gr
                 tile.quality = gt.quality;
                 tile.quality_progress = (col * 7 + row * 13) % 11 - 5; // stagger
                 tile.is_coastal = gt.is_coastal;
+                tile.river_edges = gt.river_edges.clone();
             }
         }
     }
@@ -853,6 +999,73 @@ mod tests {
         assert!(land > 0, "Map should have land tiles");
     }
 
+    #[test]
+    fn river_gen_produces_at_least_one_river_on_duel() {
+        // A 40×24 duel map has enough land + relief that we expect rivers to
+        // appear on most seeds. If this test ever flakes, raise the seed
+        // diversity rather than weakening the assertion.
+        let gen = MapGenerator::new("{}");
+        let grid = gen.generate(42, "duel");
+        let total: usize = grid.tiles.iter().map(|t| t.river_edges.len()).sum();
+        assert!(
+            total > 0,
+            "expected at least one river edge on a 40×24 duel map, got 0"
+        );
+    }
+
+    #[test]
+    fn river_gen_is_deterministic_across_runs() {
+        let gen = MapGenerator::new("{}");
+        let g1 = gen.generate(123, "duel");
+        let g2 = gen.generate(123, "duel");
+        let edges1: Vec<&Vec<i32>> = g1.tiles.iter().map(|t| &t.river_edges).collect();
+        let edges2: Vec<&Vec<i32>> = g2.tiles.iter().map(|t| &t.river_edges).collect();
+        assert_eq!(
+            edges1, edges2,
+            "river_edges differ between runs of the same seed — non-determinism in generate_rivers"
+        );
+    }
+
+    #[test]
+    fn river_edges_are_symmetric_between_neighbours() {
+        // Every river edge marked on tile A in direction `dir` must have the
+        // reciprocal direction marked on the neighbour tile B. Asymmetry would
+        // break combat and pathfinding consultation that addresses an edge
+        // from either side.
+        let gen = MapGenerator::new("{}");
+        let grid = gen.generate(7, "duel");
+        let w = grid.width;
+        let h = grid.height;
+
+        for row in 0..h {
+            for col in 0..w {
+                let idx = grid.idx(col, row);
+                let tile = &grid.tiles[idx];
+                let (q, r) = mc_core::algorithms::hex::offset_to_axial(col, row);
+
+                for &dir in &tile.river_edges {
+                    let (dq, dr) = mc_core::algorithms::hex::AXIAL_DIRECTIONS[dir as usize];
+                    let nq = q + dq;
+                    let nr = r + dr;
+                    let (ncol, nrow) = mc_core::algorithms::hex::axial_to_offset(nq, nr);
+
+                    if ncol < 0 || ncol >= w || nrow < 0 || nrow >= h {
+                        continue; // edge to off-map tile is allowed (river ends at boundary)
+                    }
+
+                    let nidx = grid.idx(ncol, nrow);
+                    let ntile = &grid.tiles[nidx];
+                    let rev = ((dir as usize + 3) % 6) as i32;
+                    assert!(
+                        ntile.river_edges.contains(&rev),
+                        "asymmetric river edge: tile ({col},{row}) has dir {dir} but neighbour \
+                         ({ncol},{nrow}) does not have reverse dir {rev}"
+                    );
+                }
+            }
+        }
+    }
+
     #[test]
     fn test_map_gen_standard_speed() {
         // Standard map must complete in reasonable time (performance regression guard).