diff --git a/changes/render3d-reload.md b/changes/render3d-reload.md
new file mode 100644
index 00000000..faf58ed2
--- /dev/null
+++ b/changes/render3d-reload.md
@@ -0,0 +1,10 @@
+bump: minor
+type: feat
+**ludic.render3d: replace the world at run time** — a game can swap its terrain, scatter, colliders, actors and water for another map's without restarting.
+
+- `terrain_reload(dem, emin, emax, base, ox, oz, ortho, half)` releases the current map's height field, survey, photograph and patch bounds and generates another at any `TERRAIN_HALF`; `terrain_unload()` is the release on its own.
+- `scatter_clear_all()` (with `stream_clear_all()`), `actor_clear_all()` and `col_reset()` empty the scene for the next map; `mesh_free()` releases a mesh's GL objects.
+- **Water bodies** — `water_body_add(level, cx, cz, ex, ez, reflect)` and `water_bodies_clear()` draw several still-water planes at their own levels; the first that reflects gets the planar reflection. `water_init` still means one reflecting plane.
+- **Fix:** the terrain's patch quadtree placed patches at a fixed 32 m leaf, so any `TERRAIN_HALF` other than 4096 put patch bounds in the wrong place; leaves now scale with the map.
+- **Fix:** `water_init` built a new mesh and compiled a new program on every call.
+- `terrain_sea(level)` separates the sea's level (the coast, the strand, the shoreline, forest and scree shading) from the carved lake's, so a lake can sit above the sea. Unset, the sea is the lake's level as before.
diff --git a/examples/rendering/reload.ludic b/examples/rendering/reload.ludic
new file mode 100644
index 00000000..16130127
--- /dev/null
+++ b/examples/rendering/reload.ludic
@@ -0,0 +1,82 @@
+# reload.ludic — the world replaced at run time.
+#
+# Generate a 2 km map, scatter onto it, give it colliders and water; release all of it and
+# generate a 3 km map in its place with two water planes; then go back. Prints RELOAD OK when
+# nothing of the previous map survived and the new one is whole.
+#
+#   bin/ludic build examples/rendering/reload.ludic --headless && ./build/reload_headless
+program Reload {
+  import "ludic.render3d/r3d.ludic"
+
+  property Marker { on: int = 1 }
+  model Anchor { Marker }
+
+  # A program only links the GL runtime render3d stands on when it names Gl.* itself
+  # (the parser's g_uses_gl), so even a check that draws nothing presents its frame.
+  # render3d calls back into the program for what stands on the terrain
+  function scene_draw() -> void { scatter_draw() }
+  function scene_draw_casters() -> void { scatter_draw_casters(shadow_cascade_vp(sh_cascade)) }
+  # no streamed cover here, but stream.ludic names the generator, so every program supplies one
+  function stream_fill(s: Stream, cx: int, cz: int, band: int) -> void { }
+
+  handler Present phase Render {
+    Gl.swap()
+  }
+
+  function check(what: string, cond: bool) -> bool {
+    if not cond { print(`reload: FAILED - {what}`) }
+    return cond
+  }
+  function populate() -> void {
+    let l = layer_cards(model_cross_card(), 1000, F_ZERO, fi(400))
+    for i in 0 .. 400 {
+      let x = fi(i * 2 - 400)
+      layer_add(l, x, terrain_height(x, fi(30)), fi(30), F_ONE, F_ZERO, F_ZERO, F_ZERO)
+      col_add(x, fi(30), F_ONE)
+    }
+    col_build()
+  }
+
+  handler Boot phase Start {
+    spawn Anchor {}
+    TERRAIN_HALF = 1000
+    ter_smooth = true
+    if not r3d_init(640, 360, "Reload") { quit() }
+    cam_set(F_ZERO, fi(80), fi(300), fi(180), f_neg(fi(10)))
+    water_init(fl(4.0), F_ZERO, F_ZERO, fi(200), fi(200))
+    populate()
+    var ok = true
+    ok = check("the first map has layers and colliders", len(sc_layers) == 1 and col_n == 400) and ok
+    # --- swap to a 3 km map with a sea and a lake above it
+    scatter_clear_all()
+    actor_clear_all()
+    col_reset()
+    water_bodies_clear()
+    terrain_lake(F_ZERO, F_ZERO, F_ZERO, F_ZERO, F_ZERO)
+    terrain_reload("", F_ZERO, F_ZERO, F_ZERO, F_ZERO, F_ZERO, "", 1500)
+    ok = check("everything of the first map released", len(sc_layers) == 0 and col_n == 0 and wb_n == 0 and not water_on) and ok
+    ok = check("the new half", TERRAIN_HALF == 1500 and ter_heights != null and ter_height_tex != 0) and ok
+    ok = check("patch bounds rebuilt", cd_min != null and len(cd_min) == CD_LEVELS) and ok
+    ok = check("patches scale with the map", cd_size(0) == f_mul(fi(32), fr(3000, 8192))) and ok
+    terrain_height(F_ZERO, F_ZERO)
+    ok = check("the cached height scale is the new map's", ter_h_scale == fr(TERRAIN_RES, 3000)) and ok
+    let sea = water_body_add(F_ZERO, F_ZERO, F_ZERO, fi(20000), fi(20000), true)
+    let lake = water_body_add(fl(12.0), fi(600), fi(-400), fi(150), fi(100), false)
+    ok = check("two water bodies, the sea mirrored", sea == 0 and lake == 1 and wb_n == 2 and wb_primary == 0 and water_level == F_ZERO and water_on) and ok
+    populate()
+    ok = check("the new map takes layers and colliders", len(sc_layers) == 1 and col_n == 400 and col_side == 3000 / COL_CELL) and ok
+    # a sea below the carved lake: set once, and not undone by a reload
+    terrain_sea(fl(-2.0))
+    terrain_lake(fl(12.0), fi(600), fi(-400), fi(150), fi(100))
+    ok = check("the sea is its own level", ter_sea_set and ter_sea_gen() == fl(-2.0) and ter_lake_level == fl(12.0)) and ok
+    # --- and back: the swap is repeatable
+    scatter_clear_all()
+    col_reset()
+    water_init(fl(4.0), F_ZERO, F_ZERO, fi(200), fi(200))
+    terrain_reload("", F_ZERO, F_ZERO, F_ZERO, F_ZERO, F_ZERO, "", 1000)
+    populate()
+    ok = check("back to 2 km", TERRAIN_HALF == 1000 and len(cd_min) == CD_LEVELS and wb_n == 1 and col_side == 2000 / COL_CELL) and ok
+    if ok { print("RELOAD OK") } else { print("RELOAD FAILED") }
+    quit()
+  }
+}
diff --git a/packages/ludic.render3d/actor.ludic b/packages/ludic.render3d/actor.ludic
index 55300c94..fe966945 100644
--- a/packages/ludic.render3d/actor.ludic
+++ b/packages/ludic.render3d/actor.ludic
@@ -307,3 +307,10 @@ function actor_draw_casters(light_vp: words) -> void {
     actor_draw_one(a, ap, true)
   }
 }
+
+# every actor off the stage at once, for a world being replaced. Actors own no GL objects;
+# their models belong to whoever loaded them.
+function actor_clear_all() -> void {
+  ac_actors = new []Actor
+  outline_clear()
+}
diff --git a/packages/ludic.render3d/collide.ludic b/packages/ludic.render3d/collide.ludic
index bb88ca17..d12da01b 100644
--- a/packages/ludic.render3d/collide.ludic
+++ b/packages/ludic.render3d/collide.ludic
@@ -108,3 +108,12 @@ function col_clear(x0: int, z0: int, x1: int, z1: int, r: int) -> bool {
   }
   return true
 }
+
+# No colliders, ready for another map's. The cell index is sized from TERRAIN_HALF on its
+# first build and kept, so it is released too: a larger map would overrun the old one.
+function col_reset() -> void {
+  col_n = 0
+  col_built = false
+  if col_start != null { free(col_start);  col_start = null }
+  if col_sorted != null { free(col_sorted);  col_sorted = null }
+}
diff --git a/packages/ludic.render3d/grass.ludic b/packages/ludic.render3d/grass.ludic
index 0101e4ee..81a8b6e1 100644
--- a/packages/ludic.render3d/grass.ludic
+++ b/packages/ludic.render3d/grass.ludic
@@ -146,6 +146,10 @@ function grass_draw() -> void {
   var lake = fl(-100000.0)
   if ter_lake_ex != 0 { lake = ter_lake_level }
   u_f(gl_uniform(p, "u_lake_level"), lake)
+  var sea = lake
+  if ter_sea_set { sea = ter_sea_level }
+  u_f(gl_uniform(p, "u_sea_level"), sea)
+  u_f4(gl_uniform(p, "u_lake"), ter_lake_cx, ter_lake_cz, ter_lake_ex, ter_lake_ez)
   u_f(gl_uniform(p, "u_snow_line"), ter_snow_line)
   sky_bind_lighting(p)
   shadow_bind(p)
diff --git a/packages/ludic.render3d/mesh.ludic b/packages/ludic.render3d/mesh.ludic
index 699a4b94..1160c234 100644
--- a/packages/ludic.render3d/mesh.ludic
+++ b/packages/ludic.render3d/mesh.ludic
@@ -148,3 +148,12 @@ function mesh_instance_buffer(m: Mesh, first_attr: int, floats_per: int, data: p
   gl_bind_vertex_array(0)
   return b
 }
+
+# release a mesh's GL objects (a mesh this package built for something being thrown away)
+function mesh_free(m: Mesh) -> void {
+  if m == null { return }
+  let ids = gl_scratch()
+  if m.vbo != 0 { ids[0] = m.vbo;  gl_delete_buffers(1, ids);  m.vbo = 0 }
+  if m.ebo != 0 { ids[0] = m.ebo;  gl_delete_buffers(1, ids);  m.ebo = 0 }
+  if m.vao != 0 { ids[0] = m.vao;  gl_delete_vertex_arrays(1, ids);  m.vao = 0 }
+}
diff --git a/packages/ludic.render3d/scatter.ludic b/packages/ludic.render3d/scatter.ludic
index b22428a6..59fae935 100644
--- a/packages/ludic.render3d/scatter.ludic
+++ b/packages/ludic.render3d/scatter.ludic
@@ -1048,3 +1048,40 @@ function carpet_bake(layers: []Layer, count: int, tile: int, res: int) -> int {
   gl_check("carpet bake")
   return tex
 }
+
+# ---- another map at run time ------------------------------------------------------------
+# Every scattered layer released - its instance arrays and GL buffers, a card layer's own
+# crossed-card mesh and baked atlas - and every stream feeding them. The models a layer drew
+# belong to whoever loaded them and are kept, and so are the programs. A game rebuilding its
+# world calls this, then places the new map's layers exactly as it did at boot.
+function scatter_clear_all() -> void {
+  stream_clear_all()
+  if sc_layers == null { return }
+  let ids = gl_scratch()
+  for i in 0 .. len(sc_layers) {
+    let l = sc_layers[i]
+    if l.buf != 0 { ids[0] = l.buf;  gl_delete_buffers(1, ids) }
+    if l.imp_buf != 0 { ids[0] = l.imp_buf;  gl_delete_buffers(1, ids) }
+    if l.sh_buf != 0 { ids[0] = l.sh_buf;  gl_delete_buffers(1, ids) }
+    if l.lod_buf != null { for k in 0 .. l.n_lods { ids[0] = l.lod_buf[k];  gl_delete_buffers(1, ids) } }
+    if l.inst != null { free(l.inst) }
+    if l.scratch != null { free(l.scratch) }
+    if l.tint != null { free(l.tint) }
+    if l.last_cam != null { free(l.last_cam) }
+    if l.gstart != null { free(l.gstart) }
+    if l.gsorted != null { free(l.gsorted) }
+    if l.gymin != null { free(l.gymin) }
+    if l.gymax != null { free(l.gymax) }
+    if l.vis != null { free(l.vis) }
+    if l.lod_dist != null { free(l.lod_dist);  free(l.lod_card);  free(l.lod_buf);  free(l.n_lod) }
+    if l.lvl != null { free(l.lvl) }
+    # layer_cards built this layer's crossed card and its atlas itself
+    if l.card and l.n_lods == 0 and l.model != null { for k in 0 .. len(l.model.prims) { mesh_free(l.model.prims[k].mesh) } }
+    if l.card and l.atlas != null {
+      ids[0] = l.atlas.albedo;  gl_delete_textures(1, ids)
+      ids[0] = l.atlas.normal;  gl_delete_textures(1, ids)
+    }
+  }
+  sc_layers = new []Layer
+  sc_view_gen += 1
+}
diff --git a/packages/ludic.render3d/shaders/grass.vert b/packages/ludic.render3d/shaders/grass.vert
index a9c37917..dbf8d966 100644
--- a/packages/ludic.render3d/shaders/grass.vert
+++ b/packages/ludic.render3d/shaders/grass.vert
@@ -20,6 +20,8 @@ uniform float u_ts_half;
 uniform sampler2D u_ortho;
 uniform float u_ortho_on;
 uniform float u_lake_level;
+uniform float u_sea_level;    // the sea (terrain_sea); the lake's level when there is no separate sea
+uniform vec4 u_lake;          // the carved lake: centre x/z, half extents (z = 0: none)
 uniform float u_snow_line;
 uniform float u_wind;
 uniform vec2 u_tile;         // world xz of this tile's corner
@@ -92,7 +94,11 @@ void main() {
   if (croot.w < -1.0 || abs(croot.x) > croot.w * 1.25 + 1.5 || abs(croot.y) > croot.w * 1.4 + 1.5) { cull(); return; }
   vec3 gn = normalize(ht.gba);
   float h3 = bladeHash(ci, j, 2), h4 = bladeHash(ci, j, 3);
-  float ok = (1.0 - smoothstep(0.30, 0.55, 1.0 - gn.y)) * smoothstep(0.0, 0.6, ht.r - u_lake_level - 0.15) * smoothstep(u_snow_line - 80.0, u_snow_line - 200.0, ht.r);
+  // No blades under water: the sea's line everywhere, and the lake's inside its outline. One
+  // line for both left a lake above the sea with grass on its bed or a valley with none.
+  float wl = u_sea_level;
+  if (u_lake.z > 0.0) { vec2 q = (xz - u_lake.xy) / u_lake.zw;  if (dot(q, q) < 1.0) wl = max(wl, u_lake_level); }
+  float ok = (1.0 - smoothstep(0.30, 0.55, 1.0 - gn.y)) * smoothstep(0.0, 0.6, ht.r - wl - 0.15) * smoothstep(u_snow_line - 80.0, u_snow_line - 200.0, ht.r);
   if (u_ortho_on > 0.5) {
     vec3 oc = textureLod(u_ortho, huv, 1.5).rgb;
     ok *= 0.25 + 0.75 * smoothstep(0.0, 0.02, oc.g - max(oc.r, oc.b));
diff --git a/packages/ludic.render3d/shaders/heightgen.frag b/packages/ludic.render3d/shaders/heightgen.frag
index 591abaca..4da0e16e 100644
--- a/packages/ludic.render3d/shaders/heightgen.frag
+++ b/packages/ludic.render3d/shaders/heightgen.frag
@@ -10,6 +10,7 @@ uniform float u_dem_base;       // the elevation that becomes y = 0
 uniform vec2 u_origin;          // world x/z of the map's centre
 uniform vec4 u_lake;            // a lake: centre x/z, half extents (zero = none)
 uniform float u_lake_level;     // its surface height; the model records the surface, the bed is carved below it
+uniform float u_sea_level;      // the sea the coast falls into (terrain_sea); the lake's level unless set
 uniform vec4 u_isle;            // an island: centre x/z, radius (or rim margin), falloff width
 uniform float u_isle_mode;      // 0 none, 1 radial (terrain_island), 2 inward from the survey's edge (terrain_coast)
 // The DEM is Copernicus GLO-30 — 30 m data resampled onto this 4 m grid — so the stored
@@ -84,15 +85,15 @@ void main() {
       t = smoothstep(u_isle.z, u_isle.z + u_isle.w, d);          // 0 inland, 1 past the fall
       shelf = 26.0 + 34.0 * smoothstep(0.0, 1.0, (d - u_isle.z - u_isle.w) / max(u_isle.w, 1.0));
     }
-    float above = h - u_lake_level;
-    h = u_lake_level + above * (1.0 - t) - t * shelf;
+    float above = h - u_sea_level;
+    h = u_sea_level + above * (1.0 - t) - t * shelf;
     // A strand. Scaling alone hands a 500 m mountainside a 40-degree plunge into the sea,
     // which is a cliff coast and nothing else. Real shores are cut flat by the water they
     // meet, so the last few metres of height either side of the line are compressed —
     // which stretches them out horizontally into beach and shallows. Only inside the
     // coastal band: an inland lake has its own bed and wants none of this.
-    float near = 1.0 - smoothstep(0.0, 48.0, abs(h - u_lake_level));
-    h = mix(h, u_lake_level + (h - u_lake_level) * 0.30, near * smoothstep(0.02, 0.30, t));
+    float near = 1.0 - smoothstep(0.0, 48.0, abs(h - u_sea_level));
+    h = mix(h, u_sea_level + (h - u_sea_level) * 0.30, near * smoothstep(0.02, 0.30, t));
   }
   o = vec4(h, 0.0, 0.0, 1.0);
 #elif defined(SMOOTH)
diff --git a/packages/ludic.render3d/shaders/terrain.frag b/packages/ludic.render3d/shaders/terrain.frag
index fe2447aa..d98afcca 100644
--- a/packages/ludic.render3d/shaders/terrain.frag
+++ b/packages/ludic.render3d/shaders/terrain.frag
@@ -18,6 +18,7 @@ uniform sampler2D u_snow_d;  uniform sampler2D u_carpet;   // the clump cards ba
 uniform float u_carpet_on;
 uniform float u_snow_line;
 uniform float u_lake_level;      // the ground just above the water is wet and dark
+uniform float u_sea_level;       // the sea's line: shore, forest and scree gates (terrain_sea)
 uniform vec2 u_origin;           // world offset of the terrain grid
 
 // The ground's own sun shadow comes from the baked height-field map (tershadow.frag),
@@ -236,14 +237,14 @@ vec3 groundShade(vec3 p, vec3 N, float slope, float dist, float viewDepth, bool
     vec3 ocf = textureLod(u_ortho, v_huv, 2.0).rgb;
     float gx = ocf.g - max(ocf.r, ocf.b);
     // (the photograph is sampled linear: sRGB 72 is 0.06, 92 is 0.11)
-    forestW = smoothstep(0.12, 0.06, max(ocf.r, max(ocf.g, ocf.b))) * smoothstep(0.004, 0.012, gx) * smoothstep(u_lake_level + 0.8, u_lake_level + 1.8, p.y);
+    forestW = smoothstep(0.12, 0.06, max(ocf.r, max(ocf.g, ocf.b))) * smoothstep(0.004, 0.012, gx) * smoothstep(u_sea_level + 0.8, u_sea_level + 1.8, p.y);
     // Classify from a ~60 m blur, never from the pixels: the survey's 10 m pixels carry
     // a foot trail as a broken line of bare ground, and thresholding them painted it
     // across the meadow as tan dashes (and, on the CPU, lined boulders up along it).
     vec3 ocl = textureLod(u_ortho, v_huv, 2.5).rgb;
     float mxc = max(ocl.r, max(ocl.g, ocl.b)), mnc = min(ocl.r, min(ocl.g, ocl.b));
     float greenEx = ocl.g - max(ocl.r, ocl.b);
-    screeC = smoothstep(0.008, -0.002, greenEx) * smoothstep(0.06, 0.12, mxc) * (1.0 - smoothstep(0.55, 0.75, mxc)) * smoothstep(u_lake_level + 0.2, u_lake_level + 1.2, p.y);
+    screeC = smoothstep(0.008, -0.002, greenEx) * smoothstep(0.06, 0.12, mxc) * (1.0 - smoothstep(0.55, 0.75, mxc)) * smoothstep(u_sea_level + 0.2, u_sea_level + 1.2, p.y);
     snowC = smoothstep(0.08, 0.04, mxc - mnc) * smoothstep(0.55, 0.8, mxc);
   }
   // snow lingering in the high gullies is drawn further down, but whether it can be
@@ -362,7 +363,7 @@ vec3 groundShade(vec3 p, vec3 N, float slope, float dist, float viewDepth, bool
   // the bank here: identical noise fields, identical time, identical phase, keyed off
   // height above the lake instead of depth below it. Across the seam the two agree, so
   // there is nothing there to read as an edge.
-  float above = p.y - u_lake_level;                       // >0 on land, metres
+  float above = p.y - u_sea_level;                        // >0 on land, metres
   // wet ground: darker and glossier near the water, fading out over ~1.2 m
   float wet = smoothstep(1.2, 0.0, above);
   alb *= mix(1.0, 0.5, wet * 0.85);
diff --git a/packages/ludic.render3d/stream.ludic b/packages/ludic.render3d/stream.ludic
index 6ffcd7f4..15db50a5 100644
--- a/packages/ludic.render3d/stream.ludic
+++ b/packages/ludic.render3d/stream.ludic
@@ -313,3 +313,16 @@ function stream_update_all() -> void {
   stream_deadline = gl_now_us() + STREAM_BUDGET_US
   for i in 0 .. len(stream_all) { stream_update(stream_all[i], cam_pos[0], cam_pos[2]) }
 }
+
+# every stream and its cached chunks, for a world being replaced (scatter_clear_all)
+function stream_clear_all() -> void {
+  if stream_all == null { return }
+  for i in 0 .. len(stream_all) {
+    let s = stream_all[i]
+    if s.chunks != null { for c in 0 .. len(s.chunks) { if s.chunks[c].data != null { free(s.chunks[c].data) } } }
+    if s.keys != null { free(s.keys) }
+    if s.htab != null { free(s.htab) }
+    if s.bands != null { free(s.bands) }
+  }
+  stream_all = null
+}
diff --git a/packages/ludic.render3d/terrain.ludic b/packages/ludic.render3d/terrain.ludic
index c81d1241..be1f5270 100644
--- a/packages/ludic.render3d/terrain.ludic
+++ b/packages/ludic.render3d/terrain.ludic
@@ -68,6 +68,15 @@ var ter_lake_ez: int = 0
 function terrain_lake(level: int, cx: int, cz: int, ex: int, ez: int) -> void {
   ter_lake_level = level;  ter_lake_cx = cx;  ter_lake_cz = cz;  ter_lake_ex = ex;  ter_lake_ez = ez
 }
+# The SEA's level, for the coast, the strand and the shoreline shading, when it is not the
+# carved lake's. They were one number, which holds only while the lake is at sea level: a
+# lake eighty metres up the valley drowned the whole coast toward its own surface. Unset,
+# the sea is the lake exactly as before, so a map with one water line needs no call.
+var ter_sea_level: int = 0
+var ter_sea_set: bool = false
+function terrain_sea(level: int) -> void { ter_sea_level = level;  ter_sea_set = true }
+# the level the generator scales the coast toward (it always read the lake's, set or not)
+function ter_sea_gen() -> int { if ter_sea_set { return ter_sea_level };  return ter_lake_level }
 # An island: land out to `r` from (cx, cz), then the terrain scaled down into the water
 # over `fall` metres and on down to a shelf. Scaling rather than blending to a fixed bed is
 # what makes the coastline come out of the terrain that is already there — low ground turns
@@ -229,6 +238,7 @@ function terrain_generate() -> void {
     u_f2(gl_uniform(p, "u_origin"), ter_ox, ter_oz)
     u_f4(gl_uniform(p, "u_lake"), ter_lake_cx, ter_lake_cz, ter_lake_ex, ter_lake_ez)
     u_f(gl_uniform(p, "u_lake_level"), ter_lake_level)
+    u_f(gl_uniform(p, "u_sea_level"), ter_sea_gen())
     u_f4(gl_uniform(p, "u_isle"), ter_isle_cx, ter_isle_cz, ter_isle_r, ter_isle_fall)
     u_f(gl_uniform(p, "u_isle_mode"), fi(ter_isle_mode))
     u_f(gl_uniform(p, "u_dem_blur"), ter_dem_blur)
@@ -453,6 +463,10 @@ function terrain_bind_prog(p: int) -> void {
   var lake = fl(-100000.0)
   if ter_lake_ex != 0 { lake = ter_lake_level }
   u_f(gl_uniform(p, "u_lake_level"), lake)
+  # the shoreline, forest and scree gates read the sea; unset it is what they always read
+  var sea = lake
+  if ter_sea_set { sea = ter_sea_level }
+  u_f(gl_uniform(p, "u_sea_level"), sea)
   u_mat4(gl_uniform(p, "u_view"), cam_view)
   u_mat4(gl_uniform(p, "u_proj"), cam_proj)
   u_f2(gl_uniform(p, "u_origin"), ter_ox, ter_oz)
@@ -660,6 +674,12 @@ function cdlod_bounds() -> void {
   }
 }
 
+# A patch's side in metres at a level. The quadtree is CD_LEAVES leaves a side over the whole
+# map, so a leaf is (2 * TERRAIN_HALF) / CD_LEAVES - 32 m only on the 8192 m map the numbers
+# were chosen for. Placing patches at a fixed 32 m put every bound in the wrong place on any
+# other TERRAIN_HALF.
+function cd_size(level: int) -> int { return f_mul(fi(32 << level), fr(TERRAIN_HALF * 2, 8192)) }
+
 # does the patch's box come within r of the camera?
 function cd_within(x0: int, z0: int, size: int, ymin: int, ymax: int, r: int) -> bool {
   let dx = f_max(f_max(f_sub(x0, cam_pos[0]), f_sub(cam_pos[0], f_add(x0, size))), F_ZERO)
@@ -679,7 +699,7 @@ function cd_inside(x0: int, z0: int, size: int, ymin: int, ymax: int, r: int) ->
 }
 
 function cdlod_draw(level: int, ix: int, iz: int) -> void {
-  let size = fi(32 << level)
+  let size = cd_size(level)
   let x0 = f_add(f_sub(ter_ox, fi(TERRAIN_HALF)), f_mul(fi(ix), size))
   let z0 = f_add(f_sub(ter_oz, fi(TERRAIN_HALF)), f_mul(fi(iz), size))
   # Which tier can run inside this patch. A patch that never comes within the split takes
@@ -723,7 +743,7 @@ function cdlod_draw(level: int, ix: int, iz: int) -> void {
 # its children, in which case each child either selects itself or is drawn at this level.
 function cdlod_select(level: int, ix: int, iz: int) -> bool {
   let n = CD_LEAVES >> level
-  let size = fi(32 << level)
+  let size = cd_size(level)
   let x0 = f_add(f_sub(ter_ox, fi(TERRAIN_HALF)), f_mul(fi(ix), size))
   let z0 = f_add(f_sub(ter_oz, fi(TERRAIN_HALF)), f_mul(fi(iz), size))
   let ymin = cd_min[level][iz * n + ix]
@@ -741,3 +761,42 @@ function cdlod_select(level: int, ix: int, iz: int) -> bool {
   }
   return true
 }
+
+# ---- another map at run time ------------------------------------------------------------
+# What belongs to ONE map - the generated height field and its CPU copy, the survey, the
+# photograph, the patch bounds and the scales cached from TERRAIN_HALF - is released here,
+# so a different survey can be generated in its place without restarting the process. What
+# belongs to the process stays: the patch mesh, the material textures, the programs, the
+# frame-sized targets and the shadow texture, whose size does not depend on the map.
+function terrain_unload() -> void {
+  let ids = gl_scratch()
+  if ter_height_tex != 0 { ids[0] = ter_height_tex;  gl_delete_textures(1, ids);  ter_height_tex = 0 }
+  if ter_dem_tex != 0 { ids[0] = ter_dem_tex;  gl_delete_textures(1, ids);  ter_dem_tex = 0 }
+  if ter_ortho_tex != 0 { ids[0] = ter_ortho_tex;  gl_delete_textures(1, ids);  ter_ortho_tex = 0 }
+  if ter_heights != null { free(ter_heights);  ter_heights = null }
+  if ter_ortho_px != null { free(ter_ortho_px);  ter_ortho_px = null }
+  ter_ortho_w = 0
+  if cd_min != null {
+    for i in 0 .. len(cd_min) { free(cd_min[i]);  free(cd_max[i]) }
+    cd_min = null;  cd_max = null
+  }
+  # derived from TERRAIN_HALF and cached on first use: stale ones would keep the old size
+  ter_h_scale = 0;  ter_o_scale = 0
+  ter_carpet = 0
+  ter_shadow_gen = -1;  ter_shadow_yaw = 0x7fffffff
+}
+# Generate another map in place. Call terrain_lake / terrain_coast / terrain_island for it
+# first - the generator reads them - then this. `half` is the new TERRAIN_HALF in metres.
+# An empty `dem` generates the analytic ground (with ter_smooth) and an empty `ortho` drapes
+# no photograph. The shadow is rebaked and the patch bounds rebuilt before it returns.
+function terrain_reload(dem: string, emin: int, emax: int, base: int, ox: int, oz: int, ortho: string, half: int) -> void {
+  terrain_unload()
+  TERRAIN_HALF = half
+  ter_ox = ox;  ter_oz = oz
+  if len(dem) > 0 { terrain_use_dem(dem, emin, emax, base, ox, oz) }
+  if len(ortho) > 0 { terrain_use_ortho(ortho) }
+  terrain_generate()
+  terrain_bake_shadow()
+  cdlod_bounds()
+  gl_check("terrain reload")
+}
diff --git a/packages/ludic.render3d/water.ludic b/packages/ludic.render3d/water.ludic
index 45f51a95..8f18da18 100644
--- a/packages/ludic.render3d/water.ludic
+++ b/packages/ludic.render3d/water.ludic
@@ -17,10 +17,25 @@ var water_refl: Target = null            # the world mirrored in the surface, ha
 var water_refl_div: int = 2              # R3D_REFLDIV overrides: 2 = half res, 4 = quarter
 var water_saved: words = null            # the real camera's matrices, restored after the pass
 var water_dumped: bool = false
+# Several still-water planes, each at its own level over its own bounds: the sea round an
+# island and a lake a hundred metres above it cannot be one surface. Each draws the same way;
+# only one - the first added with `reflect` - gets the planar reflection pass, because every
+# mirrored plane is another full scene pass. water_level / water_cx ... mirror that one, so
+# code written against a single plane still reads the surface that reflects.
+const WATER_MAX: int = 8
+var wb_n: int = 0
+var wb_level: words = null
+var wb_cx: words = null
+var wb_cz: words = null
+var wb_ex: words = null
+var wb_ez: words = null
+var wb_reflect: words = null
+var wb_primary: int = -1               # the body the reflection pass mirrors, or -1
 
 # Render the scene through a camera mirrored in the water plane into water_refl,
 # clipping everything below the surface; terrain, scattered layers and sky.
 function water_reflection_pass() -> void {
+  if wb_primary < 0 { return }            # no body reflects: nothing to mirror
   if water_refl == null {
     if Os.has_env("R3D_REFLDIV") { water_refl_div = Text.to_int(Os.env("R3D_REFLDIV")) }
     if water_refl != null { target_free(water_refl) }
@@ -81,31 +96,58 @@ function water_reflection_pass() -> void {
   gl_bind_framebuffer(GL_FRAMEBUFFER, 0)
 }
 
-function water_init(level: int, cx: int, cz: int, ex: int, ez: int) -> void {
+# the program and the plane are the process's, built once; the bodies are the map's
+function water_setup() -> void {
+  if water_prog != 0 { return }
   water_mesh = mesh_grid(2, F_HALF)
   water_prog = r3d_program("water.vert", "water.frag", "")
-  water_level = level;  water_cx = cx;  water_cz = cz;  water_ex = ex;  water_ez = ez
+  wb_level = words(WATER_MAX);  wb_cx = words(WATER_MAX);  wb_cz = words(WATER_MAX)
+  wb_ex = words(WATER_MAX);  wb_ez = words(WATER_MAX);  wb_reflect = words(WATER_MAX)
+}
+function water_bodies_clear() -> void {
+  wb_n = 0
+  wb_primary = -1
+  water_on = false
+}
+# Add a plane at `level` over (cx, cz) +- (ex, ez); true `reflect` makes it the mirrored one
+# if none is yet. Returns its index, or -1 once WATER_MAX are in use.
+function water_body_add(level: int, cx: int, cz: int, ex: int, ez: int, reflect: bool) -> int {
+  water_setup()
+  if wb_n >= WATER_MAX { return -1 }
+  let i = wb_n
+  wb_level[i] = level;  wb_cx[i] = cx;  wb_cz[i] = cz;  wb_ex[i] = ex;  wb_ez[i] = ez
+  wb_reflect[i] = 0
+  if reflect { wb_reflect[i] = 1 }
+  wb_n += 1
   water_on = true
+  if reflect and wb_primary < 0 {
+    wb_primary = i
+    water_level = level;  water_cx = cx;  water_cz = cz;  water_ex = ex;  water_ez = ez
+  }
+  return i
+}
+# one reflecting plane: what this function always meant, without a new mesh and program
+# every time it is called
+function water_init(level: int, cx: int, cz: int, ex: int, ez: int) -> void {
+  water_bodies_clear()
+  water_body_add(level, cx, cz, ex, ez, true)
 }
 
 # call after the opaque pass, before the sky: blends over the resolved depth
 function water_draw(depth_tex: int) -> void {
-  if not water_on { return }
+  if not water_on or wb_n == 0 { return }
   let p = water_prog
   gl_use_program(p)
   u_mat4(gl_uniform(p, "u_view"), cam_view)
   u_mat4(gl_uniform(p, "u_proj"), cam_proj)
   u_mat4(gl_uniform(p, "u_inv_vp"), cam_inv_vp)
-  u_f(gl_uniform(p, "u_level"), water_level)
-  u_f2(gl_uniform(p, "u_center"), water_cx, water_cz)
-  u_f2(gl_uniform(p, "u_extent"), water_ex, water_ez)
   # gl_FragCoord here runs over the scene target, which is post_w x post_h — not the
   # window. They are the same size only at a render scale of 1; at anything less, taking
   # the window's size sent the refraction and depth reads into the wrong corner of the
   # frame, and the lake showed a squashed copy of it instead of its own bed.
   u_f2(gl_uniform(p, "u_screen"), fi(post_w), fi(post_h))
   var ron = F_ZERO
-  if water_refl != null {
+  if water_refl != null and wb_primary >= 0 {
     # bind on its own unit first: generating the mip chain re-binds the texture on the active unit,
     # and it must not displace the depth texture the shader reads for the shore
     r3d_bind_2d(p, "u_refl", 1, water_refl.color);  ron = F_ONE
@@ -114,7 +156,6 @@ function water_draw(depth_tex: int) -> void {
   }
   r3d_bind_2d(p, "u_depth", 0, depth_tex)
   r3d_bind_2d(p, "u_scene", 2, post_scene.color)
-  u_f(gl_uniform(p, "u_refl_on"), ron)
   sky_bind_lighting(p)
   shadow_bind(p)
   fog_bind(p)
@@ -127,6 +168,16 @@ function water_draw(depth_tex: int) -> void {
   # and the bed 9 m below the shore would otherwise darken a band along the water line
   gl_depth_mask(1)
   gl_disable(GL_CULL_FACE)
-  mesh_draw(water_mesh)
+  # every body is the same plane at its own level and bounds; only the mirrored one samples
+  # the reflection - another body reading it would show the wrong world upside down
+  for i in 0 .. wb_n {
+    u_f(gl_uniform(p, "u_level"), wb_level[i])
+    u_f2(gl_uniform(p, "u_center"), wb_cx[i], wb_cz[i])
+    u_f2(gl_uniform(p, "u_extent"), wb_ex[i], wb_ez[i])
+    var r = F_ZERO
+    if i == wb_primary { r = ron }
+    u_f(gl_uniform(p, "u_refl_on"), r)
+    mesh_draw(water_mesh)
+  }
   gl_disable(GL_BLEND)
 }