makes it work

code/controllers/subsystem/mapping.dm

@@ -161,9 +161,11 @@ SUBSYSTEM_DEF(mapping)
 
 #endif
 	// Run map generation after ruin generation to prevent issues
-	run_map_generation()
+	run_map_terrain_generation()
 	// Generate our rivers, we do this here so the map doesn't load on top of them
 	setup_rivers()
+	// now that the terrain is generated, including rivers, we can safely populate it with objects and mobs
+	run_map_terrain_population()
 	// Add the first transit level
 	var/datum/space_level/base_transit = add_reservation_zlevel()
 	require_area_resort()
@@ -589,9 +591,15 @@ GLOBAL_LIST_EMPTY(the_station_areas)
 	if(!GLOB.the_station_areas.len)
 		log_world("ERROR: Station areas list failed to generate!")
 
-/datum/controller/subsystem/mapping/proc/run_map_generation()
+/// Generate the turfs of the area
+/datum/controller/subsystem/mapping/proc/run_map_terrain_generation()
 	for(var/area/A as anything in GLOB.areas)
-		A.RunGeneration()
+		A.RunTerrainGeneration()
+
+/// Populate the turfs of the area
+/datum/controller/subsystem/mapping/proc/run_map_terrain_population()
+	for(var/area/A as anything in GLOB.areas)
+		A.RunTerrainPopulation()
 
 /datum/controller/subsystem/mapping/proc/maprotate()
 	if(map_voted || SSmapping.next_map_config) //If voted or set by other means.

code/datums/mapgen/CaveGenerator.dm

@@ -164,8 +164,8 @@
 		var/datum/biome/selected_biome
 
 		// Here comes the meat of the biome code.
-		var/drift_x = clamp(((gen_turf.x + rand(-BIOME_RANDOM_SQUARE_DRIFT, BIOME_RANDOM_SQUARE_DRIFT)) / perlin_zoom), 1, world.maxx)
-		var/drift_y = clamp(((gen_turf.y + rand(-BIOME_RANDOM_SQUARE_DRIFT, BIOME_RANDOM_SQUARE_DRIFT)) / perlin_zoom), 2, world.maxy)
+		var/drift_x = clamp(((gen_turf.x + rand(-BIOME_RANDOM_SQUARE_DRIFT, BIOME_RANDOM_SQUARE_DRIFT))), 1, world.maxx)
+		var/drift_y = clamp(((gen_turf.y + rand(-BIOME_RANDOM_SQUARE_DRIFT, BIOME_RANDOM_SQUARE_DRIFT))), 2, world.maxy)
 
 		// Where we go in the generated string (generated outside of the loop for s p e e d)
 		var/coordinate = world.maxx * (drift_y - 1) + drift_x

code/game/area/areas.dm

@@ -207,14 +207,23 @@ GLOBAL_LIST_EMPTY(teleportlocs)
 	power_change() // all machines set to current power level, also updates icon
 	update_beauty()
 
-/area/proc/RunGeneration()
+/// Generate turfs, including cool cave wall gen
+/area/proc/RunTerrainGeneration()
 	if(map_generator)
 		map_generator = new map_generator()
 		var/list/turfs = list()
 		for(var/turf/T in contents)
 			turfs += T
 		map_generator.generate_terrain(turfs, src)
 
+/// Populate the previously generated terrain with mobs and objects
+/area/proc/RunTerrainPopulation()
+	if(map_generator)
+		var/list/turfs = list()
+		for(var/turf/T in contents)
+			turfs += T
+		map_generator.populate_terrain(turfs, src)
+
 /area/proc/test_gen()
 	if(map_generator)
 		var/list/turfs = list()

monkestation/code/modules/map_gen_expansions/forest/generator.dm

@@ -1,5 +1,6 @@
 
 /datum/map_generator/cave_generator/forest
+	buildmode_name = "Forest Generator"
 	weighted_open_turf_types = list(/turf/open/misc/asteroid/forest = 1)
 	weighted_closed_turf_types = list(/turf/closed/mineral/random/forest = 1)
 	flora_spawn_chance = 35

monkestation/code/modules/map_gen_expansions/mushroom/generator.dm

@@ -1,6 +1,7 @@
 
 /datum/map_generator/cave_generator/forest/mushroom
 	name = "Mushroom Cave Biome Generator"
+	buildmode_name = "Mushroom Cave Biome Generator"
 	weighted_open_turf_types = list(/turf/open/misc/dirt/forest = 3, /turf/open/misc/asteroid/forest/mushroom = 2)
 	weighted_closed_turf_types = list(/turf/closed/mineral/random/forest = 1)
 	initial_closed_chance = 53