diff --git a/mapgen.lua b/mapgen.lua
index 18380af..84669b7 100644
--- a/mapgen.lua
+++ b/mapgen.lua
@@ -15,7 +15,7 @@ local elevation_chill = tonumber(settings:get('elevation_chill'))
 local use_distort = settings:get_bool('distort')
 local use_biomes = settings:get_bool('biomes')
 local use_biomegen_mod = use_biomes and minetest.global_exists('biomegen')
-use_biomes = use_biomes and minetest.global_exists('default') and not use_biomegen_mod
+use_biomes = use_biomes and minetest.get_modpath("default") and not use_biomegen_mod
 
 local noiseparams = {
 	distort_x = settings:get_np_group('np_distort_x'),
@@ -77,7 +77,7 @@ local function init_mapgen(chulens)
 	end
 end
 
-local function generate(minp, maxp, seed)
+local function generate(vm, minp, maxp, seed)
 	minetest.log("info", ("[mapgen_rivers] Generating from %s to %s"):format(minetest.pos_to_string(minp), minetest.pos_to_string(maxp)))
 
 	local chulens = {
@@ -175,7 +175,7 @@ local function generate(minp, maxp, seed)
 		c_ice = minetest.get_content_id("default:ice")
 	end
 
-	local vm, emin, emax = minetest.get_mapgen_object("voxelmanip")
+	local emin, emax = vm:get_emerged_area()
 	vm:get_data(data)
 
 	local a = VoxelArea:new({MinEdge = emin, MaxEdge = emax})
@@ -276,7 +276,9 @@ local function generate(minp, maxp, seed)
 	vm:set_lighting({day = 0, night = 0})
 	vm:calc_lighting()
 	vm:update_liquids()
-	vm:write_to_map()
+	if mapgen_rivers.thread == "main" then
+		vm:write_to_map()
+	end
 
 	local t = os.clock()-t0
 	ngen = ngen + 1
@@ -285,7 +287,15 @@ local function generate(minp, maxp, seed)
 	minetest.log("verbose", ("[mapgen_rivers] Done in %5.3f s"):format(t))
 end
 
-minetest.register_on_generated(generate)
+if mapgen_rivers.thread == "main" then
+	minetest.register_on_generated(function(minp, maxp, seed)
+		local vm = minetest.get_mapgen_object("voxelmanip")
+		generate(vm, minp, maxp, seed)
+	end)
+elseif mapgen_rivers.thread == "mapgen" then
+	minetest.register_on_generated(generate)
+end
+
 minetest.register_on_shutdown(function()
 	if ngen == 0 then
 		return