diff --git a/core/src/com/unciv/logic/map/mapgenerator/MapGenerator.kt b/core/src/com/unciv/logic/map/mapgenerator/MapGenerator.kt
index 4335451c4a..0eb3ffa7ce 100644
--- a/core/src/com/unciv/logic/map/mapgenerator/MapGenerator.kt
+++ b/core/src/com/unciv/logic/map/mapgenerator/MapGenerator.kt
@@ -59,7 +59,7 @@ class MapGenerator(val ruleset: Ruleset) {
     private fun spawnLakesAndCoasts(map: TileMap) {
 
         //define lakes
-        var waterTiles = map.values.filter { it.isWater }
+        val waterTiles = map.values.filter { it.isWater }.toMutableList()
 
         val tilesInArea = ArrayList<TileInfo>()
         val tilesToCheck = ArrayList<TileInfo>()
diff --git a/core/src/com/unciv/logic/map/mapgenerator/RiverGenerator.kt b/core/src/com/unciv/logic/map/mapgenerator/RiverGenerator.kt
index 05e43143ae..655daa17b6 100644
--- a/core/src/com/unciv/logic/map/mapgenerator/RiverGenerator.kt
+++ b/core/src/com/unciv/logic/map/mapgenerator/RiverGenerator.kt
@@ -10,11 +10,11 @@ class RiverGenerator(val randomness: MapGenerationRandomness){
         val numberOfRivers = map.values.count { it.isLand } / 100
 
         var optionalTiles = map.values
-                .filter { it.baseTerrain== Constants.mountain && it.aerialDistanceTo(getClosestWaterTile(it)) > 4 }
+                .filter { it.baseTerrain== Constants.mountain && it.aerialDistanceTo(getClosestWaterTile(it)) > 4 }.toMutableList()
         if(optionalTiles.size < numberOfRivers)
-            optionalTiles += map.values.filter { it.baseTerrain== Constants.hill && it.aerialDistanceTo(getClosestWaterTile(it)) > 4 }
+            optionalTiles.addAll(map.values.filter { it.baseTerrain== Constants.hill && it.aerialDistanceTo(getClosestWaterTile(it)) > 4 })
         if(optionalTiles.size < numberOfRivers)
-            optionalTiles = map.values.filter { it.isLand && it.aerialDistanceTo(getClosestWaterTile(it)) > 4 }
+            optionalTiles = map.values.filter { it.isLand && it.aerialDistanceTo(getClosestWaterTile(it)) > 4 }.toMutableList()
 
 
         val riverStarts = randomness.chooseSpreadOutLocations(numberOfRivers, optionalTiles, 10)