Rivers at the bottom edge of maps are considered by tiles for river-adjacency

core/src/com/unciv/logic/map/mapunit/UnitMovement.kt

@@ -63,6 +63,8 @@ class UnitMovement(val unit: MapUnit) {
         // when entering territory of a city state
         val areConnectedByRoad = from.hasConnection(civInfo) && to.hasConnection(civInfo)
 
+        // You might think "wait doesn't isAdjacentToRiver() call isConnectedByRiver() anyway, why have those checks?"
+        // The answer is that the isAdjacentToRiver values are CACHED per tile, but the isConnectedByRiver are not - this is an efficiency optimization
         val areConnectedByRiver =
             from.isAdjacentToRiver() && to.isAdjacentToRiver() && from.isConnectedByRiver(to)
 

core/src/com/unciv/logic/map/tile/Tile.kt

@@ -673,7 +673,11 @@ open class Tile : IsPartOfGameInfoSerialization {
     }
 
     @delegate:Transient
-    private val isAdjacentToRiverLazy by lazy { neighbors.any { isConnectedByRiver(it) } }
+    private val isAdjacentToRiverLazy by lazy {
+        // These are so if you add a river at the bottom of the map (no neighboring tile to be connected to)
+        //   that tile is still considered adjacent to river
+        hasBottomLeftRiver || hasBottomRiver || hasBottomRightRiver
+        || neighbors.any { isConnectedByRiver(it) } }
     fun isAdjacentToRiver() = isAdjacentToRiverLazy
 
     /**