Index: water_map.h
===================================================================
--- water_map.h	(revision 27210)
+++ water_map.h	(working copy)
@@ -16,25 +16,26 @@
 #include "tile_map.h"
 
 /**
- * Bit field layout of m5 for water tiles.
+ * Bit field layout for water tiles.
  */
 enum WaterTileTypeBitLayout {
-	WBL_TYPE_BEGIN        = 4,   ///< Start of the 'type' bitfield.
-	WBL_TYPE_COUNT        = 4,   ///< Length of the 'type' bitfield.
+	WBL_TYPE_BEGIN        = 4,   ///< First bit for the structure kind, in m5 bitfield.
+	WBL_TYPE_COUNT        = 4,   ///< Number of bits, for the structure kind, in m5 bitfield.
 
-	WBL_TYPE_NORMAL       = 0x0, ///< Clear water or coast ('type' bitfield).
-	WBL_TYPE_LOCK         = 0x1, ///< Lock ('type' bitfield).
-	WBL_TYPE_DEPOT        = 0x8, ///< Depot ('type' bitfield).
+	WBL_TYPE_NORMAL       = 0x0, ///< Clear water or coast, in m5 (structure kind bitfield).
+	WBL_TYPE_LOCK         = 0x1, ///< Lock, in m5 (structure kind bitfield).
+	WBL_TYPE_DEPOT        = 0x8, ///< Depot, in m5 (structure kind bitfield).
 
-	WBL_COAST_FLAG        = 0,   ///< Flag for coast.
+	WBL_COAST_FLAG        = 0,   ///< Flag for coast, in m5.
+	WBL_RIVER_FLAG        = 0,   ///< Flag for canal on river, in m6.
 
-	WBL_LOCK_ORIENT_BEGIN = 0,   ///< Start of lock orientiation bitfield.
-	WBL_LOCK_ORIENT_COUNT = 2,   ///< Length of lock orientiation bitfield.
-	WBL_LOCK_PART_BEGIN   = 2,   ///< Start of lock part bitfield.
-	WBL_LOCK_PART_COUNT   = 2,   ///< Length of lock part bitfield.
+	WBL_LOCK_ORIENT_BEGIN = 0,   ///< Start of lock orientiation, in m5 bitfield.
+	WBL_LOCK_ORIENT_COUNT = 2,   ///< Length of lock orientiation, in m5 bitfield.
+	WBL_LOCK_PART_BEGIN   = 2,   ///< Start of lock part, in m5 bitfield.
+	WBL_LOCK_PART_COUNT   = 2,   ///< Length of lock part, in m5 bitfield.
 
-	WBL_DEPOT_PART        = 0,   ///< Depot part flag.
-	WBL_DEPOT_AXIS        = 1,   ///< Depot axis flag.
+	WBL_DEPOT_PART        = 0,   ///< Depot part flag, in m5.
+	WBL_DEPOT_AXIS        = 1,   ///< Depot axis flag, in m5.
 };
 
 /** Available water tile types. */
@@ -55,6 +56,12 @@
 /** Helper information for extract tool. */
 template <> struct EnumPropsT<WaterClass> : MakeEnumPropsT<WaterClass, byte, WATER_CLASS_SEA, WATER_CLASS_INVALID, WATER_CLASS_INVALID, 2> {};
 
+/** Bases of a canal (for #WATER_CLASS_CANAL class of water). */
+enum CanalBase {
+	WCC_WITHOUT_RIVER = 0, ///< Without a river under it
+	WCC_WITH_RIVER    = 1, ///< With a river under it
+};
+
 /** Sections of the water depot. */
 enum DepotPart {
 	DEPOT_PART_NORTH = 0, ///< Northern part of a depot.
@@ -122,6 +129,40 @@
 }
 
 /**
+ * Get the base of the canal at a tile.
+ * @param t Water tile to query.
+ * @pre GetWaterClass(t) == WATER_CLASS_CANAL
+ * @return Base of the canal at the tile.
+ */
+static inline CanalBase GetCanalBase(TileIndex t)
+{
+	assert(GetWaterClass(t) == WATER_CLASS_CANAL);
+	return (CanalBase)HasBit(_me[t].m6, WBL_RIVER_FLAG) ? WCC_WITH_RIVER : WCC_WITHOUT_RIVER;
+}
+
+/**
+ * Set the base of the canal to indicate there is a river under a tile.
+ * @param t Water tile of water class canal to query.
+ * @pre GetWaterClass(t) == WATER_CLASS_CANAL
+ */
+static inline void SetCanalOnRiver(TileIndex t)
+{
+	assert(GetWaterClass(t) == WATER_CLASS_CANAL);
+	SB(_me[t].m6, 0, WBL_RIVER_FLAG, WCC_WITH_RIVER);
+}
+
+/**
+ * Checks whether the canal at a tile has a river under it.
+ * @param t Water tile of water class canal to query.
+ * @pre GetWaterClass(t) == WATER_CLASS_CANAL
+ * @return true if the canal has a river under it.
+ */
+static inline bool HasTileCanalOnRiver(TileIndex t)
+{
+	return HasTileWaterClass(t) && GetWaterClass(t) == WATER_CLASS_CANAL && GetCanalBase(t) == WCC_WITH_RIVER;
+}
+
+/**
  * Tests if the tile was built on water.
  * @param t the tile to check
  * @pre IsTileType(t, MP_WATER) || IsTileType(t, MP_STATION) || IsTileType(t, MP_INDUSTRY) || IsTileType(t, MP_OBJECT)