--- a/src/clear_cmd.cpp	Tue Jun 19 19:49:03 2007 +0000
+++ b/src/clear_cmd.cpp	Tue Jun 19 21:15:14 2007 +0000
@@ -22,6 +22,7 @@
 #include "unmovable_map.h"
 #include "genworld.h"
 #include "industry.h"
+#include "water_map.h"
 
 struct TerraformerHeightMod {
 	TileIndex tile;
@@ -141,6 +142,13 @@
 		}
 	}
 
+	/* Canals can't be terraformed */
+	if (IsClearWaterTile(tile) && IsCanal(tile)) {
+		_terraform_err_tile = tile;
+		_error_message = STR_MUST_DEMOLISH_CANAL_FIRST;
+		return -1;
+	}
+
 	ret = DoCommand(tile, 0, 0, ts->flags & ~DC_EXEC, CMD_LANDSCAPE_CLEAR);
 
 	if (CmdFailed(ret)) {