diff --git a/src/water_cmd.cpp b/src/water_cmd.cpp
--- a/src/water_cmd.cpp
+++ b/src/water_cmd.cpp
@@ -1165,6 +1165,9 @@
 				/* do not try to flood water tiles - increases performance a lot */
 				if (IsTileType(dest, MP_WATER)) continue;
 
+				/* TREE_GROUND_SHORE is the sign of a previous flood. */
+				if (IsTileType(dest, MP_TREES) && GetTreeGround(dest) == TREE_GROUND_SHORE) continue;
+
 				int z_dest;
 				Slope slope_dest = GetFoundationSlope(dest, &z_dest) & ~SLOPE_HALFTILE_MASK & ~SLOPE_STEEP;
 				if (z_dest > 0) continue;