diff --git a/terrainlib_lua/rivermapper.lua b/terrainlib_lua/rivermapper.lua
index 92ab11f..5479d1e 100644
--- a/terrainlib_lua/rivermapper.lua
+++ b/terrainlib_lua/rivermapper.lua
@@ -111,6 +111,10 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 			local d = dirs2[i] -- Get the directions water is coming from
 
 			-- Iterate through the 4 directions
+			-- Loop is unrolled on purpose, for performance (critical part!)
+			----------
+			-- EAST --
+			----------
 			if d >= 8 then -- River coming from the East
 				d = d - 8
 				cur = cur + 1
@@ -123,7 +127,7 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 					local l2 = basin_links[b2]
 					if not l2 then
 						l2 = {b2, ib, elev=elev, i=i+1, is_y=false}
-						basin_links[b2] = l2
+						basin_links[b2] = l2 -- Potential non-linear complexity here
 					elseif l2.elev > elev then -- If this link is lower than the lowest registered link between these two basins, register it as the new lowest pass
 						l2.elev = elev
 						l2.i = i+1
@@ -146,6 +150,9 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 				end
 			end
 
+			-----------
+			-- SOUTH --
+			-----------
 			if d >= 4 then -- River coming from the South
 				d = d - 4
 				cur = cur + 1
@@ -180,6 +187,9 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 				end
 			end
 
+			----------
+			-- WEST --
+			----------
 			if d >= 2 then -- River coming from the West
 				d = d - 2
 				cur = cur + 1
@@ -214,6 +224,9 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 				end
 			end
 
+			-----------
+			-- NORTH --
+			-----------
 			if d >= 1 then -- River coming from the North
 				cur = cur + 1
 				singular[cur] = i-X
@@ -287,7 +300,10 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 	end
 
 	local basin_graph = {}
-	while cur > 1 do
+	for i=0, nbasins do
+		basin_graph[i] = {} -- Initialize (to ensure subtables don't go in the hash part)
+	end
+	for i=1, nbasins do
 		-- Iterate in lowlevel but its contents may change during the loop
 		local b1 = lowlevel[cur]
 		cur = cur - 1
@@ -307,13 +323,7 @@ local function flow_routing(dem, dirs, lakes) -- 'dirs' and 'lakes' are optional
 		-- Add link to the graph, in both directions
 		local bound = lnk1[b2]
 		local bb1, bb2 = bound[1], bound[2]
-		if not basin_graph[bb1] then
-			basin_graph[bb1] = {}
-		end
-		if not basin_graph[bb2] then
-			basin_graph[bb2] = {}
-		end
-		basin_graph[bb1][bb2] = bound
+		basin_graph[bb1][bb2] = bound -- Potential non-linear complexity here
 		basin_graph[bb2][bb1] = bound
 
 		-- Merge basin b1 into b2