diff --git a/terrainlib_lua/rivermapper.lua b/terrainlib_lua/rivermapper.lua
index 5479d1e..ac6e65e 100644
--- a/terrainlib_lua/rivermapper.lua
+++ b/terrainlib_lua/rivermapper.lua
@@ -447,31 +447,25 @@ local function accumulate(dirs, waterq)
 	-- This means: how many nodes will give their water to that given node, directly or indirectly?
 	-- This is obtained by following rivers downstream and summing up the flow of every tributary, starting with a value of 1 at the sources.
 	-- This will give non-zero values for every node but only large values will be considered to be rivers.
-	waterq = waterq or {}
 	local X, Y = dirs.X, dirs.Y
+	waterq = waterq or {X=X, Y=Y}
 
 	local ndonors = {}
-	local waterq = {X=X, Y=Y}
 	for i=1, X*Y do
 		ndonors[i] = 0
 		waterq[i] = 1
 	end
 
 	-- Calculate the number of direct donors
-	for i1=1, X*Y do
-		local i2
-		local dir = dirs[i1]
-		if dir == 1 then
-			i2 = i1+X
-		elseif dir == 2 then
-			i2 = i1+1
-		elseif dir == 3 then
-			i2 = i1-X
-		elseif dir == 4 then
-			i2 = i1-1
-		end
-		if i2 then
-			ndonors[i2] = ndonors[i2] + 1
+	for i=1, X*Y do
+		if dirs[i] == 1 then
+			ndonors[i+X] = ndonors[i+X] + 1
+		elseif dirs[i] == 2 then
+			ndonors[i+1] = ndonors[i+1] + 1
+		elseif dirs[i] == 3 then
+			ndonors[i-X] = ndonors[i-X] + 1
+		elseif dirs[i] == 4 then
+			ndonors[i-1] = ndonors[i-1] + 1
 		end
 	end