diff --git a/worldeditadditions/lib/erode/erode.lua b/worldeditadditions/lib/erode/erode.lua
index 2eb54eb..6184e6a 100644
--- a/worldeditadditions/lib/erode/erode.lua
+++ b/worldeditadditions/lib/erode/erode.lua
@@ -24,14 +24,27 @@ function worldeditadditions.erode.run(pos1, pos2, algorithm, params)
 	-- worldeditadditions.format.array_2d(heightmap, heightmap_size.x)
 	local success, msg, stats
 	if algorithm == "snowballs" then
-		success, msg = worldeditadditions.erode.snowballs(heightmap, heightmap_eroded, heightmap_size, region_height, params)
+		success, msg = worldeditadditions.erode.snowballs(
+			heightmap, heightmap_eroded,
+			heightmap_size,
+			region_height,
+			params
+		)
+		if not success then return success, msg end
+	elseif algorithm == "river" then
+		success, msg = worldeditadditions.erode.river(
+			heightmap, heightmap_eroded,
+			heightmap_size,
+			region_height,
+			params
+		)
 		if not success then return success, msg end
 	else
 		-- FUTURE: Add a new "river" algorithm here that:
 		-- * Fills in blocks that are surrounded on more than 3 horizontal sides
 		-- * Destroys blocks that have no horizontal neighbours
 		-- A bit like cellular automata actually.
-		return false, "Error: Unknown algorithm '"..algorithm.."'. Currently implemented algorithms: snowballs (2d; hydraulic-like). Ideas for algorithms to implement are welcome!"
+		return false, "Error: Unknown algorithm '"..algorithm.."'. Currently implemented algorithms: snowballs (2d; hydraulic-like), river (2d; cellular automata-like; fills potholes and lowers towers). Ideas for algorithms to implement are welcome!"
 	end
 	
 	success, stats = worldeditadditions.apply_heightmap_changes(
diff --git a/worldeditadditions/lib/erode/river.lua b/worldeditadditions/lib/erode/river.lua
new file mode 100644
index 0000000..aa788fb
--- /dev/null
+++ b/worldeditadditions/lib/erode/river.lua
@@ -0,0 +1,100 @@
+local wea = worldeditadditions
+
+--- Parses a comma-separated side numbers list out into a list of numbers.
+-- @param	list	string	The command separated list to parse.
+-- @returns	number[]		A list of side numbers.
+local function parse_sides_list(list)
+	list = list:gsub("%s", "")	-- Spaces are not permitted
+	return wea.table_unique(wea.table_map(
+		wea.split(list, ","),
+		function(value) return tonumber(value) end
+	))
+end
+
+function worldeditadditions.erode.river(heightmap_initial, heightmap, heightmap_size, region_height, params_custom)
+	local params = {
+		steps = 1,				-- Number of rounds/passes of the algorithm to run
+		remove_sides = "0,1",	-- Cells with this many adjacent horizontal neighbours will be removed
+		fill_sides = "4,3"		-- Cells with this many adjaect horizontal neighbours will be filled in
+	}
+	-- Apply the custom settings
+	wea.table_apply(params_custom, params)
+	
+	params.remove_sides = parse_sides_list(params.remove_sides)
+	params.fill_sides = parse_sides_list(params.fill_sides)
+	
+	local timings = {}
+	local filled = 0
+	local removed = 0
+	for i=1,params.steps do
+		local time_start = wea.get_ms_time()
+		
+		for z = heightmap_size.z - 1, 0, -1 do
+			for x = heightmap_size.x - 1, 0, -1 do
+				local hi = z*heightmap_size.x + x
+				local thisheight = heightmap[hi]
+				
+				local sides = 0
+				local adjacent_heights = { }
+				if x > 0 then
+					table.insert(adjacent_heights, heightmap[z*heightmap_size.x + x-1])
+					if heightmap[z*heightmap_size.x + x-1] >= thisheight then
+						sides = sides + 1
+					end
+				end
+				if x < heightmap_size.x - 1 then
+					table.insert(adjacent_heights, heightmap[z*heightmap_size.x + x+1])
+					if heightmap[z*heightmap_size.x + x+1] >= thisheight then
+						sides = sides + 1
+					end
+				end
+				if z > 0 then
+					table.insert(adjacent_heights, heightmap[(z-1)*heightmap_size.x + x])
+					if heightmap[(z-1)*heightmap_size.x + x] >= thisheight then
+						sides = sides + 1
+					end
+				end
+				if z < heightmap_size.z - 1 then
+					table.insert(adjacent_heights, heightmap[(z+1)*heightmap_size.x + x])
+					if heightmap[(z+1)*heightmap_size.x + x] >= thisheight then
+						sides = sides + 1
+					end
+				end
+				
+				local action = "none"
+				for i,sidecount in ipairs(params.fill_sides) do
+					if sidecount == sides then
+						action = "fill"
+						break
+					end
+				end
+				for i,sidecount in ipairs(params.remove_sides) do
+					if sidecount == sides then
+						action = "remove"
+						break
+					end
+				end
+				
+				if action == "fill" then
+					local above_us = wea.table_filter(adjacent_heights,
+						function(height) return height > thisheight end
+					)
+					local above_us_next = wea.min(above_us)
+					heightmap[hi] = above_us_next
+					filled = filled + 1
+				elseif action == "remove" then
+					local below_us = wea.table_filter(adjacent_heights,
+						function(height) return height < thisheight end
+					)
+					local below_us_next = wea.max(below_us)
+					heightmap[hi] = below_us_next
+					removed = removed + 1
+				end
+			end
+		end
+		
+		table.insert(timings, wea.get_ms_time() - time_start)
+	end
+	
+	return true, params.steps.." steps made, raising "..filled.." and lowering "..removed.." columns in "..wea.format.human_time(wea.sum(timings)).." (~"..wea.format.human_time(wea.average(timings)).." per step)"
+end