diff --git a/project.clj b/project.clj
index 97804ef..64282fc 100644
--- a/project.clj
+++ b/project.clj
@@ -11,8 +11,7 @@
   :jvm-opts ["-Xmx4g"]
   :license {:name "GNU General Public License v2"
             :url "http://www.gnu.org/licenses/gpl-2.0.html"}
-  :plugins [[lein-marginalia "0.7.1"]
-            [lein-gorilla "0.3.2"]]
+  :plugins [[lein-marginalia "0.7.1"]]
   :dependencies [[org.clojure/clojure "1.5.1"]
                  [org.clojure/math.combinatorics "0.0.7"]
                  [org.clojure/tools.trace "0.7.8"]
diff --git a/src/mw_engine/drainage.clj b/src/mw_engine/drainage.clj
index e8d5919..be777c0 100644
--- a/src/mw_engine/drainage.clj
+++ b/src/mw_engine/drainage.clj
@@ -28,21 +28,22 @@
                                                                   :altitude
                                                                   (or (:altitude cell) 0) >)))))
 
-(defn flow
+(def flow
   "Compute the total flow upstream of this `cell` in this `world`, and return a cell identical
    to this one but having a value of its flow property set from that computation.
 
    Flow comes from a higher cell to a lower only if the lower is the lowest neighbour of the higher."
-   [world cell]
-   (cond
-     (not (nil? (:flow cell))) cell
-     (<= (or (:altitude cell) 0) *sealevel*) cell
-     true
-      (merge cell
-           {:flow (+ (:rainfall cell)
-               (apply +
-                 (map (fn [neighbour] (:flow (flow world neighbour)))
-                      (flow-contributors world cell))))})))
+   (memoize 
+     (fn [world cell]
+       (cond
+         (not (nil? (:flow cell))) cell
+         (<= (or (:altitude cell) 0) *sealevel*) cell
+         true
+         (merge cell
+                {:flow (+ (:rainfall cell)
+                          (apply +
+                                 (map (fn [neighbour] (:flow (flow world neighbour)))
+                                      (flow-contributors world cell))))})))))
 
 (defn flow-world
   "Return a world like this `world`, but with cells tagged with the amount of
diff --git a/src/mw_engine/utils.clj b/src/mw_engine/utils.clj
index 3cdf94b..f1d308d 100644
--- a/src/mw_engine/utils.clj
+++ b/src/mw_engine/utils.clj
@@ -2,7 +2,7 @@
 
 (ns mw-engine.utils
   (:require
-    [clojure.core.reducers :as r]
+;;    [clojure.core.reducers :as r]
     [clojure.math.combinatorics :as combo]))
 
 (defn abs
@@ -32,13 +32,16 @@
 (defn map-world
   "Apply this `function` to each cell in this `world` to produce a new world.
    the arguments to the function will be the world, the cell, and any
-   `additional-args` supplied"
+   `additional-args` supplied. Note that we parallel map over rows but
+   just map over cells within a row. That's because it isn't worth starting
+   a new thread for each cell, but there may be efficiency gains in 
+   running rows in parallel."
   ([world function]
     (map-world world function nil))
   ([world function additional-args]
     (into []
-           (r/map (fn [row]
-                    (into [] (r/map
+           (pmap (fn [row]
+                    (into [] (map
                              #(apply function
                                      (cons world (cons % additional-args)))
                              row)))