diff --git a/src/mw_engine/core.clj b/src/mw_engine/core.clj
index 8a0ca95..4d946a6 100644
--- a/src/mw_engine/core.clj
+++ b/src/mw_engine/core.clj
@@ -23,12 +23,12 @@
 ;; that every cell's :x and :y properties reflect its place in the matrix.
 ;; See `world.clj`.
 ;;
-;; Each time the world is transformed (see `transform-world`, for each cell, 
+;; Each time the world is transformed (see `transform-world`, for each cell,
 ;; rules are applied in turn until one matches. Once one rule has matched no
 ;; further rules can be applied.
 
 
-(defn apply-rule 
+(defn apply-rule
   "Apply a single `rule` to a `cell`. What this is about is that I want to be able,
    for debugging purposes, to tag a cell with the rule text of the rule which
    fired (and especially so when an exception is thrown. So a rule may be either
@@ -41,7 +41,7 @@
      (seq? rule) (let [[afn src] rule] (apply-rule cell world afn src))))
   ([cell world rule source]
     (let [result (apply rule (list cell world))]
-      (cond 
+      (cond
         (and result source) (merge result {:rule source})
         true result))))
 
@@ -58,11 +58,11 @@
    exception is thrown, cache its message on the cell and set it's state to error"
   [world cell rules]
   (try
-    (merge 
-      (apply-rules world cell rules) 
+    (merge
+      (apply-rules world cell rules)
       {:generation (+ (or (:generation cell) 0) 1)})
-    (catch Exception e 
-      (merge cell {:error 
+    (catch Exception e
+      (merge cell {:error
                    (format "%s at generation %d when in state %s"
                            (.getMessage e)
                            (:generation cell)
@@ -93,28 +93,10 @@
   * `generations` an (integer) number of generations.
 
   Return the final generation of the world."
-  [world init-rules rules generations]  
-  (let [state {:world (transform-world world init-rules) :rules rules}]
-    (:world 
-      (last 
-        (doall 
-          (take generations 
-                (iterate transform-world-state state)))))))
-
-(defn run-world2
-  "Doesn't work yet"
   [world init-rules rules generations]
-  (with-local-vars [r (ref (transform-world world init-rules))]
-    (dotimes [g generations]
-      (dosync
-        (ref-set r (transform-world (deref r) rules))))
-    (deref r)))
-
-(defn run-world3
-  [world init-rules rules generations]
-  (reduce (fn [world _iteration] 
+  (reduce (fn [world _iteration]
             (transform-world world rules))
         (transform-world world init-rules)
         (range generations)))
-        
-  
+
+
diff --git a/src/mw_engine/drainage.clj b/src/mw_engine/drainage.clj
index c9d5874..e50f6ad 100644
--- a/src/mw_engine/drainage.clj
+++ b/src/mw_engine/drainage.clj
@@ -2,9 +2,13 @@
 ;; assumed to have altitudes already set from a heighmap.
 
 (ns mw-engine.drainage
+  (:require
+    [clojure.core.reducers :as r])
   (:use mw-engine.utils
         mw-engine.world))
 
+(def ^:dynamic *sealevel* 10)
+
 (defn rain-world
   "Simulate rainfall on this `world`. TODO: Doesn't really work just now - should
    rain more on west-facing slopes, and less to the east of high ground"
@@ -16,12 +20,13 @@
   `cell` and for which this cell is the lowest neighbour"
   [world cell]
   (remove nil?
-          (map
-           (fn [n]
-             (cond (= cell (get-least-cell (get-neighbours world n) :altitude)) n))
-           (get-neighbours-with-property-value world (:x cell) (:y cell) 1
+          (into []
+            (r/map
+             (fn [n]
+               (cond (= cell (get-least-cell (get-neighbours world n) :altitude)) n))
+             (get-neighbours-with-property-value world (:x cell) (:y cell) 1
                                                                   :altitude
-                                                                  (or (:altitude cell) 0) >))))
+                                                                  (or (:altitude cell) 0) >)))))
 
 (defn flow
   "Compute the total flow upstream of this `cell` in this `world`, and return a cell identical
@@ -29,11 +34,14 @@
 
    Flow comes from a higher cell to a lower only if the lower is the lowest neighbour of the higher."
    [world cell]
+   (cond
+    (> (or (:altitude cell) 0) *sealevel*)
       (merge cell
            {:flow (+ (:rainfall cell)
                (apply +
                  (map (fn [neighbour] (:flow (flow world neighbour)))
-                      (flow-contributors world cell))))}))
+                      (flow-contributors world cell))))})
+    true 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 8c6fee5..3cdf94b 100644
--- a/src/mw_engine/utils.clj
+++ b/src/mw_engine/utils.clj
@@ -1,7 +1,7 @@
 ;; Utility functions needed by MicroWorld and, specifically, in the interpretation of MicroWorld rule.
 
 (ns mw-engine.utils
-  (:require 
+  (:require
     [clojure.core.reducers :as r]
     [clojure.math.combinatorics :as combo]))
 
@@ -36,10 +36,10 @@
   ([world function]
     (map-world world function nil))
   ([world function additional-args]
-    (into [] ;; vectors are more efficient for scanning, which we do a lot.
+    (into []
            (r/map (fn [row]
-                    (into [] (r/map 
-                             #(apply function 
+                    (into [] (r/map
+                             #(apply function
                                      (cons world (cons % additional-args)))
                              row)))
                   world))))