diff --git a/terranostra.lisp b/terranostra.lisp
index 0e647bb..cba9b40 100644
--- a/terranostra.lisp
+++ b/terranostra.lisp
@@ -9,13 +9,13 @@
 
 (defvar *world* NIL)
 (defconstant *directions* '(N NE E SE S SW W NW))
-
+(defconstant *base-height* 50)
 
 (defstruct patch
 	(pos '(0 0)) ;position
-	(alt 50) ;altitude
-	(streams-in NIL)
-	(streams-out NIL))
+	(alt *base-height*) ;altitude
+	(stream-in NIL)
+	(stream-out NIL))
 
 
 (defun init-matrix (size)
@@ -95,9 +95,8 @@
 
 (defun create-peak (x y height world)
 	"Create a mountain peak at the specified location"
-	(let ((base-height (patch-alt (make-patch))))
-		;;TODO
-		))
+	;;TODO
+	)
 
 (defun create-valleys (world &optional (aqueousness 5))
 	"Create valleys through streams. Aqueousness determines their number."
@@ -105,32 +104,38 @@
 											(* (length world)
 												(length (first world)))))))
 		((= n nstreams) world)
-		(setf world (create-stream (random (first world))
-						(random (length world)) world))))
+		(create-stream (random (first world))
+			(random (length world)) world)))
 
 (defun create-stream (x y world)
 	"Create a stream, starting at x/y"
-	(let ((dir NIL) (min 10000) (nextpatch NIL))
-		;;XXX min=1000 is is a hack to get a hypothetical maximum altitude
+	(let* ((minheight (* *base-height* 1000)) (here (coord x y world))
+			  ;;XXX hack: minheight must initially be higher than any patch
+			  (nextpatch NIL) (dir NIL))
 		;;Figure out the lowest neighbouring patch
 		(dolist (d *directions*)
-			(let ((np (patchindir x y d)))
-				(when (and np (< (patch-alt np) min))
-					(setf min (patch-alt np))
+			(let* ((np (patchindir x y d)) (alt (when np (patch-alt np))))
+				(when (and np (not (eq d (patch-stream-in here)))
+						  (or (< alt minheight)
+								 (and (= alt minheight) (zerop (random 2)))))
+					(setf minheight alt)
 					(setf dir d nextpatch np))))
-		(unless (and dir nextpatch) (return-from create-stream world))
-		;;Set streams-in and streams-out
-		(setf (patch-streams-out (coord x y))
-			(append (patch-streams-out (coord x y)) (list dir)))
-		(setf (patch-streams-in nextpatch)
-			(append (patch-streams-out nextpatch)
-				(list (opposite-dir dir))))
 		;;Erode this patch
-		(setf (patch-alt (coord x y))
-			(- (patch-alt (coord x y))
-				(random (round (/ (patch-alt (coord x y)) 10)))))
+		(let* ((instream (patch-stream-in here))
+				  (maxheight (if instream
+								 (patch-alt (neighbour here instream))
+								 (patch-alt here))))
+			(setf (patch-alt here)
+				(- maxheight (random (round (/ maxheight 10))))))
+		(format t "~&Creating a stream at ~S/~S, flowing ~S." x y dir)
+		;; Test for break conditions (hitting another stream or the edge
+		;; of the world)
+		(unless (and dir nextpatch (null (patch-stream-out nextpatch)))
+			(return-from create-stream))
+		;;Set streams-in and streams-out
+		(setf (patch-stream-out (coord x y)) dir)
+		(setf (patch-stream-in nextpatch) (opposite-dir dir))
 		;;Continue forming the stream
-		;;FIXME causes an endless loop -> lowest neighbour becomes this patch
 		(create-stream (first (patch-pos nextpatch))
 			(second (patch-pos nextpatch)) world)))