Henning Thielemann
Sounds like a generator for scientific articles. :-) Maybe http://hackage.haskell.org/cgi-bin/hackage-scripts/package/markov-chain can be of help for you. It's also free of randomIO.
I once invented this, though ungeneralised, for a map generator of a RTS... the river always went from the left to the right, at approximate the middle of the map, its direction being dependant on its current offset from that middle, its width (that is, a wide river can bend upwards and downwards more than one tile) and a random factor. You can also express this using a markoff chain. -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.