[Therion] How do you draw objects outside cave?

Hi everyone! What trick can be used to draw cave objects like rivers, trees outside actual cave like here: http://www.thailandcaves.shepton.org.uk/files/cavesurveys/chumphon/cp0054-th... (a piece of river near Resurgence entrance) Drawing invisible walls did not help me. My rivers always gets cut by line connecting entrance sides.