Can you place landmarks in lake?
No, there is not. The only stipulations for landmarks are the following, according to the game setup sheet:


  
  Each space may only contain one object.
  All similar objects must be placed at least four spaces apart.
  Statues may not be placed adjacent to the ocean.
  


If you're making the choice to not put trees or huts on lake tiles, while it is practical for "realism", there's no rule against it.