You can create a particle effect using HL2's Particle editor, Import it into the game with a info_particlesystem entity, Trigger it on with a button, and after a few seconds, kill the fire entity.
Gimme a few while i dig up examples on how to do each.. I know there's a particle tutorial floating around TWHL somewhere.Particle Editor InformationOther Related LinksGood Waterfall TutorialThat should be everything you need to create the particle system and import it into the map.
Now are you versed with the Input/Output system? If you're not doing the other triggers can be tricky. Lemme know