By the Arbiter Snipers, and the filthy water with pipes. (Right before you head into the place where the Snipers are scattered on the trees.
Either at the end of the level with the big battle, or the par that had the two pelicans and the banshee.
umm its the part where theres the broken hydroelectric dam where the pelicans hit each other and you've got lots of little PIA grunts runnin everywhere throwing them blam sticky grenades(I play with catch on all the time). you can get another pretty weird lookin shot from there too, where the water is gone but you can see the riverbed floor...