A sprinkler includes an outer case having an upper end and a tubular riser telescopically mounted in the outer case for extension and retraction from the upper end of the case. The riser has an upper portion with a plurality of circumferentially spaced apertures sized to allow debris to be carried through the apertures with flowing water. A riser seal is positioned and configured to substantially prevent water from escaping between the riser and the outer case when the riser is extended. A drive assembly is mounted inside the riser and includes a drive shaft that is co-axially mounted at an upper end of the riser and rotates when pressurized water is supplied to the outer case. A cylindrical nozzle housing is co-axially coupled to the drive shaft and is rotatable relative to the riser. The nozzle housing is positioned and configured to define a gap between an outer cylindrical wall of the nozzle housing and a complementary outer cylindrical wall of the tubular riser. The apertures in the upper portion of the riser intersect the gap and are configured and sized to receive pressurized water below the riser seal when the riser is retracted.