Take Exit 1 on I-68 and turn north onto US 119. Drive 2.4 miles and turn right on Callen Ave. Take an immediate right on Mississippi St. and go just under .5 mile to a kiosk and parking. Cross the street and follow the trail along the reservoir. The falls is at a spill way at the dam at the end of the reservoir. Access is not too hard, but you should be careful as some of the perches for photography are dubious at best.
It appears this waterfall has been removed from the park. Overhead view from google maps indicates the creek has been filled in at the dam.