Ricketts Glen Falls

Ricketts Glen State Park, near Benton, PA, has 28 named falls including the namesake Ricketts Glen Falls. If you don't mind climbing down from the trail and don't mind placing your tripod in the water, Ricketts Glen Falls is an easy location to get a keeper. Pick a cloudy day and use a circular polarizer filter.
 
What is the ideal exposure duration for motion-blurred water? That answer is both situational and personal preference. In this location, my personal preference is around half a second. Experiment to learn what works well and what doesn't. Watch the details in the water (typically air bubbles) go from sharp to smeared to an indistinguishably smooth color as exposure times increase. When the right amount of blur is obtained, that is the right shutter speed.