Generally, it flows year-round. If you want to go under the falls, you continue on the trail for about a mile until the trail comes back down to the river, and then you turn and hike back upstream to the falls. I should point out that the hiking upstream is strenuous in parts, and small children are not recommended here. In fact, as you hike upstream to the main waterfall, you'll pass several smaller waterfalls and good swimming holes underneath. On a hot day, this canyon is a wonderful place to be.
Weekdays are the best times to visit because the area is less crowded then.
On some summer weekends, this place can get crowded and you might as well be at a crowded public beach. The Millard Canyon waterfall is reached by driving to the Lincoln Avenue offramp from the 210 freeway in Pasadena. Drive north on Lincoln Avenue to Loma Alta, and then travel east on Loma Alta. Look for the hanging orange light in the street, which is Chaney Trail, and then travel up Chaney Trail for a mile. In one mile of this curving mountain road, you come to the T-intersection, where you go left, or down the road to the campground.