Grassy Lake Rd
Many of the 3 million visitors to Yellowstone go and see the tallest falls in the park, Lower Falls on the Yellowstone River. However, lesser known but just as impressive is the second tallest falls. Union Falls, which derives its name from the fact that two rivers join just at the edge of the falls, is a spectacular 250 feet tall.
Union Falls is less visited than other waterfalls in Yellowstone due to it being in the backcountry. It is a long day hike, or a relatively straighforward overnight trip. Either way, don't forget to bring a bathing suit!
To get to the trailhead, follow the Grassy Lake Road. If you are coming from the east, you will start at Flagg Ranch and pass through the John D. Rockefeller, Jr. Memorial Parkway. You will end up passing into the National Forest before you get to the trailhead. From the west, you will also travel along the Grassy Lake Road, but if you enter the Parkway, you have gone too far. The Grassy Lake Trailhead is on the western edge of Grassy Lake Reservoir on the north side of the road. Also, please remember that the Grassy Lake Road is not open year-round, and is difficult traveling all year. A high clearance vehicle is highly recommended.
The trail begins on the west side of the parking lot. Less than a half mile in you will cross the boundary into Yellowstone National Park. About a mile after that, there is a large stream crossing. Remember to bring and extra pair of shoes, preferably ones that strap on (not flip-flops) because the water is swift. The water is high and can be dangerous in early season, but by mid-July, crossing is pretty straight forward.