From I-40, 15 miles east of Flagstaff, take exit 211 at Winona. Drive 2.3 miles north to Leupp Rd. Turn right and drive 20.3 miles to unpaved and unsigned Indian Road 6910 (between mileposts 5 and 6). Turn left and drive along this rough road 9.4 miles to the turnoff on the left, which leads to the falls overlook (do not cross the Little Colorado River). Grand Falls can also be accessed from Indian Road 70 (instead of 6910). This road is located 15.0 miles along Leupp Rd (5.3 miles before Road 6910). Follow Road 70 8.4 miles to the same turnoff on the left before the Little Colorado River (Roads 6910 and 70 join together just before the turnoff). I recommend driving along Road 6910 instead of Road 70. Both roads are very rough going and you will need a 4x4 to drive them. But Road 6910 is much easier going and faster than Road 70.
The Little Colorado River runs south to north in central Arizona. Starting at near 10,000 feet surrounded by Ponderosa Pines it runs through the high and dry Navajo Indian Reservation before it meets the Colorado River in the Grand Canyon. Most of the year the "river" is dry at Grand Falls. But when the summer monsoon erupts the river floods.