Early in the River course, you'll see a large, sleeping Vileplume blocking a path. To get a good photo and potentially open a new route, you need to wake it up.
Play the Pokémon Flute near the Vileplume. It will slowly wake up and start dancing. This is your prime opportunity to get a high-scoring photo. If you manage to wake it up and get a good shot, it might move, revealing a different path later in the course.