Royal Arch Cascade is a stringy waterfall tumbling between the Ahwahnee Hotel and the Royal Arches, which formed from the calving of the granite wall on which it resides leaving behind arches (though they're not natural arches).
Although we've only seen this waterfall during the annual spring snowmelt, I've read that it can expand and decorate the granite wall on which it tumbles during a heavy thunderstorm (which might occur during the Summer months when pop-up thunderstorms become the norm). We haven't seen the cascade do the expansion due to a sudden downpour from a thunderstorm so we can only imagine how much of a spectacle it might become should it occur. Instead, we only spotted this cascade in its narrow state, which you can see from the photos on this page. Given its generally weak flow, I don't think it can be reliably seen by June unless the snow pack is high. When it's dry or trickling, only streaks are left on the wall.
Until we started to consciously look for this waterfall, it was easy to miss it because it neighbored other granite formations (in addition to the Royal Arches) such as North Dome and the Washington Column. I also heard that rock climbers like to ascend the Royal Arches as well so it might be possible to spot them from the wide open expanse of Stoneman Meadow.
As for photographing the waterfall, I found that it wasn't terribly interesting by itself. However, when it's composed with other subjects (such as North Dome), then it becomes a bit more interesting.
Directions: There are a couple of ways to get decent views of this waterfall. The first is from the Stoneman Meadow in front of Curry Village (signpost V23). From this vantage point, you can see not only the streaky cascade, but you can also see the rock formations like the Royal Arch Cascade, the Washington Column, and North Dome all in one shot.
Alternatively, you can get a closer and more angular view of the waterfall as you drive north across the Stoneman Bridge near the LeConte Memorial and towards Yosemite Village. There's a pullout in front of another grassy area (no signpost) where you can stop the car and get the view you see at the top of this page.