Water flow when I was there was 4x normal summer flow rate through the narrows ... pretty tough going. Rain eased up the day I was there, but runoff was still too high to get to the subway from the bottom side. We hiked down to the stream, and debated, but even the first crossing would have been challenging ... doing it 20 or 30 more times up the canyon would have most likely resulted in a disastrous ending to either us, our camera gear or both. I don't know how long it takes the water to drop after the rain subsides, and I don't know if the rain returned or it's been dryer.
We hiked back out and by that time of day our only choice was to join the other tourists on a hike through Emerald Pools, which at least offered a few nice opportunities. Waterfalls were similar to early spring flows. I'm targeting 1st Saturday of November to try the subway again.