I went to the Arches in October - 9th October 2015 to be precise. There was a queue over half a mile long to enter (I measured it with my GPS - I had NEVER been in a queue that long - I figured that the gate was half a mile from the road to cater for long queues at the entrance). Every trail was littered with people. If you don't call that overcrowded, I don't know what is. I went on to Dead Horse Gully and Canyonlands that day, after deciding that The Arches was really, seriously far too overcrowded for me to enjoy it. This was several days before I decided that the Grand Canyon was even more overcrowded, but it didn't have much of a queue at the entrance.
Last weekend I went to the Mount Samaria Park - roughly 1/4 the size of The Arches - and there was NO-ONE there except me (not even a ranger), in the entire park. The day was beautiful - sunny and not a cloud in the sky. The birds were amazing, the waterfalls were in full flow and I was there ALL BY MYSELF. I even saw one of the rare wallabies. And it is in the most populous state in Australia.
But crowded is relative of course. Having spent time in Acadia, Shenandoah, and Great smokies pretty much any Park West of the Mississippi never feels crowded to me.
I recently went to Uluru (which is known for being crowded, being one of the major Australian draw cards), and it had far fewer people than The Arches, Bryce Canyon, or The Grand Canyon. If there are places that are more crowded, I'm not interested.
If I come to this occasion, I wouldn't be able to bring much. Would I be able to come?
That's why going in autumn (I was just there) and staying in the park itself key. Staying in the park gives us easy access to the less crowded times of day. If we bring headlamps/flashlights we can also spend sunrise/sunset at some awesome locations. Note: the Needles section of Canyonlands is amazing and wasn't at all crowded when I was there last week (people don't take the time to go since it's further away from Moab).
When is a less crowded time of the day? I was there early in the morning. I agree, the Needles wasn't crowded.