- Go to the hot river as early as possible to avoid crowds. This river is fairly close to Reykjavik so it gets quite a bit of tourists
- When I went there in October, I got to the parking lot at the base around 7:30am, it was still dark (sunrise in October was around 8:40am). The hike up to the river took me 50 minutes, so I got to the river around 8:20am, and I was the first one there
- When I started hiking down around 9:30am, I went past 15 people on their way up, and another 10 or so people at the parking lot. So I estimate that the river starts to get crowded around 10am.
- Keep in mind that when I went in October it's considered low season and it was already pretty cold and snowy there, so I imagine in the summer people will start going up even earlier
- So my recommendation, if you want to avoid crowds, is to get to the parking lot around 1 hour before sunrise in the winter (October - March; google sunrise time), and at around 6:30am in the summer
Tip: you're not allowed to camp up there overnight, but I did see a group camped there when I got to the hot river by 8:20am (they camped on the wooden path on the side of the hot river; with the steam coming out from the river I imagine it's quite warm and comfortable). So technically I wasn't there first; the campers were there first before anyone else. Iceland is so sparsely populated and this hot river is so far out from vehicle access point that it's hard for them to enforce the no camp rule