Check google. We were there on a sunny weekday in september and site was ~25% full. $5/car entrance fee keeps out the riff raff.
Actually I was there 2 weeks ago visiting family and there were plenty of tourists. People come from all over to see the falls. There were canadians and Asians and I was pleasantly surprised because there didn't used to be so much diversity. The viewing area isn't very large so it seems crowded, but even crowded it's worth going.
I cannot find information on average tourist visit annualy but when I was there, there wasn't a lot of tourists. Probably because it's not a popular destination.
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