September is great...even if its hot the days are shorter so it's less suffering in general. More chances for rain/cool weather. Lower water in September also means more beaches and camping spots. Some of the rapids can be kinda bony...but if you're a competent boater it shouldn't be a problem. Weekends can be busy, and there tends to be a little more jet boat traffic because of the fishing is better, but fewer multiday folks and fewer outfitters on the river. It's good to note that on October 1 (or near there) chukar season opens and you get the "cast and blast" folks on the river...which is fine if that's your thing...I prefer to avoid guns around inflatable boats
. BTW the Lower is great for kids at any season...it's my kids favorite activity!