September on Deso can vary widely in temps. depending of whether you are going in early September or late. I'd bring layers for all temps. Could be shorts & polypro T-shirt, or Smartwool and splash pants/jacket. Wet suit shorts for early Sept. and maybe a full suit for late Sept. If you are a rookie, you might swim more, so maybe err on the side of warmth?
My outfit of choice for September Middle Fork trips is Smartwool, and splash wear if it's cool, and polypro layers if it's warm.
"We are all visitors to this time, this place. We are just passing through. Our purpose here is to observe, to learn, to grow, to love....and then we return home."
Australian Aboriginal Proverb