Layers work well. Hypothermia is not uncommon. Don't drink and paddle. Drinking on the river is never done. Yet, a beer and some munchies at the take out are not uncommon.
Paddlers often face more dangers on land than on the water.
These are usually narrow rocky paths that can be treacherous. Poison ivy and poison oak are abundant on the riverbanks. Learn to distinguish and bring lotion if you're highly allergic. Always double check that your spray skirt release loop is out and free to pull. Your boat should have float bags in the stern and bow.