Specific needs depend on the river paddled and local weather

Much of this topic has already been covered above in Emergency Shelter and Survival Kit sections. Boaters always need to strive for the right balance between weight and function. Whitewater kayaks are much smaller these days than 30 years ago so we need to be a bit more selective in what we bring. You can pack much lighter for your typical day trip than longer and more remote overnight expeditions. Often, expeditions will use larger boats like rafts and canoes that can safely store more gear and still run whitewater. Some expeditions cache supplies along the run to cut down on the carrying load. It is always a good practice to select a somewhat easier run when help is very far away - Consider limiting yourself to class III+ instead your normal class IV/V runs. Larger parties can carry more group gear by spreading the load. Always consider the weather when planning a trip. Winter paddling mandates extra clothing, food, and perhaps a temporary shelter. Summer paddling requires extra water to avoid dehydration. Always think "What If?".

Food, water, extra clothing, shelter, fire making supplies, …

Trust your gut, if there's any chance of any overnight stay if an incident takes place - be prepared.  Carrying extra high energy foods doesn't take up much weight or space.  These snacks also mitigate a second issue - low blood sugar and hypothermia.  Extra clothing is a wise precaution.  When someone takes a bad swim, getting them in dry clothes pays huge dividends.  Extra thermal gear like an NRS hood is a great precaution (I carry one in my PFD vest).  I also carry Glacier Gloves in my pin kit.  These are often loaned out when someone has cold hands or forgot their pogies.  A Tyvek tarp weighs practically nothing and has all sorts of first aid and shelter uses.  Heck, getting out of the wind for a spell can be real nice.  Packing some waterproof matches can be beneficial and they are a part of many repair kits.  Extra water is certainly smart any time of the year.