If your buddies are not so great at estimating time (while on the sticks), an egg timer will keep them honest. Lack of invites will have the same effect.
A spare oar lock is a must and a breakdown oar is a good idea too.
Head lamp with spare batteries.
A garbage can so plastic bags, cans, old leaders etc. don't blow all over the river - the small narrow recycling bins work well (doubles as a bailer).
Garbage bags to pack out others people's plastic bags, cans, old leaders etc. you're going to find.
Spare trailer light, bearing, electrical tape/wire nuts, Crescent wrench for bush fixes.
Toque/gloves.
Common sense.