Dogs will cover far more ground than we do during a walk and will use up huge amounts of energy so they will need food and water for energy and hydration throughout the day. It is a good idea to give your dog breakfast so they start the walk with plenty of energy on board. Throughout the day they will need regular sustenance: some dry kibble or a pouch of moist dog food are ideal for this. Do not give your dog human snacks as dogs do not metabolize these in the same way as humans.
Most dogs will be happy to drink out of burns or pools but take care to avoid letting them drink stagnant water. In hot weather or when climbing hills which are particularly rocky or dry (e.g. the Cairngorms or Torridon) it is advisable to carry water for your dog and a collapsible drinking bowl.
Always ensure your dog has a meal during the evening to replace lost nutrients. You may find your dog is hungrier than usual in the couple of days following a hillwalk and may need additional food to avoid losing condition.