Final answer:
Paddling a canoe upstream is easier near the shores where the current is weaker, while canoeing downstream is more efficient in the middle where the current is stronger.
Step-by-step explanation:
When paddling a canoe upstream, it is advised to stay close to the shore because the current is weaker near the edges of a river. This is caused by friction between the water and the riverbed as well as the riverbanks, which slows down the flow. Conversely, when canoeing downstream, the middle of the river is usually the deepest part, resulting in a swifter current that can make the trip easier and faster. The central flow can carry the canoe more effectively, allowing for better maneuverability and less paddling effort.