Final answer:
The greatest threat at a low head dam is the downstream side due to the dangerous recirculating currents that can trap objects and individuals. When canoeing, the shore is preferred upstream for less resistance, while the middle of the stream is preferred downstream for increased velocity.
Step-by-step explanation:
The part of a low head dam that poses the greatest threat to river users is the downstream side. This is because of the backwash or 'boil' that occurs just downstream of the dam, which can create a dangerous recirculating current. This current can trap objects, debris, and even people in a continuous cycle that keeps them from escaping.
Moreover, as water flows over the dam, it accelerates and can push river users into this hazardous area. When canoeing, staying near the shore when traveling upstream minimizes the force against the canoe due to the slower water speed and less resistance compared to the center of the stream.
Conversely, when canoeing downstream, the faster-moving water in the middle pushes the canoe, increasing its velocity, which is why it may be best to stay near the middle to maximize this effect.