Final answer:
Low head dams are dangerous to small boats and paddle craft mainly due to the backwash they create, which can trap boats and paddlers against the dam indefinitely. This hazard is often not visible or well-marked, contributing to unexpected and dangerous situations for river users. Option b is the correct answer.
Step-by-step explanation:
Low head dams, also known as "weirs" or "run-of-the-river dams," are particularly dangerous to small boats and paddle craft. One of the main reasons for their danger is that they create a backwash or "boil" that can churn endlessly below the dam, creating a hazardous condition known as a "hydraulic." This backwash can potentially trap boats and paddlers against the dam indefinitely, leading to fatigue, injury, or drowning as escape can be extremely difficult. This dangerous feature is often invisible to approaching paddlers, especially from upstream, which aggravates the risk.
Moreover, the areas around low head dams are not always marked clearly, leading to a false sense of security amongst paddlers and boat operators. This lack of signage or warnings contributes to the unexpected encounters with the hazardous dam backwash. Additionally, low head dams impact the ecology and physical characteristics of rivers. They can obstruct fish migration, alter water temperatures, change flow characteristics, and increase sediment loads, which affect the river's ecosystem both upstream and downstream.
In summary, the key danger point associated with low head dams is the relentless backwash that can pin boats and paddlers, rather than confusion from the roar of the water, the presence or absence of water on the other side of the dam, or overconfidence due to them being well-marked. Therefore, the correct option for why low head dams are dangerous to small boats and paddle craft is b. Backwash can pin you and your boat indefinitely.