What aspect of stream flow is typically associated with pool areas?

Pool areas in a stream are characterized by low flow velocity. This is because pools form in sections of the stream where the current slows down, allowing water to collect more deeply. The reduced velocity in these areas can lead to various ecological benefits, including providing habitat for different species of fish and other aquatic organisms that prefer calmer waters.

Additionally, the slow movement of water allows for the settling of suspended sediments, which can lead to a build-up of organic matter and a rich substrate over time. Thus, pool areas often support a diverse ecosystem due to their unique hydrodynamic conditions. The other choices pertain to characteristics or processes that may not be inherently linked specifically to pools. For instance, while substrate size can vary widely and sedimentation can be affected by various factors in a stream environment, they are not defining traits of pool areas like low velocity is.