What is a poised stream?

A poised stream is characterized by stable sediment transport and a balanced system, making this the correct choice. In this context, the term "poised" refers to a state where the stream maintains a healthy equilibrium between the sediment it carries and the sediment available in its environment. This balance implies that neither erosion nor sediment deposition is occurring at an untenable rate, which can lead to environmental degradation.

When a stream is in this poised state, it effectively transports sediment without overwhelming its banks, leading to less erosion and a more stable ecosystem. On the contrary, other scenarios such as rapid erosion of banks or temporary low flow conditions do not represent a stable sediment transport system and may lead to significant ecological disturbances. A gain in tributaries generally suggests an increase in water input but does not inherently indicate that the stream is poised; in fact, it may disturb the balance of flow and sediment transport. Therefore, identifying a poised stream helps in understanding and managing waterway health effectively.