Which of the following is a key characteristic of boulders in a stream?

The reason that boulders in a stream are identified as large and often roll in the surf is rooted in their physical characteristics and behavior within flowing water. Boulders are typically defined as large rock fragments that are larger than cobbles, often exceeding 10 inches in diameter. Due to their mass, boulders have the capacity to move within a stream, particularly in high-flow conditions like floods or rapid currents.

When water flows around these large rocks, the force of the current can cause them to roll or shift, which can further influence the stream's channel morphology and sediment dynamics. This rolling action not only contributes to erosion but also affects the habitat within the stream as it creates diverse structural environments for aquatic organisms.

Other characteristics of boulders, such as their formation or transport mechanisms, do not apply here. For instance, boulders are not primarily formed from sediment; rather, they are solid rock. Similarly, they are not the smallest rock type in water bodies, as that classification typically refers to sand or silt particles. Lastly, while some materials can be easily transported by wind, boulders, due to their size and weight, are far more influenced by water flow than by wind.