Final answer:
The difficulty of navigating rivers at the Fall Line is primarily due to the presence of waterfalls and cascades, which create barriers to transportation and movement in the water.
Step-by-step explanation:
The characteristics of the Fall Line that contribute to the difficulty of navigating rivers are best described by option C) Presence of waterfalls and cascades. The Fall Line is a natural boundary in the eastern United States where harder crystalline rocks to the west meet softer sedimentary rocks of the coastal plain to the east. At this junction, rivers often have steep drops in elevation, leading to the formation of waterfalls and cascades. These features create a vertical barrier that interferes with river navigation and transport, hindering activities such as boating and the passage of migratory fish species.
Rivers at the Fall Line often experience a significant change in gradient. When the elevation of the riverbed decreases abruptly, the water flows more quickly and forcefully, leading to the appearance of these dramatic features that pose challenges to navigation. Unlike options A, B, and D, C specifically mentions waterfalls and cascades, which are characteristic of the Fall Line and are the primary contributors to the navigational difficulties.