Which of the following accurately describes the concept of "navigable rivers"?

The concept of "navigable rivers" refers specifically to the characteristics that make a river suitable for navigation by vessels. The correct option highlights a key measurement criterion—navigability often requires a certain width and depth to accommodate boat traffic safely.

In many jurisdictions, a navigable river can indeed be defined by its width, which helps ensure that vessels can traverse it without obstruction. The mention of "30 feet wide" relates to certain legal definitions that often set a threshold to distinguish between navigable and non-navigable waters, thereby impacting jurisdiction over these bodies of water.

Understanding navigability is crucial for numerous reasons including transport, navigation rights, and environmental regulations. Recognizing the legal implications of a river's navigable status allows for better management of water resources and related activities.

The other options do not capture the concept accurately; for instance, the crossing by foot does not address the minimum depth or width for navigation by boats, and simply stating "small boats" does not imply the required channel dimensions needed for broader navigation. The width required for larger vessels is also a critical factor, which is why the final answer holds more significance in the context of navigability.