* The current: The river is described as a "swift and swirling" current, which would make it difficult to navigate, especially for those unaccustomed to water travel. The river "dashed and bubbled" which would make it difficult to maintain your balance and steer a boat.
* The magic: The river itself is enchanted and is said to "carry away" those who fall into it. This suggests that the river has a magical ability to pull people into its depths.
* The danger of the journey: The journey down the river is meant to be a shortcut, but it is clearly dangerous. The hobbits are going against the current, and the river itself is unpredictable. The fact that they had to be careful not to capsize their boat shows the danger of the situation.
While the specific danger of the river isn't explicitly stated, the text makes it clear that the hobbits were taking a risk by taking this route. The enchantment and the turbulent nature of the river combined made for a perilous journey.