Submerged trees can sometimes be snags in waterways which increases the difficulty in navigating waterways, particularly for larger boats.

Snags in waterways are an issue for large boats, restricting access or causing damage in collisions, as mentioned in another answer. But they can also be deadly to small boats like kayaks and canoes. The water flows through the limbs of downed trees and the surprisingly strong hydraulic pressure can trap boaters underwater in these "strainers". This can happen with stream current or tidal flow.