The area has suffered for years from water courses not cleared of naturally fallen trees and branches. This includes the Den of Alyth, where natural tree debris has built up over many years (20 years, to my knowledge).
Both the Dean Water and the River Isla are now blocked in several places due to very large dams of naturally fallen trees and other rubbish, not cleared by landowners.
In a minor way, canoeists attempt to remove some of these blockages.
In England and Wales, it is a clear duty of landowners to keep rivers clear of debris – if they don’t, the Environment Agency steps in, does the work, and charges said landowners.