Excessive sediments washing off land can change the biological, physical and chemical properties of river environments.
Excessive, unnatural amounts can change the biological, properties of a river environment and is considered a significant stressor on aquatic ecosystems.
For fish eggs and juveniles, a supply of clean, oxygenated water flowing through gravels is vital for their survival. Excessive fine sediments entering a river can prevent this, entrapping and suffocating eggs and juveniles along with invertebrates too. Fine sediments can also damage aquatic plants and diatoms.
There can be physical changes too, including a reduction in water depth and changes in water temperature, which is particularly concerning as the effects of climate change take effect on Welsh rivers.
More reading:
"Exceedance of modern ‘background’ fine-grained sediment delivery to rivers due to current agricultural land use and uptake of water pollution mitigation options across England and Wales." A.L. Collins & Y. Zhang, Environmental Science & Policy, 2016