Welsh farmers and landowners have been told not to undertake 'unconsented work' to streams and rivers following an increase reports of damage.
Natural Resources Wales (NRW) has issued the warning amid what it says is a rise in reports of damage caused by gravel removal, reprofiling of banks and straightening channels.
These activities can destabilise rivers, Wales' environment watchdog said, causing changes to erosion and deposition processes, potentially increasing flood risk.
It warned that altering river channels could destroy fish habitat and bird nesting sites, disturb protected species and cause the spread of invasive non-native species.
Hilary Foster, NRW's specialist advisor, said alterations by landowners should be limited to situations where there was a 'clear justification', such as alleviating flood risk.
"You will also require a permit or consent from NRW or the Local Authority in most cases," she explained.
“Any work that is carried out on a river or stream will affect the characteristics of the channel both upstream and downstream.
"And river channel alterations carried out by one landowner can potentially cause problems over considerable distances for their neighbours."
According to NRW, Wales has lost more than 50 percent of its river gravel shoals over the last century.
It says gravel shoals support more than 500 species of invertebrates, half of which are only found in these shoal habitats.
Ms Foster went on to issue a warning: “If you do not consult NRW prior to undertaking works on a river or stream, you are at risk of committing an offence."
