Despite its small size it is one of the few rivers in Britain which has a bore when the incoming tide cause a tidal wave to run upstream. The bore is only a few inches high and it runs only for a short distance upstream; beyond the road bridge in Aberarth the terrain rises steeply, preventing the bore propagating any further.
In the 1970s, the River Arth suffered severe intermittent agricultural pollution from a dairy unit not far from the village but improved pollution control and a change of ownership of the farm rectified that position.
