The River Taff at the Western Avenue bridge:
The River Ely at Ely Bridge:
And the Rhymney at Rhymney Bridge:
The Shoothill GaugeMap shows the water level at other places on the Taff and other rivers in-and-around Cardiff — just click the required location in the “Water Level” menu on the left-hand side or click a marker ‘pin’ to see the monitoring station at that location.
And there are gauges for rivers all over the UK and Ireland, too.