As a Severn user I think the most important information is the river level and flow.
The best source I've found is the Environment Agency's gauges. Ok, they don't tell you flow, only level, but the charts of level vs time do give you a good idea of flow.
But the E A charts are a pig to find, so I put together a web page (http://www.rwilson.org.uk/severn.html) which shows the levels on the navigable part of the Severn.
Also there are links to Farsons water cams.
It would be useful to have Met office weather warnings and forecasts too!