This is exactly what has happened in many of the South Wales rivers. I grew up next to a fairly short industrial river which was virtually dead when I was a boy. It’s now ‘relatively’ healthy. Past industrial urban rivers like the Sirhowy, Ebbw, Rhymney and Taff are also now ‘relatively’ clean and fish consistently well.While rivers in industrialised areas have recovered to some (and sometimes great) extent, many of those in agricultural areas appear to have seen a significant decline in water quality (and/or the number and size of game fish they support) since the 1960s/70s.
The stark reality is, there's not much point in rejoicing at the news of a salmon being seen in an inner-city river "for the first time in over a hundred years" if several hundred salmon have gone missing from a rural river system during that time.
Further west it’s a different picture and similarly on the Eastern border we have the Usk, Wye and tributaries all showing decline. Agricultural rivers are struggling.