Wildflowers are less abundant by late summer particularly in a drought year (though most wild plants are well adapted to survive long dry spells). Watery places are usually a good bet to find something in bloom at this point in the season.

 Admittedly this stretch of the River Hiz between Hitchin and Ickleford is not laden with flowers. Nonetheless there are flowers to be found among the vegetation where the muddy bank meets the shallows.

 Looks like Hedge Woundwort but it's by the water so I think this is Marsh Woundwort (Stachys palustris).

 Looks like a Field or Wood Forget-me-not but it's growing in the water so I think this must be Water Forget-me-not (Myosotis scorpioides).

 Looks like a mint but it's growing in and around the water so I think this will be Water Mint (Mentha aquatica).