As regards rivers drying up - there are rivers that do this naturally = esp in chalk or limstone strata - it does not mean that all of the river dries up necessarily.
For instance - in the River Mole in Surrey, considerable flow dissapears into "swallow holes" along the chalkly North Downs sections and some flow re-appears later downstream (basically the river goes underground for a while). In very dry drought years, the river bed can be dry (around Leatherhead) as water losses are total through the swallow holes, yet upstream and downstream there is flow. The fish just move away during drought as it's not an overnight thing.
You can see the whirlpools where the flow goes underground.
The Herts rivers like the Minram, Oughton, Beane, Gade, Hiz are chalk streams and may suffer from similar underground flow losses in sections. Abstraction too makes things worse.
Mark