The River Ash is a tributary of the River Lea which flows through Hertfordshire, England, before joining the Lea near Stanstead Abbots.

The Ash has elements of both a chalk stream and a more erosive river.

It is the only tributary of the River Lea to achieve "Good" ecological status along some of its course.

[1] The River Ash originates near the village of Brent Pelham in North Hertfordshire and flows through The Hadhams (Little, Ford and Much), Widford, Wareside, until it reaches the River Lea near Stanstead Abbots.

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