The Lee Valley Regional Park extends some 25 miles down the valley of the River Lea, from Ware in the north deep into London as far as the Thames in the south, straddling the borders of Hertfordshire and Essex.
The gravel beds of the valley were left behind by retreating ice at the end of the last Ice Age 10,000 years ago. There has been massive extraction since the 1920s and this has created a huge complex of lakes and marshes in the northern part of the park, while in the south there are a number of large water supply reservoirs. Together these provide an area of open water to rival even the Norfolk Broads.
It is a spectacular place to watch birds, both on the open waters of the gravel pits and reservoirs, on the many channels and streams, and in the surrounding marshes and meadows. But you do not need to be especially interested in birds to enjoy wandering around in this tremendous (in both senses) wetland.
The largest areas of accessible open space are around Waltham Abbey, including the River Lee Country Park and, just south of Waltham Abbey, Gunpowder Park, a new country park mainly on reclaimed land once used for testing munitions.
Further south between Walthamstow and Hackney is a small complex of sites including Walthamstow Marsh and two little jewels of nature reserves on former water company filter beds.
There are a number of other good wildlife sites within the Regional Park. For birdwatchers, the most interesting of these are the Thames Water Authority's reservoirs at Chingford and Walthamstow, for which permits can be obtained from Thames Water. There are also nature reserves just across the border in Hertfordshire, notably at Rye House Marsh and Amwell.
Train to Cheshunt station from Liverpool St for River Lee Country Park or to Waltham Cross for Cornmill Meadows (bikes welcome). Many bus services serve the area: routes 505 and 250 should be most useful for access from Essex.