Cambridgeshire is a county in England. It is on land which in the last ice age was tidal estuary and swamp. Drainage was improved after that, initially by the Romans, releasing a lot of flat fertile land.
Since the Change, as the electrical pumps and weirs failed and the population died away, the land area has increasingly and rapidly reverted to swamp, to the point where escaped and released hippopotami are able to live and breed in the wild.
The Lorings and John Hordle are canoeing the Great Ouse River, and have decided to avoid the first herd of hippo they have met when the 'Netherfield Avengers' spring an ambush. The Lorings head toward the nearest building, and Hordle paddles into the swamp to circle the pack of "Brushwood Men".
While the Lorings shoot at the Avengers, Hordle takes out a rear guard and comes out behind the hippo, now alert and agitated. Hordle fires an arrow into the rear of one, and its' scream of pain attracts the attention of the Netherfield Avengers in their boat. Some open fire on the herd, giving the hippo a target for their aggression. Hordle makes his way back to the Lorings, and they get their two surviving canoes over a lock and into the drainage canal beyond.
The Avengers pursue them over the county border into Norfolk and King's Lynn, back on the Ouse.