A seasonal river, the Lavant springs resurface near Singleton once the chalky water aquifer under the North and South Downs reaches a certain level.
Observer nature expert Richard Williamson said the river was a late re-emerging this year after lower than average rainfall over winter, but water extraction had depleted the water reserves that fed the watercourse.
He said: “A lot of little animals used to live quite happily in the Lavant like frogs and newts and so on.
“This extraction has lowered the aquifer all over the downs quite dramatically, so the Ems and the Lavant aren’t really flowing as well as they would have done in the old days.”
He said the river was still home to Miller’s Thumbs fish, or European Bullhead, distinguishable by their ‘squashed flat’ appearance, as well as being an important water course for birds such as egrets, water rails and moorhens.
Kingfishers are also thought to use the river to migrate from Chichester Harbour to the Weald, where they breed in the Rother.
“There are a lot of movements in these freshwater runs that people don’t know a lot about, but I do know they use that migration route because I’ve seen them,” explained Richard.