The River Itchen is a celebrated 28-mile chalk stream flowing through Hampshire, England, from springs near New Cheriton to its tidal mouth at Southampton Water. Fed entirely by the chalk aquifer, it carries naturally hard, alkaline water at a near-constant temperature with a stable base flow, creating a physically uniform and rare habitat. The river holds designations as both a Site of Special Scientific Interest and a Special Area of Conservation, underscoring its ecological importance. It is renowned for its wild trout fisheries and is a historic cradle of modern fly fishing techniques, having been fished by pioneers like F. M. Halford, G. S. Marryat, and G. E. M. Skues. The river offers a mix of private estate beats and day-ticket waters, providing top-quality fly fishing throughout the season.