The River Itchen (previously also known as the River Alre) is a river in Hampshire, England. It flows from mid-Hampshire to join with Southampton Water below the Itchen Bridge in the city of Southampton. The river has a total length of , and is noted as one of the world's premier chalk streams for fly fishing, especially using dry fly or nymphing techniques. The local chalk in the earth provides excellent filtration and thus watercress thrives all along the Itchen valley in its once pristine, crystal clear waters, now affected by pollution by some farming practices. It is designated as a Site of Special Scientific Interest and is noted for its high quality habitats, supporting a range of protected species including the endangered water vole,otter, brook lamprey and white-clawed crayfish.