To pronounce 長谷川 as Hasegawa is the special case. Usually it is pronounced as naga-tani-gawa which meaning is long river valley. There is the place 初瀬 Hase or Hatsuse in Nara prefecture where famous for long valley. By association with this place 長谷川 is pronounced as Hase-gawa. 初hatsu:first 瀬se:shoal by the way. Also descripted as 泊haku:mooring 瀬se:shoal. Hase in Nara was important river port when Nara was Japanese capital.
Usually it is pronounced as naga-tani-gawa which meaning is long river valley.
There is the place 初瀬 Hase or Hatsuse in Nara prefecture where famous for long valley. By association with this place 長谷川 is pronounced as Hase-gawa.
初hatsu:first 瀬se:shoal by the way. Also descripted as 泊haku:mooring 瀬se:shoal. Hase in Nara was important river port when Nara was Japanese capital.