Complex sentence: Ibid The Lerez river is the principle river to flow into the Ría de Pontevedra and at the mouth are islands known as Isla Ons and Isla Onza.
Split into simpler sentences: Ibid The Lerez river is the principle river to flow into the Ría de Pontevedra. At the mouth of the ria are islands known as Isla Ons and Isla Onza.