Exactly this.
I walked the Central route from Lisbon four years ago with no diversions and to be honest I don't have any definitive memories of the town of Valença and have no knowledge of the walk from Caminha to Valença, as Albertinho does.
Maybe the question is : how do you want to cross from that river from Portugal to Spain? By foot over the bridge between Valença and Tui (Central) or by passenger ferry from Caminha to A Pasaxe/Guarda (Coastal)?
About the ferry - i posted a picture below of the schedule when I crossed over a week ago. However, sometimes it does not run because of low tide but you can cross by private fishing boat. Or walk, as Albertinho describes, towards Valença and cross at the first bridge you encounter and try to get back to the coast or indeed continue all the way to Valença.