Humm... I went to a speaking dictionary but couldn't find the word "Riva", but I did find "rivage" [riv'-ij] (has a mix of 'a' and 'e' sound as in 'river'), "rival" [rì´vel] (has a distinctive 'i' sound) and "rive" [riv] (also with a distinctive 'i' sound).
Could not find any word that had a distinctive 'e' sound, so, maybe the correct pronunciation is
"R 'eye' va" or "R 'a+e' va" as in the southern pronunciation of "River", ie, "Riv 'ah'".