How does 哇 affect the meaning of 大河向东流哇?
哇, in this case interchangeable with 啊 or 呀, does not change the meaning but can affect the intonation and atmosphere.

The way you pronounce will totally change how your utterance is perceived.

For example, with a powerful sound (as in the song). It provides a background of the rest of the song. Since the song is about history/hero/war etc. It has a sense of the flow of time and history. In Chinese, river are often used as a symbol of time and history.

On the other hand, with a soft, emotional tone like a sigh (of course the rhythm of the whole sentence should be adjusted accordingly, e.g. slower), it could be perceived as full of regret and sorrow.