I'm not a PS pro so I'm not sure how to taper them, but I think it might be easier if you draw the rivers on a separate layer, rather than erasing them out of your main land shape layer. It's also non-destructive to do them that way, so you have more room to change them later without messing up your whole map. Ascension, Pasis and Tear have some great beginning tutorials in Photoshop (and there are a ton more that are awesome); check those out for some step-by-step action.