At this scale most rivers wouldn't show up at all . Since you have chosen to represent the mountains as hand-drawn as opposed to say photo realistic ; the other elements should be in the same style. Draw in the rivers based on importance just keep the river widths conservative. For the hills use a suitable symbol matching the mountains style and make the line work much more subdued. Tolkien detailed areas that were the most important . This is how most maps would have been historically made. After all you can't / shouldn't fill in what you don't know.