Mm....maybe. It would depend on a few things -- namely your map's and menu's layout and what you want these "bubbles" to look like. Keep in mind about DVD menu buttons:

1. The actual "button" is a rectangular area, and these rectangles may not overlap.
2. The rectangle can contain a highlight that "lights up" when that button is selected.
3. The highlight does not have to fill the entire rectangle (you can mask away part of it).
4. The highlight can only contain a few colors. These are determined by color sets which you define in DVD Architect.

If your map and menu can be set up with these rules in mind, then I suspect you could do what you want.

Here's a [url=http://www.vegasvideohelp.com/custom-highlight-masks/]tutorial on custom highlight masks[/link] to get you started. Off the top of my head, I'd say you'd want to have the background with the map and your menu already included. Then insert empty buttons for each river -- each one would need to be wide enough to cover both the "bubble" and the menu button. As a result (and keeping in mind #1 above), each river will have to be relatively in line with its button. If the rivers are all over the place, this might be difficult (or impossible).

As per #4 above, the "bubbles" created this way will be pretty simple looking. If you're hoping for slick graphical bubbles, I'm afraid the chances are slim. I have a method for using images as highlights (instead of simple color highlights), but it wouldn't work over a map.

Rob