Kevin, without actually seeing the modeled scene firsthand, it is very difficult to offer much in the way of sound advice. A great deal matters regarding the placement of your bridge and how steep the sides of your river gorge are, as well as its width.
I can tell you that, even for a fairly narrow river gorge, any mirror will likely have to be set to at least a 30-degree angle with the backdrop and more likely around 45-degrees, if you don't wish to see your own reflection in it. In addition, hiding the top edge of a mirror can prove quite difficult, especially when it is set at such a dramatic angle in a fairly shallow scene. Likewise, appreciate that a common, rear-surface, mirror will likely do a poor job and may result in a double image. Only front-surface mirrors are well suited to such modeling purposes and, in the larger sizes, can prove quite expensive.
Try getting that photo of your scene posted and it will be a lot easier to offer advice.
CNJ999