Well, you could put in retaining walls, as suggested, but I'd opt for trees and underbrush. First, round off the top edge of the foam on either side of the abutments, to give the impression that it's the top edge of a ravine. (I'm referring to the layer of foam that forms the sub-roadbed, beneath the cork on either side of the bridge.) Next, paint the vertical sides of the ravine either black, dark grey or green, or dark brown. Apply some ground foam weeds, grass, etc. to the top edge of the foam that you rounded-off, along with the banks of the stream, then start making trees and bushes. If you place enough trees and bushes, the exact nature of the ravine walls will not be apparent, as the dark painted background areas simply appear as shadows. In the three pictures that I posted on Page 2 of this thread, the stream bank on the far side of the water consists of a very low bank only about 1" in width, from the "water" to a cliff rising to track level. The cliff (it's light grey, and glimpses of it can be seen through the trees, especially under the bottom of the bridge trusses in the first photo, both high and low in the second, and up high in the third photo) is absolutely vertical, but the trees and the distance from the viewer disguise this fact. Because your scene is right at the layout edge, the dark vertical walls will disguise this even more effectively. Depending on what method you intend to use to make your "water", it's generally best to do the water first, as it's very easy to accidently knock bits of ground foam off the trees and onto the still-wet "water".
Wayne