First I need some reference. I say how about the Hoover Dam? That gave me lots of reference including this illustration of the basics.
Hoover Dam Reference Illustration
After drawing a sketch I now block out the basics.
Roughing dam
Had some strange problems with importing flight files that turn the normals inside out in the screen display. Anyway here is the road and Dam layout. Roughly modeled after the Hoover. I added a little water on the other side of the dam so you will be able to drive over it and see both sides. Put a hair pin turn in the road to the dam too. Now I need the guard shack below. I have one but its 1200 verts and like one about 250 virts.
Dam Layout
Starting to paint the topical Map. I have it at 8000 pixels x 8000 pixels and then reduce it to what I need. That way I save my high res work and if needed II an reduce as necessary.
topical map
Checking the map on the mesh often to see how it fits the terrain.
painting the terrain
Continuing to paint out the terrain.
Painting the River
Technical Graphic Design and 3D Media 
Awesome… I really need to get with you before the Fall semester so I can put together a mapping / low poly modeling class. The kids really love this stuff but working with the community made modding tools can be a little stressful and Max is too expensive. Too bad they stopped developing the free Gmax before it even got off the ground. People still use it but who knows how long it will work.
yes I would love to visit again. lets make a plan.
I need to figure out how to make the city of Saskatoon in Canada. I have gis info coming on dvd. the down load function did work on the site so I just had them send a disk.