In Civil 3D, you should be able to get that SDF (which I believe you are correct in noting that although the extension looks like the Autodesk Spatial Data Format, it in fact isn't) into the program by doing the following:
Then you should be able to work with this data in Map 3D if you apply the Civil object enabler to your Map 3D.
Alternatively, if you have access to the 2012 release of Map 3D, you could try out the River Analysis Extension. It's good until June 30th, but it might be extended or offered as an add-on to the current product release. If you do this, please refer to the resources listed on the Labs site for support.