Neil - I do something similiar that might help you, although it's not exactly what you're looking for. I sometimes use gauge readings to model stream heights. A gauge is a point, and has an attribute for the elevation of the stream. I take the points shapefile, use IDW to create a surface of the stream height, and then I get a raster of the interpolated stream height over an area. Then I subtract the ground elevation DEM from the interpolated stream height DEM - they're both rasters so I use the raster calculator or the 'minus' tool [raster calculator needs spatial analyst, 'minus' tool needs 3D analyst]. The result is a raster of stream depth. So, what you could do, is create a point file of your stream boundary lines, then get elevation attribution for the points from a ground elevation DEM, then add 1ft to the stream points elevation attribute, then IDW to create a raster surface of the increased stream elevation, then subtract the ground elevation DEM raster from the increased stream elevation raster, with the result being an approximate stream depth with 1m added. There's a little bit of cleanup needed to get the boundary as a vector line. Reclass the raster to two classes of stream and non-stream, then convert to a vector polygon file, then remove all the polygons not attached to the stream, then display your stream polygon as a boundary line or convert it to a line. There are many different ways to do this problem. You also could have taken the stream elevation raster and created contours from that. Or you could do this with TINs instead of rasters. Just think of this problem as you want to make the stream go up, so you need the stream to be a 3D feature. You need to work on the stream, not your ground elevation contours.
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