my problem is quite complex, though I hope that solution may be simple or at least possible.
I do have .img DEM 0.5x0.5m. DEM consists of river bed, river valley and adjacent areas (it is strip of 5km with the river in the centre). DEM was derived from ALS, so it does not show depth of water feartures and of course that something I am interested in. I do have some cross-section of the river bed and will have surveys of certain sections, lets say I have various point data for cross-section of the river bed, which will be 200-300m apart. My aim is to interpolate points for those 200-300 m gaps, convert it in to DEM and somehow combine with the original DEM. So as you see there are several issues.
How to extract shape of the river corridor from original DEM? How to interpolate points within such irregular shape as river corridor, so I can receive valid DEM for river bed? How to combine original and river bed DEM?