Set value for interpolation barrier for IDW/Spline/etc?

[MattTrueheart]

I am attempting to "burn in" a river channel into a terrain model from depth soundings. The DEM is from a Lidar scan, so the river channel bottom is the water surface at the time of the flight. I have sonar depth-soundings of the river channel (point data) that I would like to use to correct the DEM. 

So what I have done is add many points along the river banks at zero depth, and tried kriging, IDW, spline, etc., to create an interpolation of the channel bottom from the depth soundings. The idea is to then subtract the resulting raster from the original DEM, to incorporate approximate bathymetry into the terrain.

However, the interpolation "overshoots" the zero value at the banks. I have tried using the "barrier" option on the IDW and spline functions, but this still leaves some spurious artifacts and I am not sure how to proceed. 

Is it possible to delineate the river banks with polylines, assign them a 0-value, and enforce them as boundaries to the interpolation? Or any other thoughts or ideas?

Thanks for any advice!

Matt

[ChrisDonohue__GISP]

Just an idea on why things are going awry:

I forget off the top of my head which ones, but some of the interpolation methods do not strictly enforce the values, in other words they extrapolate a line that may not necessarily pass through the data point.  So that will be why some attempts will not have the zero result where one expects it even though there are zero value data points present.  So one possibility may be to stick to just the interpolation methods that are true to the data points and then make sure one has a large amount of zero data points where the banks are.

EDIT:

For example, Spline does honor the values (underline added for emphasis):

Spline

The Spline tool uses an interpolation method that estimates values using a mathematical function that minimizes overall surface curvature, resulting in a smooth surface that passes exactly through the input points. 

Source:  Comparing interpolation methods—Help | ArcGIS Desktop 

Chris Donohue, GISP