QGIS SAGA Channel network error: Input layers do not have the same grid extent

I am trying to follow along with this exercise using my own DEM.

I have done the first step, creating the catchment area, with no trouble. When I try to run the Channel network algorithm, however, I get the error message “Input layers do not have the same grid extent.”

I don’t see anywhere to specify a change in extent or cell size, so I’m puzzled about why the extents are different at all. And then if they are different, how can I get them to be the same? I’ve tried using the raster layer clipping to make new layers from the DEM and the catchment area to the same extent, but this gave me the same error.

I did try running the Channel network and drainage basins algorithm, and this worked but the channels seemed to be incomplete − it looked like only the top two order channels were there, with nothing below these.

I’m using QGIS 2.6.0 on Windows 8.1.

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I’ve been using TauDEM to do this work since I haven’t resolved the issue described above.