A stream complex is made up of rocks that are the result of overlapping streams. Because of how quickly the water moves through a stream channel, the channels frequently have rocks with large particles. Flow only occurs between stream channels during floods. Because flood flow is unrestricted and slower, overbank rocks with smaller particles can result. This image shows a contemporary stream complex.

Suppose the vertical cross section shown looks like this:

Suppose the reservoir has just two permeabilities: a permeability along the channels and a permeability over the bank (in between the channels). One measures 100 mD, the other 20 mD. What average permeability will be observed if flow away from the scene creates this particular reservoir?