In literature some times to account for semi-pervious sediments at bottom of stream, streambed leakance also called retardation coefficient ($L$) is used. This is equal to the ratio of conductivity of aquifer to streambed multiplied by thickness of streambed.
On other hand, another term seems to describe the effect of same less permeable sediments on bottom of river named as "Riverbed conductance" ($L2/T$) used in MODFLOW. This is equal to $K \cdot A / L$, where $K$ is conductivity of riverbed, $A$ is area of cell and $L$ is length. Is this the only difference between these two terms?