First, assume laminar flow, then you can assume the velocity profile is linear, except for close to the bottom boundary. T = Shear stress mu= dynamic viscosity du/dy = velocity flow profile T = (mu) * (du/dy) You do not need to know the surface area becuase the shear stress on a surface would be the same throughout, no matter how big the surface is. You do need to know the depth of the water, becuase then you can divide the velocity by the depth and you would have the velocity profile, du/dy. For an excelent overview on fluid dynamics check out this power point presentation: terra.rice.edu/courses/esci321/Earth_S..... Since, your problem deals with flow on a microscopic level, the principles of external flow are not valid. The reason is becuase the reynolds number for fluid arround cells is very low and therefore the fluid flow is extremely viscous which implies the shear stress would be very high.