Laboratory experiments were performed to study the forces acting on flexible plants such as reed in river. The drag forces and lift forces acting on the model plants were measured by using load cells installed under the bed of open channel. Drag coefficient decreased with increase of Reynolds number because projected frontal area of plants in still water decreased with increase of incident flow velocity. The drag forces on the leaves of reed were small in the case of fast flow. The deformation of very flexible plants under the fluid flow are analyzed. Manning's n and Hydraulic permeability of plants were also estimated by using the Drag coefficient of plans deforming in fluid flow.