If the flow is unsteady then the configuration of the stream tubes and filaments changes from time to time. However, if the flow is steady then the stream tubes and filaments are stationary. In the latter case, a stream tube acts like an actual tube through which the fluid is flowing. This follows because there can be no flow across the walls, and into the tube, because the flow is, by definition, always tangential to these walls. Moreover, the walls are fixed in space and time, because the motion is steady. Thus, the motion of the fluid within the tube would be unchanged were the walls replaced by a rigid frictionless boundary.
Consider a stream filament of an incompressible fluid whose motion is steady. Suppose that the cross-sectional area of the filament is sufficiently small that the fluid velocity is the same at each point on the cross-section. Moreover, let the cross-section be everywhere normal to the direction of this common velocity. Suppose that and are the flow speeds at two points on the filament where the cross-sectional areas are and , respectively. Consider the section of the filament lying between these points. Because the fluid is incompressible, the same volume of fluid must flow into one end of the section, in a given time interval, as flows out of the other, which implies that