Final answer:
The velocity of water in a stream is fastest at the center due to reduced resistance and it follows the principles of fluid dynamics where streamlines closest together indicate higher velocity.
Step-by-step explanation:
The velocity of water flow in a stream tends to be fastest near the stream's center. This can be explained through fluid dynamics, where the flow of the water is affected by resistance from the walls of the stream channel and interactions between the layers of fluid.
The speed at the walls is effectively zero, increasing steadily to its maximum at the center of the tube, a phenomenon sometimes referred to as no slip boundary conditions. When streamlines, which represent the path of a small volume of fluid as it flows, are closest together, the fluid velocity is greatest.
This is due to the relationship between fluid velocity and the cross-sectional area through which it flows - a concept that relates to the conservation of mass and Bernoulli's principle.