I've been thinking about a hypothetical problem concerning fluid flow across two distant countries. Assume that country A wants to share water from a river with country B. Country A builds a dam to collect water in a large reservoir. A horizontal pipe is connected from the bottom of the reservoir to country B in an attempt to transport water from the reservoir to country B (We are talking a 1000km~2000km long pipe). No pumps are used in this assembly. Since the reservoir is constantly collecting water from the river, we can assume an infinite supply of water (water level in the reservoir does not decrease) My question is: Can the friction forces in the pipe be so great that it hinders the flow to impractical levels or stop the flow all together?
My first guess is that, using energy conservation, the potential energy in the reservoir must be greater than the friction work done by the pipe. If the pipe is very long, then at some point the friction work done by the pipe will overcome the potential energy and the pressure difference energy available in the reservoir, and thus flow will stop. Pumps will most likely be needed for water to reach country B.
However, I've been discussing this with a friend and he theorises that water will reach country B eventually regardless of friction in the pipe. Because the pipe is horizontal, and water supply is unlimited, water will eventually reach the destination even if it takes weeks or months. After flow reaches the other side, country B can extract and use water from the assembly.
What will most likely happen? Will flow stop or will it continue? Will pumps be needed in this assembly? And why? And if my friend's theory is correct, What will happen when country B tries to collect water at a certain mass flow rate?