No, but it would be best if it did. It would also help if the slope did not change over the length of the pipe. If the pipe can run downhill for a bit at the beginning, it will make it much easier to start the water flowing as a valve can be installed at the low point and opened to start the flow. A pipeline can also run uphill for a bit and this may require adding a small valve to vent the line if it traps air. Automatic valves are available for this. Some pipeline will require venting to start the flow and small holes can be drilled in the pipe to allow for air to escape. These can be plugged with small screws after the air is purged. If the pipeline is buried, either the automatic valves can be used or a small valve can be installed and a vertical length of pipe installed so that you can reach in with a long handle to operate this valve and release the air.