A closed modular ditch liner includes a plurality of closed liner sections, a plurality of covers, and at least one alignment key. The cross section of each closed liner section includes at least one trough contour and a single cover retention lip formed on a top end of each side thereof. Each cover is laterally retained between the two cover retention lips. A key slot is preferably formed on at least one side of the closed liner section to receive a single alignment key. However, alignment keys that do not require a key slot may also be used.