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Why do summit-level canals need reservoirs to supply water?

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View of Utrecht's Oude Gracht canal lined with historic brick buildings, arched doorways at water level, and boats moored along the canal edge in the Netherlands. — canals and inland waterways The *Oudegracht* (Old Canal), Utrecht, Netherlands.

Summit-level canals require reservoirs to ensure a consistent water supply for the canal's highest point, known as the summit pound.1

Because the summit pound is the highest elevation of the canal, it cannot be fed by gravity from other sections of the waterway. Reservoirs are necessary to provide enough water to compensate for two primary types of loss:

Lockage losses: Each time a vessel passes through a lock, a significant volume of water is required to fill or empty the chamber, which is then discharged to lower levels.12
Environmental losses: Water is continuously lost from the canal through evaporation and seepage into the surrounding soil or rock.31

If natural water supplies are insufficient to offset these losses, the reservoir acts as a critical storage facility to maintain the water level required for navigation. In cases where even these supplies are inadequate, pumps may be employed to return water from lower levels back to the summit. A historic example of this engineering necessity is the Canal du Midi, which required the construction of the Saint-Ferréol Dam to create a reservoir capable of feeding its summit.14