When are pumps needed to return water to the upper levels of a canal?

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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.

Pumps are used in artificial canals when the natural water supply is insufficient to offset the water lost during operation.1

Every time a lock is operated, water is consumed as the chamber is filled and emptied to move vessels between different levels. In addition to these lockage losses, water is also lost through evaporation. If the available water supply—typically provided by storage reservoirs—cannot adequately replenish these losses, pumps are required to return water from lower levels back to the upper levels of the canal.231