The majority of power plants use one of two types of cooling water systems. In a once-through or open-cycle cooling system, water is withdrawn from a water source, such as a lake, river or reservoir. The water passes through the condenser and is then returned to its original source, with a negligible amount of heat transferred to the aquatic environment.
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When open cycle cooling is used, the temperature of the nearby body of water can rise noticeably, which can be harmful to its ecosystem. If the regulations specify an upper limit for water temperature, the reactor may need to reduce power or cease operation in hot weather.
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Open-Cycle Cooling — The system that uses water in the circulating system
for once-through cooling. Water is taken from a the river, lake, or ocean
and used to cool the condenser. It is then discharged back to the saee
body of water with the added heat.

Closed-Loop Cooling — Operation of the circulating water system so that
water is circulated within a closed loop for condensing steam in the
condenser and cooling the water in cooling lakes, cooling towers, or
other heat sinks.

https://www.osti.gov/scitech/servlets/purl/7220266-sMuOFB/