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All bodies of water have multiple inflows, but often, one inflow may predominate and be the largest source of water. However, in many cases, no single inflow will predominate and there will be multiple primary inflows. For a lake, the inflow may be a river or stream that literally flows into the lake. Inflow may also be, strictly speaking, not flows, but rather precipitation, like rain.
Inflow can also be used to refer to groundwater recharge.
The dictionary definition of inflow at Wiktionary