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Semi-arid southern California relies heavily on imported water for domestic use. A synthesis of river discharge data in this region reveals that summer (June, July, and August) river discharge in watersheds that have at least 50% urban, suburban, and/or commercial land cover has increased by 250% or more over the past half-century, without any substantial precipitation during these months. Total annual discharge in the Los Angeles River has also increased at levels up to several hundred percent. Three factors likely contribute to our observations: (1) increased groundwater recharge rates from leaking water pipelines, (2) inputs of treated wastewater into streams and rivers, and (3) increased runoff or recharge due to over-irrigation of ornamental landscaping. In the southwestern United States, water importation consumes large amounts of energy and contributes to decline of river flows in source regions. Here we show that water importation also increases river flows in urban areas.