The hydrogeological conditions of coal mines in China are quite complex, and water inrush accidents occur frequently with disastrous consequences during coal extraction. Among them, the risk of coal mining under a river is the highest due to the high water transmissivity and lateral charge capacity of the unconfined aquifer under the river. The danger of mining under a river requires the accurate determination of the developmental mechanisms of the water flowing fractured zone (WFFZ) and the water flow mechanisms influenced by the specific geological conditions of a coal mine. This paper first used the transient electromagnetic (TEM) method to monitor the development of the WFFZ and the water flow mechanisms following the mining of a longwall face under a river. The TEM survey results showed that the middle Jurassic coarse sandstone aquifer and the Klzh unconfined aquifer were the main aquifers of the 8101 longwall panel, and the WFFZ reached the aquifers during the mining process. Due to the limited water reserves in the dry season, the downward flowing water mainly came from the lateral recharge in the aquifer. The water inrush mechanisms of the 8101 longwall panel in Selian No.1 Coal mine were analyzed based on the water flow mechanisms of the aquifer and the numerical simulation results. This provides theoretical and technical guidance to enact safety measures for mining beneath aquifers.