Rainfall and water level fluctuations are critical triggering factors for reservoir bank failures. Taking the soil reservoir bank in the middle reaches of the Yellow River as the case study, this paper investigates the effects of rainfall action and water level fluctuations on the source of sediment deposition and the soil accumulation characteristics of the offshore direction in the reservoir area. By means of the rare earth element (REE) tracer technique, concept bank slopes containing the gentle slope of 3°, steep slope of 40° and abruptly steep sidewall of 70° were investigated. Four different conditions are designed for the analysis, including falling water level, rising water level, the coupled effect of ongoing rainfall and falling water level, and the coupled effect of ongoing rainfall and rising water level. The results show that after five experiments in the same group of terrain, soil productions from the gentle slope, steep slope, abruptly steep sidewall, and the total soil accumulation are strongly correlated with the distance between the sediment deposition and the bank, which can be expressed by the negative exponential function. The abruptly steep sidewall is a dominant source of sediment deposition in reservoir areas, particularly under the condition of water level fluctuations. The average mass of sediment deposition from the abruptly steep sidewall accounted for 71% of the total sediment deposition mass after five experiments in the same terrain group under rising and falling water levels. Furthermore, the sediment contribution of the abruptly steep sidewall to the mass of sediment deposition is basically highest at the toe of the slope, and its average contribution accounted for approximately 66%. Hence, it is advisable to protect the abruptly steep sidewall of the reservoir banks when impounding and releasing.