Frequent earthquakes along the eastern margin of the Qinghai-Xizang Plateau have led to the frequent occurrence of debris flow disasters in regional gullies. The damming and and breaching hazards formed by the accumulation of such debris flows pose serious threats to the safety of upstream and downstream areas. An improved depth-integrated “debris flow-water flow” coupled equation and numerical model were developed, and breaching hazards were conducted to provide parameters for the numerical simulations and serve as calibration controls. Multiple sets of numerical simulations were carried out to study debris flow-induced damming in gullies. The simulations analyzed the relationships between backwater tongue length, debris flow velocity, and varying channel widths, longitudinal slopes, and debris flow masses. Results show that the steeper the channel slope and the narrower the channel, the faster the debris flow and the longer the backwater tongue, increasing the likelihood of dam formation. Under otherwise constant conditions, for every 39 kg increase in debris flow mass, the backwater tongue length increases by an average of 39.4 cm, and the flow velocity increases by 0.1 m/s. For every 10 cm increase in channel width, the backwater tongue length decreases by an average of 8.2 cm, and flow velocity decreases by 0.22 m/s. For every 3° increase in channel slope, the backwater tongue length decreases by an average of 2.3 cm, and flow velocity increases by 0.17 m/s. These results indicate that debris flow mass and channel width are the primary factors influencing the backwater tongue length and flow velocity, respectively, while the channel slope is a secondary factor. The findings contribute to the scientific prediction of river-blocking landslide dam hazards caused by debris flows.