Debris flows are rapidly moving mixtures of water, rock, and soil that often occur in mountainous regions. When entering and depositing in river channels, they can obstruct the incoming water flow and generate secondary hazards such as outburst floods. Understanding the formation and failure of debris-flow dams is therefore critical for developing effective mitigation strategies and engineering countermeasures. This study investigates river blockage mechanisms through controlled flume experiments of debris flows with varying water contents. Further insights on the dam formation mechanisms are obtained from comparisons with recent findings on dry avalanches. The experiments show that the viscous pore fluid in debris flows reduces deflection upon entering the river, enabling them to form dams more readily than dry avalanches. Debris flow dams are generally thinner but extend across a wider section of the channel. These morphological differences also lead to contrasting outburst behaviors: dry-avalanche dams typically collapse abruptly under the impounded water load, whereas debris-flow dams are simply gradually eroded. The degree of blockage is governed by the relative dynamics of the debris flow and river flow. Based on this, we propose a predictive criterion using relative Froude numbers of the river and the debris flows that reliably distinguishes between blockage and no blockage in both experimental and real-world cases. These findings advance the understanding of river blockage processes and support more accurate hazard assessments in mountainous terrain.