Abstract The velocity of the density current head in the expansion reach in which a river enters a dam reservoir, lake or sea is a key parameter for evaluating the extent to which suspended material travels, and for determining the type and distribution of sediment in the water body. This study experimentally evaluated the effects of the reach degree of expansion on the density current head velocity. Experiments were conducted in a 6.0-m-long, 0.72-m-wide flume. The head velocity was measured at three expansion degrees (8; 12; 26) and two slopes (0.009; 0.016) for various discharges. For the same slope and discharge, the results illustrated that the head velocity increases in the reaches expanded up to 20 degrees, compared to that for a uniform cross-section reach. As anticipated, the velocity head increased directly with the bed slope.