Technical Abstract: The formation of ephemeral gullies can significantly increase soil losses from agricultural lands and severely impact farm productivity. Yet current soil-loss assessment programs are unable to address these erosional channels. Moreover, few data exist on the physical characteristics of actively eroding gullies. To this end, pre-formed ephemeral gullies were constructed at field scale in a large experimental channel using the same cohesive soil but at two different moisture contents and bulk densities. For the bed with relatively high soil moisture content and bulk density, clear-water flows caused low rates of erosion, detachment-limited flows prevailed, and uniform bed degradation along the flume. For the bed with relatively low soil moisture content and bulk density, comparable clear-water flows caused high rates of erosion, both detachment-limited and transport-limited flows prevailed, and bed degradation was greater in the upstream reaches. In both experiments, erosion caused the gullies to incise, to significantly increase the gully bottom width, and to steepen the gully sidewalls. These results compare favorably with field studies of ephemeral gullies, and the hydraulics and sediment transport processes observed are similar to those reported for actively eroding rills.