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The formation process and timing of a dam embankment breach caused by flood overtopping can dramatically impact the rate at which water is released from a reservoir and directly impacts the hazard to life and property downstream of a breached dam. Therefore, dam embankment erosion processes and the rate of breaching from overtopping events are important to both engineers and planners alike, who must predict impacts on local communities and surrounding areas affected by flooding. There is a distinct difference between the erosion processes of non-cohesive and cohesive embankments during overtopping. The USDA-ARS has conducted seven large-scale overtopping failure tests on cohesive embankments ranging in height from 1.5 to 2.3 m. The purpose of this study is to: (1) establish a better understanding of the erosion process of overtopped cohesive embankments, and (2) provide detailed data for future numerical model development, validation, calibration, and testing. Three soils were tested, two non-plastic SM silty sand materials and a CL lean clay. A four-stage breach erosion process was observed for cohesive embankments. The primary erosion processes observed and reported in this article for cohesive embankments during stages 2 and 3 were headcut migration and erosion widening. The rate of these two processes was observed to vary by several orders of magnitude and was observed to be strongly dependent on the soil material properties.