The Cabled Observatory Vent Imaging Sonar (COVIS) was installed on the Ocean Observatories Initiative’s Cabled Array observatory at ASHES hydrothermal vent field on Axial Seamount in July 2018. The acoustic backscatter data recorded by COVIS, in conjunction with in-situ temperature measurements, are used to investigate the temporal and spatial variations of hydrothermal discharge within ASHES. Specifically, sonar data processing generates three-dimensional backscatter images of the buoyant plumes above major sulfide structures and two-dimensional maps of diffuse hydrothermal sources within COVIS’s field-of-view. The backscatter images show drastic changes of plume appearance and behavior that potentially reflect episodic variabilities of vent fluid composition and/or outflow fluxes. The diffuse-flow maps show that the areal extent of hydrothermal discharge on the seafloor varies significantly with time, which is largely driven by bottom currents and potentially tidal loading. These findings demonstrate COVIS’s ability to quantitatively monitor hydrothermal discharge with sufficient spatial and temporal coverage to provide the research community with key observational data for studying the linkage of hydrothermal activity with oceanic and geological processes during the dynamic period leading up to the next eruption of Axial Seamount.