The first mathematical study of the propagation of gravity currents can be attributed to T. B. Benjamin. 
Observations of intrusions and collisions between fluids of differing density were made well before T. B. Benjamin's study, see for example those by Ellison and Tuner, by M. B. Abbot 
or D. I. H. Barr. 
J. E. Simpson from the Department of Applied Mathematics and Theoretical Physics of Cambridge University in the UK carried out longstanding research on gravity currents and issued a multitude of papers on the subject. He published an article 
in 1982 for Annual Review of Fluid Mechanics which summarizes the state of research in the domain of gravity currents at the time. Simpson also published a more detailed book on the topic.