LABORATORY gravity currents are frequently used to model a range of en
vironmental and industrial flows1. The manner in which these flows bec
ome diluted with distance by the surrounding fluid has important impli
cations for turbidity currents2, pyroclastic flows3,4, avalanches5, ac
cidental dense gas releases6, fire propagation7 and emission of indust
rial pollutants. Here we present an experimental technique for quantif
ying the entrainment of ambient fluid into the head of a gravity curre
nt propagating along a horizontal surface. The technique relies on the
neutralization of an alkaline current by entrainment of acidic ambien
t fluid, and is visualized by using a pH indicator. Dimensional analys
is indicates that the proportion of ambient fluid entrained into a gra
vity current head depends only on the initial volume of the current an
d distance from the release point, and is independent of the initial v
alue of the density difference. This result is confirmed by the experi
mental data, which also show that little dilution of the head takes pl
ace during the slumping phase8,9. Thereafter the dilution increases wi
th the downstream distance, in quantitative agreement with the predict
ions of a theoretical model which evaluates the volume of entrained fl
uid. We apply the results to show that sediment slumps of initially hi
gh sediment concentrations will become dilute turbidity currents owing
to entrainment of sea water before they have propagated extensively o
ver the floors of ocean basins.