Explosive eruptions of mafic magmas produce lava fountains whose heights are a function of the exsolved volatile content of the magma, its erupted mass flux, and the geometry of the vent (which may be an elongate fissure or a localized, near-circular conduit). The geometry of the initial vent (and the eruptive behaviour) can be distinctly modified by lava drainback and accumulating ejecta. Hot pyroclasts landing near the vent may coalesce to form rootless flows, some of which may drain back into the vent to be recycled into the eruption products. Rootless flows may be at least partially confined by pre-existing topographic features, or by spatter or cinder ramparts being built up by the eruption itself, so that they accumulate into a lava pond over and around the vent. The erupting jet of magmatic gas and pyroclasts must force its way through such a pond and will entrain some of the pond lava as it does so. The energy expended in entraining and accelerating previously erupted materials will reduce the eruption velocity and the lava fountain height by an amount which can be calculated as a function of the eruption conditions and the lava pond depth (or lava drainback rate). The results of such calculations are presented, and are used to assess the influence of this process on attempts to infer magma volatile contents from field observations of lava fountain heights.