Multi-decadal patterns of fluvial morphodynamics and lahar hazard in response to episodic eruption of Soufrière Hills Volcano, Montserrat, Eastern Caribbean

Volcanic eruptions disturb the hydrogeomorphic dynamics of proximal fluvial systems, often with hazardous consequences. Studies to date have examined the effect of either transient (i.e., relatively short-lived, lasting days to a few years) or persistent (i.e., lasting decades/centuries) volcanic sediment production on rivers. By contrast, Soufrière Hills Volcano (Montserrat, Eastern Caribbean) has exhibited distinctly episodic activity – phases of sediment production via lava dome growth/explosions lasting up to two years, punctuated by phases of quiescence – since the onset of its most recent eruptive period in 1995. Here, we examine the Belham River Valley, a small fluvial system affected by this eruption. We synthesise a range of data relating to this valley to develop a conceptual model of catchment-scale controls on fluvial responses to episodic volcanic disturbance. We observe that morphodynamics are controlled by the interplay of variable water supply, sediment supply, and vegetation cover. In general, successive episodes of sediment production are accompanied by heightened lahar hazard and downstream valley floor aggradation, followed by channel incision and reduced hazard during quiescent periods. We propose that episodic eruptions induce distinct hydrogeomorphic responses that could be considered hybrids of those driven by transient and persistent modes of volcanic sediment production.