CATASTROPHIC VALLEY ENTRENCHMENT AND DEBRIS FAN FORMATION IN THE BLUEFIELDS RIVER, WESTMORELAND JAMAICA

Little is known about the geomorphic response of Jamaican rivers to climate, geology, and historical human disturbance. Extreme rainfall (32 cm of rain on June 12, 1979) along the southwestern coast of Jamaica led to valley entrenchment and debris fan formation along the Bluefields River. Reports indicated that valley areas formed temporary lakes that overflowed drainage divides to concentrate floodwaters to record stages in steep streams producing debris flows and sediment fans. This study investigates the effects of this extreme rainfall and flooding on the geomorphology of the present-day Bluefields River near Belmont, Westmoreland Jamaica. The river drains 4.9 km² of limestone foothills and mountain slopes from a peak elevation of 760 masl. The Bluefields River was entrenched up to 9 meters along its middle and lower reaches and formed a large debris fan out into Bluefields Bay. Cross sections, longitudinal profiles, soil profiles, and alluvial stratigraphy are used to identify landforms and variations in bed and bank morphology along the Bluefields River. The objectives of this study are to: (i) describe the geomorphic effects of a catastrophic flood in 1979; (ii) calculate sediment delivery to the debris fan from valley incision; and (iii) evaluate the potential role of colonial land disturbance and soil erosion leading to the destabilization of the Bluefields River in 1979. Bluefields Bay is now a fish sanctuary and understanding sediment inputs into the bay over different timescales is important for the future protection of the bay and the local economy.