SEDIMENTATION AND RUNOFF ONTO THE REEF SURROUNDING ROATAN, BAY ISLANDS, HONDURAS

The island of Roatan , the largest of the Bay Islands, a chain of Caribbean islands roughly 60 km off the north coast of Honduras, is surrounded by a barrier and fringing reef. It has been estimated that this small island of approximately 160km² has as many as 82 watersheds. The reef surrounding Roatan, one of the world's most beautiful, is experiencing sedimentation rates that seasonally are 10X those reported from other reefs in the Caribbean. As part of a larger study on the Roatan reef, we are examining the rate of sedimentation, total suspended solids in the reef water, and sediment composition. Some of the sediment accumulating in traps on the fore reef are terrigenous in composition, which raised the question, "what is the contribution of terrignenous material to the reef from the numerous small watersheds?" a question made more relevant by the rapid land use change the island is currently experiencing. Initially we chose 5 watersheds for study however difficulty of access led to only 2 watersheds being studied in detail, seasonally over a 2 year time period. We did not measure stream bedload, so terrigenous loading values presented here represent suspended load only. Because our previous studies indicate that the bedload is trapped on the shoreface and does not mix into the adjacent lagoon we feel that measuring the suspended load is an adequate estimate for what material might ultimately move across, and impact, the reef.

permanent stream				ephemeral stream
1999 fairweather		2001 fairweather		1999 fairweather	2001 fairweather
Q=13,105cm3/sec		Q=56,800cm3/sec		Q=0	Q=0
tss=4mg/l		tss=234mg/l
loading=52.5 g/sec		loading=13,291g/sec
1999 storm		2001 storm		1999 storm	2001 storm
Q=2,289cm3/sec		Q=537,882cm3/sec		Q=639cm3/sec	Q=1,185cm3/sec
tss=74mgl/l		tss=2,280mg/l		tss=14,170mg/l	tss=15,000mg/l
loading=170g/sec		loading=1,226,391g/sec		loading=9,054,630g/sec	loading=17,775000g/sec

We are currently using a fingerprinting technique on selected trace elements in the suspended sediment to see if variations in concentrations reflect bedrock composition on the island. This will enable us to determine how "closed" individual watersheds on the island may be, an important factor on an island characterized by highly variable land use.