Sedimentation as a control of reef development: St. Croix, U.S.V.I.

Abstract

Sediment-transport rates in and around Salt River submarine canyon, St. Croix, U.S.V.I., were measured over a 2-year period using bedload traps and Legrangian sediment-tracer experiments. Sodiments generatly move in a westerly direction along the north shore of St. Croix. Consequently, sediment moves into the canyon at a higher rate over the eastern margin (47,000 kg/m-year) than over the west wall (19,000 kg/m-year). As a result, extensive reef growth is limited to the west wall, and the inner portion of the eastern margin is a cobblecovered slope inhabited primarily by gorgonians and a few sediment-tolerant corals. Sedimentation exhibits similar controls on other reefs around the island. At Cane Bay, sedimentation rates are lower than at Salt River, and reef development is accordingly greater. Along the east side of Christiansted canyon and the downdrift margin at Sandy Point, extreme sedimentation has completely buried the reefs. Sediment transport was an order of magnitude higher during storms than during fair weather. Under all conditions measured, the amount of sediment exported from the canyon was considerably below that entering over the canyon walls. It is hypothesized that major hurricanes periodically flush the excess, and keep long-term sediment influx and export in balance.