long, 0.75 km at its widest point, and covers an area of around 1.5 km 2 . A narrow, shallow-water moat extends along the inner edge of the bay, adjacent to the coastline. This gives way to an extensive reef flat, and a reef margin characterized by a well-devel- oped spur and grove formation (Randall and Holloman, 1974). Bot- tom substrates within the bay range from soft alluvial mud around the river mouth at the southern end, to coarse carbonate sands and coral rubble further north. The bay harbours a relatively rich diver- sity of marine life and supports a variety of scientific, commercial and recreational activities, including harvesting by local residents of many of its fisheries resources for food. Pago Bay receives continuous drainage from the Pago River sys- tem, a complex of three rivers that drains a catchment area of approximately 27 square kilometres inland. One of these rivers, the Lonfit River, receives leachate from the island's only municipal landfill located in the village of Ordot, about 3.5 km upstream of the estuary. The Ordot landfill has been in continuous use for over 50 years and receives about 75 m tally sensitive area have never been investigated despite some con- siderable speculation by concerned members of the local community. The study described herein is the first of its kind to ad- dress these issues. The Pago River drains into the southern half of Pago Bay and is highly seasonal. The river has formed a channel that cuts com- pletely though the reef flat and drains much of the water coming over the reef margin at high tide back into the ocean. Sedimentary deposits within the bay are largely confined to the moat and inter- tidal zone and are virtually absent on much of the reef flat. The composition of these deposits varies appreciably within the bay, with surface sediments at the north-eastern end composed largely of bioclastic (biogenic) carbonates (e.g., foraminifera, coral, shells, Halimeda debris and calcareous red algae) while volcanic detrital material predominates at the south-eastern end, adjacent to the river mouth (Randall and Holloman, 1974). A mixture of the two sediment types occurs to varying degrees in between. Currently, conspicuous banks of silt and clay have accumulated in the inter- tidal zone on both sides of the river mouth as a result of soil ero- sion processes further upstream. The extent of deposition of this material is controlled largely by rain events, which also play a sig- nificant role in purging the bay of accumulated sediments when major storms occur. Groundwater seepage occurs at various points along the beach north of the river mouth for 1.5 km and a major spring discharges into the bay 200 m south of the river mouth. During the wet season, the central part of the bay is heavily inun- dated with urban runoff from a nearby residential area. The north- ern part of the bay also receives runoff and septic system wastes from the lower University of Guam campus. Biota are unevenly distributed throughout the bay. At the time of this study, conspicuous patches of seagrass (Enhalus acoroides) occurred in the muddy moat sediments on both sides of the river channel in the southern half of the bay and provided a suitable habitat for several species of bivalves including Ctena bella, Gafrar- ium pectinatum, and Quidnipagus palatum. Of the common brown algae encountered, Sargassum cristafolium dominated the outer reef flat along the entire length of the reef margin, while Padina boryana was the most abundant species in the moat area, where the seacu- cumber, Holothuria atra, was also reasonably well represented.