Mapping the Sea Floor

Figure 2. Top: Sea-floor topography in the region of the future ocean outfall. The submerged hills (tops about 20 m from the surface and shown capped in red) in this region are similar in size and shape to the harbor islands. They formed during the last glaciation and submerged during the rise in sea level after the glaciers melted. Bottom: Reflectivity of bottom sediments, measured by sidescan sonar and superimposed on the bathymetry, showing a complex pattern that indicates differences in the sea-floor environment. Areas of boulders are represented by the lightest tone and are typically found on the crests of the small submerged hills. The intermediate gray tone is sand with varying amounts of gravel. The darkest areas represent deposits of fine-grained sediments, often in the topographic lows between the hills. Long-term sampling of the sediments is conducted in two areas of fine-grained sediments. Currents and suspended sediments are monitored at the long-term mooring site. The stripes represent the ship track and are about 150 m apart. (See Bothner and others, 1992.)

Maps of the sea-floor geology identify sampling sites for monitoring of long-term environmental change (fig. 2). Areas of fine-grained sediments are often the best areas to monitor changes in contaminant concentrations. Using high-resolution sea-floor maps ensures selection of sites having similar texture and sufficient size that they can accommodate long-term monitoring and provide samples that are directly comparable. The maps also ensure that samples can be efficiently obtained without damage to sampling equipment, as frequently occurs in areas of rough or hard bottom. The USGS, as part of the Massachusetts Water Resources Authority (MWRA) Outfall Monitoring Program, has established sediment monitoring stations at two sites in the vicinity of the new ocean outfall. Samples have been obtained at these sites three times each year since 1989.