Water depth and bathymetry are important for shipping in the Duluth/Superior Harbor. When harbor water levels are low, freighters must lighten their loads in order to use the harbor. Lighter loads represent less freight carried and less revenue for shippers.
Researchers from the Large Lakes Observatory used the R/V Blue Heron's multibeam sonar to survey the harbor in July and September of 2007, a period of record low lake levels on Lake Superior. The survey area is illustrated on the image to the right, with the dark lines representing the survey tracklines. The survey area is divided up into four subsections, each of which can be viewed separately by selecting the subsection's name on the chart.
The most prominent features on the multibeam images are the areas with especially deep water (>11 meters) found just inside both the Duluth and Superior entries and at the northern edge of the East Gate Basin. These areas are where the large freighters that enter the Harbor must brake and turn, and are probably an expression of bottom sediment being eroded by currents created by the ships' propellers as they increase speed after making the turn.
Additional man-made features include anchor drag marks, as well as the deeper water found in a channel to the east of the main channel of the Duluth Harbor Basin, probably an indication of sediment being moved by currents created by the propellers of freighters that are 'cutting the corner' and taking a shorter route to the East Gate Basin. More natural features include scour marks found near the Duluth and Superior entries presumably caused by inflowing and outflowing water generated by the Nemadji and St. Louis Rivers. These features are approximately 20 cm in depth, 4 meters wide and 100 meters long and are best seen near the Superior entry.
The survey area represents approximately 130 km of survey lines covering an area of 7km2. While a multibeam swath system does give us multibeam data from a wide area underneath the boat, because of the shallow waters in the harbor the swath is not as wide as it would be in deeper waters, therefore there are sometimes gaps of missing data between tracklines. Even so, multibeam data is much more efficient than single track echosounding, which only collects data directly underneath the vessel.