Offshore of Park Point
Park Point is a barrier island that has formed across the mouths of the St. Louis and Nemadji Rivers at the western end of Lake Superior, enclosing the Duluth/Superior harbor. Researchers from the Large Lakes Observatory have used the R/V Blue Heron's multibeam sonar and side scan/CHIRP sonar systems to image many features in the shallow nearshore waters off of Park Point. Both of these systems use sound to create images of the lake bottom.
The lake bottom feature that the R/V Blue Heron travels over the most is the wreck of the Thomas Wilson. The Wilson was a whaleback freighter that sank in 1902 outside of the Duluth entry to the Duluth/Superior Harbor after being struck by another ship. To the right is a side scan image of the Wilson, which is currently in ~70 feet of water (you can click on the image to see the ship from a different angle). You can clearly see the bow and stern of the vessel as well as several of the hatches to the ship's hold. The dark area to the right of the ship is the 'acoustic shadow', an area of the lake floor that sound waves from the side scan sonar can not image because the ship blocks them.
The Nemadji River flows from Wisconsin into Lake Superior through the Superior entry of the Duluth/Superior Harbor. Approximately 4,000 years ago, lake level in Superior was significantly lower, allowing the Nemadji River to form a river channel to the north of the current Superior entry to the harbor. This channel is currently submerged and Large Lakes Observatory researchers have imaged the channel out to 20 meters of water depth. Multibeam sonar (left) and side scan sonar (right) images of the submerged channel are below.
The multibeam image is a representation of water depth. Lighter colors (white, yellow) are shallower water depths (13 to 15 meters), while blues represent deeper waters (18-20 meters). We can see from this image that the sinuous river channel is delineated by river banks that are 1 to 2 meters higher than the bottom of the channel. The side scan image is an indication of the amount or strength of the sound reflecting from the lake floor. Darker grays represent stronger reflectors such as sands or bedrock while lighter grays reflect weaker reflectors such as clays. The side scan image indicates that the banks of the submerged channel consists of stronger reflectors such as sand.
The image to the right is a side scan image of the municipal water intake off of Park Point. The line from the lower left to the upper right in the image is the pipeline from the shoreline to the water intake crib in the upper right corner of the image. The two white marks in the upper right corner of the image are the acoustic shadows of the water intake crib, giving an indication (calculated from the location of the side scan relative to the crib) that the crib stands several meters above the lake floor.
The linear and curvilinear marks on the two side scan images below are examples of anchor drags in the shallow water sediments off of Park Point. These features are found throughout the side scan images of the Park Point area. Freighters often anchor off of Park Point. When retrieving their anchors or during storms, the anchors will make indentations in the lake bottom sediment.
To the right is an image of sand waves on the bottom of the lake in approximately 20 meters of water depth. We find more sand waves on the lake floor off of Park Point near the Duluth entry. They are less common off of Park Point near the Superior entry, where there is more exposed bedrock on the lake bottom.
Below is a chart of western Lake Superior showing the location of the wreck of the Thomas Wilson, the Nemadji submerged river channel and the public water intake. This is a section of NOAA chart 14966. You can look at more NOAA charts and the User's agreement online.