Numerous anthropogenic features and deposits were imaged, including cable crossings, anchor drags, old piers, debris fields, and several obstacles, some of which are clearly imaged shipwrecks (Fig. 5.12 and color plate 2). The wrecks appear to be most easily located on the multibeam data because there is often a topographic anomaly such as a moat or sediment drift created by flow around the obstacle. The existence and images of the shipwrecks in the river provides some new challenges. Publishing maps that reveal wreck locations is prohibited until they can be given an appropriate protective status. Additional studies are now underway to inventory, identify, andprotect these submerged artifacts. A major debris field is found adjacent to Hook Mountain, close to the old Rockland Ice Pier, which may be rock debris dumped offshore during construction of the Hook Mountain ice chute in the early 1800s. The Rockland Ice Company provided ice for places as far away as Florida through the late 1800s. This debris field overlays the west channel bank sediments. This field is comprised of a series of doughnut-shaped structures, some of which form linear chains. These structures are 35-45 m in diameter and rise to 50 cm above the riverbed. The field extends for over 2.5 km along the channel margin and is ~500 m wide. A series of five narrow (<5 m) linear mounds ~350 m long that extend at oblique angles to the channel margin are found south of the major debris field. Anchor drags are imaged within the southern portion of the channel axis along the northwestern edge of the channel margin. Cable crossings are imaged off Nyack as three undulating low-backscatter lineations that extend diagonally across the western marginal flats toward Tarrytown. Mooring fields associated with Peterson's Boat Yard and the Nyack Boat Club are also identified.
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