BEYOND THE CHARTS: THE GEOLOGICAL LEGACY OF THE HISTORIC NORTH RIVER INLET, MASSACHUSETTS

Former tidal inlets and river mouths comprise substantial portions of many coastal lithosomes, however their locations and dimensions, even when documented historically, are often in question. Due to continuing intense development of barrier coasts, and because channels may reoccupy their former locations, identifying these features is an important component of coastal hazards management. We present an example of a reconnaissance field investigation, consisting of geomorphic mapping and grounding-penetrating radar (GPR) surveys, which complements the historical evidence of the closure of the Old North River inlet in Marshfield, Massachusetts. The breaching of the barrier is documented to have occurred several times since the 1600s. The most recent paleo-channel was located at the confluence of North and South Rivers and closed naturally as a result of a new breach (New Inlet), which formed several kilometers to the north during the 1898 Portland Gale and remains open today. Geomorphic evidence of the former inlet includes a short, 100 m-wide channel remnant in the rear portion of the barrier and a vegetated ridge along the seaward edge of the saltmarsh landward of this feature. Since inlet closure, the area has been occupied by a large dunefield, a common feature at a number of former inlet sites along the U.S. Atlantic Coast. Along the rear portion of the barrier, shore-parallel GPR records show a complex, 600 m-wide channel fill represented by steep sigmoidal-oblique reflections. The deepest part of the fill coincides with the last position of the Old North River inlet and is at least 5 m below present mean high water level. A smaller, 40-50 m-wide channel structure truncates the northernmost portion of the barrier-inlet sequence. The upper part of the sequence is dominated by sub-horizontal and hummocky reflection geometries characteristic of beach and dune stratification. This study suggests that throughout its history, the mouth of the North River has occupied a wide segment of the barrier and, following its closure, contributed large volumes of sediment to the barrier superstructure.