MIOCENE TO QUATERNARY PALEOCHANNELS OF WASHINGTON D.C. - STRUCTUAL INFLUENCES, GEOMORPHIC AND HYDROLOGICAL CONSEQUENCES

The valleys, terraces, scarps (up to 18 m), and hills of Washington D.C. provide a visible record of erosional downcutting (~130 m relief) by the Potomac and Anacostia River paleochannels and their tributaries, despite urban modification that removed hill tops, partly filled valleys, made streams into sewer lines, and covered downtown and water-front areas with artificial fill up to 9 m thick. Beneath this modified landscape is a complex partially preserved record of incision and infilling. Compilation of available subsurface data including recent core and geophysical data from geotechnical investigations for Washington D.C.’s new sewer system reveals numerous paleochannels with complex drainage patterns and buried scarps up to 12 m. Structural influences explain both the sharp turns of the paleochannels occur across documented and proposed fault zones (includes now buried streams like “Slusher Run”), and paleochannel entrenching on the down side of the faults. Thicker fill amplifies the potential hazard from seismic shaking. Miocene Potomac River paleochannels preserved on many highest hilltops and Pliocene to early Pleistocene at midlevel terraces (10-30 m elev.) have mostly narrow, shallow (up to 15-m-thick) paleochannels filled with sand and gravel (locally arkosic) indicating a braided-alluvial fan deposition. The preserved river systems show a change during the Pleistocene from braided-alluvial fans and small tributaries to large meandering rivers and tributaries. The lowland terraces (<8 m elev.) have broad, deep (up to 45-m-thick) late Pleistocene and Holocene paleochannels. These paleochannels are filled with a complex pattern of regressive, fluvial, poorly sorted sand and gravel; and transgressive, estuarine to swamp, clay and silt indicating meandering rivers. In addition to the fill’s highly variable lithology, the base of these paleochannels is very irregular and can change abruptly in only a few meters which is important to ground water modeling and subsurface engineering projects. The highly permeable sand and gravel facies in these paleochannels acts as conduits for groundwater, shown by numerous springs located along paleochannel scarps. Structure also apparently influenced intersection of the Potomac and Anacostia River paleochannels across the lower Anacostia River Valley.