CONTRIBUTION OF COARSE-GRAINED SEDIMENT TO THE NEARSHORE AND INNER SHELF BY LARGE ESTUARIES: NEW ENGLAND, USA

An examination of lower reaches of three large river systems in northern New England has shown that high-latitude estuaries can be effective coarse-grained sediment conduits to the coastal ocean. Hydrographic measurements, sedimentological analyses, side-scan sonar records, and LIDAR surveys were used to document the bedload transport in the Kennebec and Saco River estuaries, Maine, and Merrimack River estuary, Massachusetts. The lowermost Kennebec River is a relatively narrow, deep, bedrock-bound estuary, whereas Saco and Merrimack estuaries have lower relief and are jettied. The freshwater discharge ranges from 85 m³ /sec for the Saco River to 280 m³ /sec for the Kennebec River. Long-term bedload discharge from the rivers is evidenced by lowstand deltas, associated shelf sand bodies, and extensive onshore barrier systems containing 22-115x10⁶ m³ of sand.

Estuarine sediments are compositionally immature ranging from medium sand to fine gravel. These deposits are traced to the nearshore region immediately seaward of the estuary mouths and grade seaward into fine to very fine sands. Bedforms in the estuaries range from megaripples and sandwaves to transverse bars reaching 0.3-0.5 km in length and over 10 m in height. Under normal discharge conditions, the smaller bedforms exhibit variable orientations, however, the transverse bars remain ebb-oriented. During major discharge events all bedforms exhibit ebb dominance. Riverine sediment bypasses dams and is introduced to the estuary during spring freshets when freshwater discharge may increase by an order of magnitude. The influx of sediment infills harbors and jettied channels. During this time, some of the bedload is exported to the nearshore. The welding of nearshore bar complexes at the mouth of the Kennebec River adds 2-300,000 m³ of sand to the barrier shoreline every 6-9 years. The active fluvial bedload supply is also proposed to be an important source of sediment to the adjacent barrier systems since mid-Holocene. In New England, the effects of large floods and strong bedrock influences produce patterns of sedimentation and a distribution of sand bodies at the mouths of major estuaries that differ markedly from accepted estuarine models.