SEDIMENT TRANSPORT VARIABILITY IN A BEDROCK-CHANNELED, HIGH-LATITUDE ESTUARY, SACO BAY, MAINE

The Saco River is a major source of sediment for the Saco Bay beach system, providing an estimated 10,000-16,000m³ of sand per year, primarily during the spring freshet. The 1867 construction of a jetty on the north side of the inlet has altered the Saco River's ability to deliver sediment to the bay and changed the rate and patterns of sand movement throughout the bay. The jetty system now channels the Saco River and extends the estuary 2032 m out to sea and the river mouth must be dredged every 10 years. Beaches adjacent to the northern jetty have eroded severely in the past century, with neighboring Camp Ellis suffering the loss of 33 properties since 1968.

Previous estimates of river sediment output were based on limited measurements of upstream sediment discharge and the shoaling rate of the Camp Ellis anchorage at the river mouth. To better constrain the rate of sand introduction to the Saco Bay from the Saco River in April 2005 a series of instruments were deployed on three moorings inside and outside the Saco River jetty system. Elevated nearbed current velocities and diminished nearbed salinity characterized high fluvial discharge events. From these measurements a discharge threshold was constructed for net offshore bed-load sediment transport. The threshold envelope was then put into a historical context with the 89-year discharge record obtained from the US Geological Survey's stream gauge station on the Saco River at Cornish, ME. These results indicate that the role rock-bound estuaries play in sediment delivery and dispersion varies along seasonal and annual time scales.