Response of the Length and Stratification of the North River Estuary to Changes in Forcing

Abstract:

The response of the length and stratification of the North River estuary (MA) to variations in river discharge Q_f and tidal amplitude U_T is analyzed using time series from four along-channel locations in conjunction with cross- and along-channel boat board surveys collected from April 4 to June 22, 2006. The North River is a generally well-mixed estuary with a channel depth of 5 m and width of 70 m. Its length L₅ is defined as distance between the mouth and the location where bottom salinity reaches a value of 5 psu, determined by approximating a linear along-channel salinity gradient. During the period of low discharge (Q_f < 5 m³/s) spanning April, the average length of the estuary is relatively high and varies based on tidal amplitude. Lengths are typically 7-9 km for neap, 8-11 km for intermediate, and 10-14 km for spring tides. During two storm events marked by high discharge (Q_f >15, reaching 85 m³/s) mean length remains shorter, at 6-9 km. Length of the estuary has a weak dependence on discharge and a positive relationship to tidal amplitude, L₅ ~ Q_f ^-0.135 U_T ^0.611. Stratification exhibits an increase with increasing discharge and a decrease with increasing tidal amplitude at mid-estuarine locations as ΔS ~ Q_f ^0.443 U_T ^-1.22. Within tidal cycles, peak stratification is typically observed at the start of ebb. Using a one-dimensional salt balance, the along-channel diffusivity K is defined as K = Q_f L/A, where A is cross-sectional area. K has a strong dependence on river discharge, and less so on tidal amplitude. It ranges from 100-200 m²/s during low discharge periods and peaks at 1200 m²/s during storm events. Although a simplified linear model of the salinity gradient was assumed, insight was gained as to how the estuary responds to river discharge and tidal variability.