SUSPENDED SEDIMENT DYNAMICS IN THE GREAT BAY ESTUARY, NH: PATTERNS AND CONTROLLING PROCESSES

Describing temporal and spatial variations of suspended sediments in estuaries and determining how the major physical forcings influence these distributions require detailed observations to be made at several scales of space and time. For this study, observations of temperature, salinity, turbidity, and suspended sediments were made over the entire length of Great Bay Estuary, New Hampshire from the marine boundary at Portsmouth Harbor to the major source of freshwater in Great Bay. Sampling was done during different seasons, river discharge events, and weather conditions. In total, eleven cruises were conducted from February 22, 1997 to April 19, 1999. From these observations variations in the overall distribution and composition of suspended sediments in response to seasonal changes, fluctuating river discharge, tides, and winds were determined.

An important finding of this study was the strong temporal and spatial variability of the water column turbidity, the concentrations of suspended sediment, and the salinity structure (both longitudinal gradients and stratification). For instance, simple up-estuary gradients of suspended sediments were the exception. In addition, the importance of major shoal areas (Little Bay and Great Bay) as source regions of suspended sediments to the estuary was apparent. Despite this variability, the sampling protocol allowed the local and estuary-wide impacts of two of the major physical forcings (river discharge and wave resuspension) to be evaluated. The results of this study indicate that even though river discharge increased suspended sediment concentrations, the magnitude appeared to be secondary to resuspension events. However, the very highest turbidity and suspended sediment concentrations occurred when river discharge was higher than normal (bringing particulate material into the estuary from the rivers) and wind events caused resuspension of bottom sediments in the major shoal areas.