CIRCULATION MODELING, RESIDENCE TIME AND WATER QUALITY IN THE HOUSATONIC RIVER ESTUARY, SOUTHERN NEW ENGLAND

The Housatonic River drains 4,970 km² in southwestern New England and is the largest source of freshwater to the western basin of Long Island Sound. The hydro-chemical estuary extends 22 km north from its mouth at Milford Point to the confluence of the Naugatuck River, while tidal influence extends north to Derby Dam which controls the main freshwater input to the estuary. The circulation of water impacts water quality in the estuary. Exchange processes also influence water quality in the western Sound, which suffers from seasonal hypoxia driven by nutrients that produce algal blooms. Dredging has altered the natural bathymetry of the Housatonic River. The mouth of the estuary was dredged for navigation from a maximum depth of 4 m in 1855 to depths ranging from 6 to 11 m, while the head of the system was dredged for aggregate beginning in 1958 deepening a shallow 3 m deep thalweg to up to 13 m. The resulting bathymetric profile consists of a shallow 7 km long reach in the middle of the estuary separating the two dredged reaches. This unusual profile dramatically affects the normal partially mixed circulation pattern that occurs at high river flow. During periods of low freshwater discharge, brackish water enters the northern reach of the estuary where it causes long-term stratification and bottom water anoxia. Field measurements and flow modeling demonstrate the relationship between water discharge, tidal range, tidal phase and the resulting residence time for advected water particles at the head of the estuary. In general, flows greater than 140 m³/s are necessary to break up deep retention gyres in the bottom waters of the upper basin and limit adverse water quality conditions in the estuary. Because flows of this magnitude occur infrequently during low flow periods in the summer, anoxic conditions persist for weeks at a time. Tidal range and tidal phase during the water discharge events have a smaller influence on circulation residence time in this part of the estuary. Interestingly, nutrient input from offshore is seen to play an important role in the water quality within the upper basin.