SEDIMENTARY RESPONSE OF TIDAL FRESHWATER RIVER COVES TO SALT INTRUSION – IMPLICATIONS FOR SEA LEVEL RISE

Tidal floodplain water bodies (TFWB’s) are pervasive features of rivers draining passive continental margins. These backwater environments connect to tidal freshwater river channels by means of tie channels, many of which have been excavated and cleared of debris to facilitate harbor development and fishing access. Therefore, these coves and ponds now receive sediment and accompanying nutrients and contaminants from the broader watershed due to enhanced tidal pumping. While sediment storage within estuaries and subaerial floodplains has been heavily studied, we have found that relatively understudied TFWB’s play a disproportionately large sediment storage role relative to their small area. Furthermore, levels of mercury within TFWB’s of the Connecticut River reach levels more than three times the maximum reported from San Francisco Bay, often regarded as the poster child for mercury contamination.

Here we present preliminary results of coupled water column monitoring and sediment coring at Hamburg Cove, Connecticut, USA, which lies at the very upstream extent of salt intrusion into the Connecticut River estuary. Observations suggest that a combination of factors acting in concert may serve to maximize tidal pumping of sediment in TFWB’s. Maximum tidal ranges and low summer discharge serve to efficiently introduce turbid saline estuary water into the cove. Sediment accumulation rates are therefore highly seasonal, responding to predictable annual river hydrograph patterns. Accompanying changes in sediment chemistry due to the presence/absence of seawater may cause these sediments to display annual lamina that allow for detailed paleohydrological reconstruction. Because this cove lies directly at the upstream-most extent of salt, it serves as an excellent natural laboratory of changes likely to occur in purely fresh TFWB’s as sea level rises and the salt wedge pushes further up tidal rivers.