INFILLING OF TIDAL FRESHWATER RIVER COVES – OBSERVATIONS AT MULTIPLE TIME SCALES

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 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. At monthly time scales, samples from sediment traps suggest that higher accumulation rates correlate with low freshwater river discharge and more negative δ­­¹³C values. However, neither river turbidity nor salinity values display strong correlations with monthly accumulation rates. Observations at the sub-tidal timescale suggest that small temperature differences between river and cove water may play a large role in efficiently storing remobilized estuarine sediment within these coves. At current rates of infilling, many TFWB’s will not persist beyond the end of this century as suitable harbors or fish rearing environments.