USE OF CONTINUOUS WATER-QUALITY MONITORS TO EXAMINE SEDIMENT AND NUTRIENT TRANSPORT IN THE LOWER CONNECTICUT RIVER DURING TROPICAL STORM IRENE 2011

On August 28-September 5, 2011 tropical storm Irene delivered between 3 and 8 inches of rainfall in the Connecticut River watershed, with higher amounts occurring in the upper part of the watershed in Vermont and New Hampshire. Peak streamflow for tributaries in the upper watershed reached an annual exceedance probability of greater than 1%. In addition to streamflow, continuous water-quality monitors were operated at the USGS streamgaging station at Middle Haddam, Connecticut. The station monitored continuous turbidity in formazin nephelometric units (FNUs), fluorescent dissolved organic matter (FDOM) in quinine sulfate equivalent units, and used an automatic sampler to collect samples for sediments and nutrients through the duration of the storm event.

The monitors recorded changes in the turbidity of the Connecticut River through the storm that were not coincident with the changes in streamflow. During the event the turbidity had two pronounced peaks; the turbidity rose rapidly to over 400 FNUs and then receded to under 300 FNUs and then rose again to 600 FNUs. The streamflow rose steadily from 142 m³/s to a single peak of 340 m³/s , and then receded to 142 m³/s. The changes in the turbidity corresponded well with the changes in suspended sediment and phosphorus concentrations measured in samples collected during the event. Both constituents showed an early peak in concentration that corresponded to the first peak in turbidity and then a second larger peak in concentration that corresponded to the second peak in turbidity and in streamflow.

Continuous turbidity data from the monitors also were used to correct the continuous FDOM data for attenuation of signal due to high concentrations of suspended sediment. This allowed for the corrected FDOM data to be used and calculate peak values of dissolved organic carbon (DOC) of 6.0 mg/L. DOC concentrations also had a double peak in concentration during the event with the second, larger peak in concentration occurring coincident with the peak in streamflow.