CHANGES IN SUSPENDED SEDIMENT CONCENTRATION IN THE URBANIZED BLACKSTONE RIVER WATERSHED DURING RAINFALL EVENTS, MILLBURY, MASSACHUSETTS

Human activities can have profound effects on drainage basin hydrology and sediment transport. We investigated the relationship between suspended sediment concentration (SSC) and discharge (Q) during nine precipitation events between 9/30/2014 and 11/1/2014. Our study reach was on the highly urbanized Blackstone River located in Millbury, Massachusetts, one third of a mile downstream of USGS Gauge #01109730. We collected samples with a depth-integrated sampler, separated the solid fraction using vacuum filtration, and measured SSC and POC using standard oven and furnace procedures. Finally, we compared the SSC and POC values to instantaneous discharge. We find that SSCs are significantly affected by Q. Average low-water Q is 53.4 cfs, and typical low-water SSC is ~ 3 mg/L. However, once a threshold of 75 cfs is exceeded, SSC first drops, then increases rapidly, peaking up to 65 times the low-water value. Once the rate of change for Q is zero, SSC begins to drop back to its typical low-water value. We observe this pattern of SSC rise and fall in all nine rainfall events. We had predicted that the non-urbanized tributaries and distal sources such as roads would add sediment to the Blackstone River affecting the relationship between Q and SSC and that sediment would arrive late in the storm to our main-stem study site. However, our results suggest instead that suspended sediment in this reach of the Blackstone River is likely being resuspended from the channel bed into the water column during storm events; it does so when it crosses the Q threshold of 75 cfs. The river’s steep, step-pool river profile and presence of several defunct mill dams upstream may be the predominant factors controlling the relationship between SSC and Q. We propose that during storm events, SSC first decreases as Q increases due to dilution. Then fine sediment in the channel and trapped behind these dams are mobilized when Q reaches 75 cfs at the Millbury gauge. SSC can reach values as high as 195 mg/L during moderate rainfall events, potentially influencing the quality of aquatic habitats in the region several times a year. This resuspension of channel sediment during moderate hydrologic events has implications for stream restoration and water quality, and suggests that the importance of mill dam sediment should be investigated further.