Particulate matter in urban rivers transports a significant fraction of pollutants, changes rapidly during storm events and is difficult to characterize. In this study, the physical speciation of trace metals and organic carbon in an urban river and upstream headwaters site in Torrington, CT during a winter rain-on-snow event were measured and compared. In addition, a selective fractionation scheme, using membrane and tangential-flow ultrafiltration methods to separate suspended particulate matter into sand, silt, clay, and colloid fractions, was evaluated based on the relevance and appropriateness of the chosen size categories. During peak runoff at the urban river site, total-recoverable concentrations of the metals Cu and Pb increased 6 and 13 fold to 16.9 and 9.5 5gL -1 , respectively, compared with baseflow concentrations. Concentrations of Cu and Pb reached only 0.9 and 0.86 5gL -1 at the headwaters site. For the measured storm event, the majority of metals were transported by the urban river in association with coarse silt (20 to 80 5m particle diameter) during peak runoff. Fine silt (1 to 20 5m particle diameter) and large colloids (0.1 to 1.0 5m particle diameter) were also important to the physical speciation of metals during storm runoff. During peak runoff at the urban site, organic C associated with the large colloid fraction (0.1 to 1.0 5m) increased from 5% (at baseflow) to 54% of the total C in transport, while dissolved organic C and that associated with smaller colloids decreased from 91.5% (at baseflow) to 41% of the total. Other elements that were monitored as part of the urban stormwater study were Na, K, Ca, Mg, Fe, Mn, Al, Cd, Cl - , NO 3 - , and SO 4 2- . The chosen fractionation scheme was useful to characterize metal transport during this event, but it should be revised to cover the entire size range of aquatic particulate matter. Further testing should be undertaken to determine the most appropriate size range categories, and to ensure that the sizes measured are comparable to those used in other studies.