Watershed Best Management Practices (BMPs) are often designed to reduce loading from particle-borne contaminants, but the temporal lag between BMP implementation and improvement in receiving water quality is difficult to assess because particles are only moved downstream episodically, resting for long periods in storage between transport events. A theory is developed that describes the downstream movement of suspended sediment particles accounting for the time particles spend in storage given sediment budget data (by grain size fraction) and information on particle transit times through storage reservoirs. The theory is used to define a suspended sediment transport length scale that describes how far particles are carried during transport events, and to estimate a downstream particle velocity that includes time spent in storage. At 5 upland watersheds of the mid-Atlantic region, transport length scales for silt-clay range from 4 to 60 km, while those for sand range from 0.4 to 113 km. Mean sediment velocities for silt-clay range from 0.0072 km/yr to 0.12 km/yr, while those for sand range from 0.0008 km/yr to 0.20 km/yr, 4-6 orders of magnitude slower than the velocity of water in the channel. These results suggest lag times of 100-1000 years between BMP implementation and effectiveness in receiving waters such as the Chesapeake Bay (where BMPs are located upstream of the characteristic transport length scale). Many particles likely travel much faster than these average values, so further research is needed to determine the complete distribution of suspended sediment velocities in real watersheds.

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Fallout radionuclides (FRNs) and their ratios, such as Beryllium‐7 (7Be) and excess Lead‐210 (210Pbxs), have been used to determine suspended sediment source and age in catchments. These models are based on numerous assumptions, for example, that channel deposition of FRNs from precipitation is negligible in comparison to their delivery to the channel from land surface erosion during individual storm events. We test this assumption using a mass balance approach during eight storms from summer 2011 to fall 2012 in a mid‐Atlantic United States piedmont region watershed with mixed land use. Event peak discharge and storm type corresponded to the importance of direct channel FRN deposition from precipitation. During relatively low discharge summer thunderstorms, with minimal overland flow, less than 1% of 7Be and 210Pbxs flux deposited on the watershed exits the watershed associated with suspended sediment. The majority but not all deposited on the stream channel exits the watershed associated with suspended sediment (60% of 7Be and 80% of 210Pbxs). Here precipitation and throughfall onto the wetted channel area can be responsible for any FRN newly associated with suspended sediment, as opposed to landscape surface erosion. Furthermore, FRNs can be stored with sediments in the channel between events. Events with higher discharges, including hurricanes, show the opposite pattern—FRN flux associated with suspended sediment exported from the reach is greater than channel FRN wet deposition, suggesting net erosion from the watershed landscape and/or stored material during these types of storms.