Joint isotopic mass balance: a novel approach to quantifying channel bed to channel margins sediment transfer during storm events

The important role of floodplains and the broader riparian zone in providing temporary storage for a large fraction of the annual sediment load of rivers is well established, but this understanding is largely based on observations of the long‐term average behavior of the catchment. Here we combine measurements of the fallout radionuclides ⁷Be and ²¹⁰Pb and the stable isotopes of hydrogen in water to quantify fine sediment mobilization and storage in a stream and its channel margins during individual intermediate‐sized storm events with recurrence intervals of a few months or less. We demonstrate this method using five storm events in a small (~15 km²), undeveloped, gravel‐bedded tributary of the Connecticut River (USA). We estimate that in each storm, the mass of sediment deposited onto the margins accounts for almost 90% of the sediment mobilized from the bed, with the remainder of the mobilized bed sediment transported downstream as suspended load. The result that the bed is a net source of sediment to the stream and the margins a net sink is robust, but estimates of the mass of material eroded from the bed and deposited on the margins are less certain. The source of sediment to the bed remains unclear as, consistent with earlier studies, we observe only limited deposition of sediment to the bed during the storm events. The suspended sediment is organic‐rich and thus its source may be associated with in‐channel organic decay between storm events. Understanding the coupled interactions between discharge magnitude and frequency and sediment resupply at the event time scale has important implications for stream restoration efforts seeking to connect the channel and the broader riparian zone, and for the development of accurate sediment budgets and predictions of sediment flux from a watershed. Copyright © 2015 John Wiley & Sons, Ltd.