Assessing the Sources and Age of Fine-grained Channel and Suspended Sediment using Radionuclides

Abstract:

Understanding the sources and ages of stream sediment is important in developing conceptual models of landscape evolution, as well as in developing management strategies to reduce sediment flux. Atmospheric derived cosmogenic radionuclides (⁷Be and ²¹⁰Pb_ex), along with bomb-fallout ¹³⁷Cs, have been shown to differentiate between surface and non-surface derived sediment. Non-surface derived sediment includes streambanks and in-channel sediment storage. Two techniques were used: (1) Presence or absence of ²¹⁰Pb_ex and ¹³⁷Cs indicated surface and non-surface sources and (2) ⁷Be to ²¹⁰Pb_ex ratios were used to date recent (surface) sediment. We have tested the use of ⁷Be, ²¹⁰Pb_ex, and ¹³⁷Cs to source and age fine-grained sediment (<0.063 mm) in small streams (<250 km²) draining regional areas of the United States (Midwest, Southeast, and Pacific Northwest) and in a small watershed of the Chesapeake Bay, Smith Creek (242 km²), Virginia, U.S.A. Land uses in the watersheds vary between agriculture, forest, and urban, and sampling included both suspended-sediment and bed material. Results indicate that non-surface derived sediment (e.g. streambanks) is an important source of sediment in all settings. For example, in 80% of 114 Midwest streams, non-surface derived sediment contributed greater than 70% of sediment. Age determination results indicate that many of the samples include a component of sediment that is 200 days old or younger. We discuss the assumptions associated with using radionuclides and whether rainfall or surface soil should be used as boundary conditions to date sediment. Management implications of these results are also discussed.