Sorting and gravel bed-penetration of silt during fluvial transport

Understanding the transport behavior of fine particles informs our understanding of the sedimentary record as well as the transport and fate of weathering products, contaminants and nutrients. Sorting and bed-penetration of silts during fluvial transport are poorly-understood functions of the rates of particle arrival to, entrainment from and penetration and sequestration immediately below the sediment-water interface. We conducted laboratory and field experiments to examine these processes, and report the following: 1) Rate of silt arrival at a stream bed is a function of flow and turbulence intensity. 2) Rate of re-entrainment of particles newly arrived at a bed is a function of flow intensity and bed porosity. 3) There is no evidence of dynamic sorting of silt in laboratory channel flows. 4) Cores, comprised of fine-gravel matrix, were introduced to the gravel beds of two New England upland-alluvial streams. Core infiltration by particles less than 2 mm in diameter and tagged by the naturally-occurring, short-lived radionuclide ⁷Be (half-life ~ 53.3 days) during a three-week deployment was related to directly-measured, time-averaged channel flow conditions. Depths of particle penetration and total inventory of ⁷Be were maximal in beds underlying mean flow speeds in the range of 10-40 cm/s. These results are interpreted in the light of our laboratory experiments: in lesser flows, little mass is mobilized by the flow and thus available for bed penetration; in more intense flows, fine-particle arrival at, and subsequent penetration of a bed are limited as full-suspension transport becomes dominant. Thus, the greatest potential for stream-bed sequestration and biogeochemical alteration of silts, if not their sorting, occurs in stream flows of moderate intensity.