Continental Shelf Research 20 (2000) 2267–2294 Development and reworking of a seasonal flood deposit on the inner continental shelf off the Atchafalaya River Mead A. Allison a, *, Gail C. Kineke b , Elizabeth S. Gordon c , Miguel A. Gon˜i c a Department of Geology, Tulane University, New Orleans, LA 70118, USA b Department of Geology and Geophysics, Boston College, 140 Commonwealth Ave., Chestnut Hill, MA 02167, USA c Marine Science Program and Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA Abstract Sediment cores and water column measurements of suspended sediment and flow conditions were taken on the continental shelf off the Atchafalaya River in Louisiana to examine the development and reworking of a seabed flood layer with seasonal variations in river discharge and hydrodynamics. Five stations in water depths of 5–23 m were occupied on the Atchafalaya inner shelf on four cruises from October 1997 to March 1999 representing a range of freshwater input and wave energy conditions. Downcore profiles of the short half-life (53 d) cosmogenic radiotracer 7 Be showed a three to fivefold increase in seabed inventory and an increase in depth of penetration during the 1998 high Atchafalaya discharge period (April) at two inshore stations (5–7 m water depth). X-radiograph evidence of the absence of biological mixing at these sites suggests that the 7 Be data is recording the deposition of a 1–3 cm thick annual flood deposit. The organic carbon contents and stable carbon isotopic compositions of this flood deposit are distinct and reflect the increased terrestrial influence of the riverine sediment flux. 210 Pb and 137 Cs sediment profiles indicate that this seasonal deposit is two to six times the long-term (e.g., decadal) accumulation at these sites. Passage of cold fronts on 3–7 d timescales interrupts the formation of these flood deposits, particularly during the rising to early high discharge period (December–March). The depth of sediment resuspension landward of 10 m water depth during these events may reach 1 cm and decreases offshore. Offshore stations (  20 m water depth) show only a small increase in deposition during the high Atchafalaya discharge period. Redistribution of sediment from shallower parts of the shelf during the remainder of the year is likely a major supplier to these areas. A station east of the *Corresponding author. E-mail address: malliso@tulane.edu (M.A. Allison). 0278-4343/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII:S0278-4343(00)00070-4