Residence times of alluvium in an east Texas stream as indicated by sediment color

The relationships between sediment production, storage, and transport in fl uvial systems are complex and variable. Key issues in addressing these rela tionships are the residence times of sediment delivered to the channel, and the proportion derived from recent upland erosion as opposed to remobilize d alluvium. The systematic changes in iron geochemistry often experienced b y sediments deposited in an anaerobic environment, such as a stream channel or waterlogged floodplain, are used here as an indicator of residence time over contemporary time scales, In areas such as east Texas, where upland s oils are high in iron oxide content, these changes are reflected in soil co lor. Alluvium with red, yellow, or brown colors indicating ferric (oxidized ) iron and sufficient organic matter for reduction to occur indicates a sho rt (< 1 year) residence time. Redox Features along root channels may indica te the residence time of oxidized material without organic matter. Alluvium with gley colors (Munsell chroma < 3) indicates a longer residence time (> 1 year). Sediments with the longest residence times in alluvial environmen ts (>> 1 year) will not oxidize on exposure to the atmosphere due to the lo ss of iron, while those with ferrous iron remaining will experience oxidati on and color change on exposure. In Loco Bayou, Texas, these indicators of residence time are shown to be generally consistent with other field eviden ce of erosion and sedimentation. Further, the color indicators correctly in dicate the residence time in several cases where the latter is known from f ield observations. Published by Elsevier Science B.V.