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

The relationships between sediment production, storage, and transport in fluvial systems are complex and variable. Key issues in addressing these relationships are the residence times of sediment delivered to the channel, and the proportion derived from recent upland erosion as opposed to remobilized alluvium. The systematic changes in iron geochemistry often experienced by 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 soils are high in iron oxide content, these changes are reflected in soil color. Alluvium with red, yellow, or brown colors indicating ferric (oxidized) iron and sufficient organic matter for reduction to occur indicates a short (<1 year) residence time. Redox features along root channels may indicate 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 environments (≫1 year) will not oxidize on exposure to the atmosphere due to the loss of iron, while those with ferrous iron remaining will experience oxidation and color change on exposure. In Loco Bayou, Texas, these indicators of residence time are shown to be generally consistent with other field evidence of erosion and sedimentation. Further, the color indicators correctly indicate the residence time in several cases where the latter is known from field observations.