Near-Channel Sources and Sinks along a Mountainous Stream: Establishing the Controls and Time Scales of the Lateral Transfer of Sediment and Carbon

Abstract:

River channels exchange sediment, carbon, and other matter with hillslopes and floodplains. An ongoing challenge is to quantify the time and length scales of these lateral interactions, and to establish physical controls on direction of transfer. Here we investigate whether downstream changes in stream power (Ω) can predict near-channel sources or sinks of matter on decadal time scales in a case study of Mink Brook, a 50 km² watershed in New Hampshire, USA. Building on the Exner equation, we hypothesize that reaches with downstream increases in stream power (Ω↑) exhibit near-channel deposition and accumulation of organic matter, and reaches of downstream decreases in stream power (Ω↓) exhibit near-channel erosion and stripping of organic matter. We measured ²¹⁰Pb_ex inventory (an indicator of erosion versus deposition), organic matter inventory, grain size, and depth of alluvium/colluvium in 29 soil pits at 6 cross sections along the brook. Sites had equivalent total Ω for a given storm event. However, 3 cross sections exhibited Ω↑, and 3 exhibited Ω↓. All cross sections showed a general trend of stripping of organic matter and fine sediment particles in the channel, paired with loading of matter at the ~2-year flood elevation. From the ~2- to ~25-year flood elevation, a marked difference appeared between sites. The Ω↑ cross sections exhibited several locations of erosion and stripping of organic matter, as evidenced by low ²¹⁰Pb_ex inventories (70 to 1,000 bq m^-2), low organic matter inventories (17 to 219 kg m^-2), and thin alluvial cover (average 23 cm). The low ²¹⁰Pb_ex inventories, below the characteristic 6,000 bq m^-2 of stable soil profiles in this region, suggest no areas had consistent deposition over the last century. In contrast, the Ω↓ cross sections exhibited deposition of fine particles and organic matter from the ~2- to ~25-year flood elevation, as evidenced by elevated ²¹⁰Pb_ex inventories (up to 9,100 bq m^-2), elevated organic matter inventories (up to 3,300 kg m^-2), and thicker alluvial cover (average 41 cm). This analysis has direct management implications for predicting sources and sinks of sediment, carbon, and likely other nutrients and pollutants along rivers—with broader implications for riparian ecology and restoration practices.