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首页> 外文期刊>Geomorphology >Time and depth scales of fine sediment delivery into gravel stream beds: Constraints from fallout radionuclides on fine sediment residence time and delivery
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Time and depth scales of fine sediment delivery into gravel stream beds: Constraints from fallout radionuclides on fine sediment residence time and delivery

机译：细沙沉积物进入砾石流床的时间和深度尺度：放射性放射性核素对细沙沉积物停留时间和输送的限制

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Particles of fine sediment can clog interstitial pore spaces of coarser grained sediment in river beds and thereby impede the exchange of water, dissolved constituents, and particulate matter with consequent ecological impacts. The extent to which fine sediment reduces connectivity between the stream bed and overlying channel is a function, in part, of sediment residence time. Short residence times imply frequent exchange of matter and reduced impact of fine sediment on stream bed habitat, while long residence times indicate that with respect to fine sediment delivery, the stream bed is more isolated from the overlying channel. Here we present a novel technique to quantify the residence time of river bed sediment at various depths over annual and decadal timescales using the fallout radionuclides (FRNs) ~7Be (t_(1/2) = 53 days) and ~(210)Pb_(ex) (t_(1/2) = 22.3 years). We placed mesh cylinders filled with ~7Be-free sediment into a stream bed to quantify the capture of ~7Be-tagged particles in the absence of scour and fill. We also took cores to the depth of refusal in alluvial sediment in unregulated and regulated rivers in Vermont and New Hampshire. Sampled watershed areas ranged from 29 to 410 km~2, and core depths ranged from 19 to 77 cm. The ~(210)Pb_(ex) activity profiles of cores show that bed sediment is exchanged to the depth of refusal at decadal timescales. In contrast, ~7Be activity profiles indicate that fine sediment infiltrating into the bed had residence times ranging from 4 to > 300 days in unregulated rivers. Cores from a regulated river are notably different-subsurface sediment residence times were always longer than in unregulated rivers at comparable depths, likely owing to restriction of bed mobilization and clogging of bed material by fine sediment These results suggest that filtration can be an important component of bed material delivery to stream beds, but filtration does not deliver material as deeply into the bed as scour and fill. We find that fallout radionuclides provide direct measurements of time and depth scales of active exchange of particulate matter, which are key controls on hyporheic function.
机译：细颗粒的沉积物会堵塞河床中较粗颗粒沉积物的间隙孔隙，从而阻碍水，溶解成分和颗粒物的交换，从而对生态产生影响。细颗粒沉积物降低流床与上覆通道之间的连通性的程度部分取决于沉积物停留时间。短停留时间意味着物质交换频繁，细颗粒沉积物对河床生境的影响减小，而长停留时间表明，对于细颗粒沉积物的输送，河床与上覆河道更加隔离。在这里，我们提出了一种新技术，可以使用落射放射性核素（FRNs）〜7Be（t_（1/2）= 53天）和〜（210）Pb_（在年际和年代际尺度上量化不同深度的河床沉积物的停留时间。例如）（t_（1/2）= 22.3年）。我们将装有〜7Be的无沉淀物的网状圆柱体放入流床中，以量化在没有冲刷和填充的情况下〜7Be标记的颗粒的捕获量。我们还深入研究了佛蒙特州和新罕布什尔州不受管制和受管制的河流冲积沉积物被拒绝的深度。采样流域面积为29至410 km〜2，岩心深度为19至77 cm。岩心的〜（210）Pb_（ex）活性曲线表明，年代际时间尺度上床沉积物交换为拒绝深度。相比之下，〜7Be的活动曲线表明，在不受管制的河流中，渗透到河床中的细沙沉积物的停留时间为4天到> 300天。调节河道的岩心明显不同，在可比深度处，地下沉积物的停留时间总是比未调节河道的时间长，这可能是由于床层动员的限制和细颗粒物堵塞床层材料所致。将物料输送到流化床中，但过滤不会像冲刷和填充一样将物料输送到床中。我们发现，放射性放射性核素可以直接测量颗粒物质主动交换的时间和深度尺度，这是对流变功能的关键控制。

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