San Lorenzo River Floodplain Abandonment

This resource contains relevant data and programs (.py and .ipynb) for the paper \"River floodplain abandonment and channel deepening coincide with the onset of clear-cut logging in a coastal California redwood forest\" (https://doi.org/10.1002/esp.5299). Programs are generalizable to other studies and may be used with proper citation. Article abstract: Changes in both land use and climate can alter the balance of transport capacity and sediment supply in rivers. Hence, the primary driver of recent incision or aggradation in alluvial channels is often unclear. The San Lorenzo River on the central coast of California is one location where both climate and land use—specifically, clear-cut forestry of coastal redwoods—could explain recent vertical incision and floodplain abandonment. At our field site on the San Lorenzo, we estimate the magnitude of recent incision using both the ratio of bankfull to critical Shields numbers (𝜏∗𝑏𝑓/𝜏∗𝑐) and the geomorphically effective discharge, calculated from historical gauge data. The Shields number ratio suggests that the normalized bankfull stress of the San Lorenzo River is in the upper 1–2% of West Coast rivers, and the effective discharge corresponds to flow depths ~2–4 m below current bankfull conditions. Radiocarbon ages from detrital charcoal in floodplain sediment reveal active floodplain deposition during the 1600s and possibly into the 1800s, constraining the timing of incision to the last few centuries. Multiple hanging tributaries above the mainstem San Lorenzo River, along with patterns in vegetation on terrace surfaces, corroborate our estimates of the magnitude and timing of incision. Taken together, our findings suggest that floodplain abandonment in this reach was mainly due to methods employed during logging that increased shear stress on the channel bed and reduced sediment storage capacity. We suggest that direct channel modifications in rivers can counterbalance increases in sediment delivery due to clear-cutting, resulting in channel incision rather than aggradation. Today, a young, lower surface appears to be forming adjacent to the San Lorenzo River, which we interpret as an incipient floodplain that is in equilibrium with modern sediment supply and transport capacity.本数据集包含与论文《加利福尼亚海岸红杉林皆伐（clear-cut logging）起始与河漫滩废弃及河道加深同步》（https://doi.org/10.1002/esp.5299）相关的数据与程序代码（格式为.py及.ipynb）。本程序具备可推广性，可应用于其他研究场景，使用时需进行规范引用。 论文摘要： 土地利用与气候变化均可改变河流输沙能力与泥沙供给之间的平衡，因此冲积河道近期下切或加积的主导驱动因素往往难以明确。位于加利福尼亚中部海岸的圣洛伦索河（San Lorenzo River）便是一处同时受气候与土地利用（具体为海岸红杉林皆伐伐木）影响，可用于解释近期河道垂直下切与河漫滩废弃现象的研究区域。 在圣洛伦索河的野外研究点，我们通过两种方法估算了近期河道下切的幅度：一是满岸希尔兹数（Shields number）与临界希尔兹数之比（τ*_bf/τ*_c）；二是基于历史水文站数据计算得到的地貌有效流量（geomorphically effective discharge）。希尔兹数比值结果显示，圣洛伦索河的归一化满岸水流应力处于美国西海岸河流的前1%~2%区间，而地貌有效流量对应的水流深度较当前满岸条件低约2~4米。 通过对河漫滩沉积物中碎屑炭（detrital charcoal）进行放射性碳定年（radiocarbon ages），我们发现河漫滩活跃沉积期为17世纪，且可能延续至18世纪，由此将河道下切的时间限定在近数百年间。圣洛伦索河干流上方多处悬谷支流，以及阶地表面的植被分布模式，均佐证了我们对河道下切幅度与时间的估算结果。 综合来看，本研究结果表明该河段的河漫滩废弃主要源于伐木作业所采用的施工方式：这类作业增大了河道床面的剪切应力，同时降低了泥沙存储容量。我们提出，河流的河道直接改造可抵消皆伐作业导致的泥沙输运增量，最终引发河道下切而非加积。目前，圣洛伦索河沿岸正形成一处年轻的低位地表，我们将其解释为与现代泥沙供给及输沙能力处于平衡状态的初始河漫滩（incipient floodplain）。