Extreme floods are underrepresented in stream gauge records. Sedimentological evidence of past floods (paleofloods) yields longer records, allowing extreme floods to be examined over several Holocene climate periods. This study examines the influence of hydrogeomorphic complexity (floodplain aggradation and spatially variable flood deposition) on paleoflood record “completeness” and their implications for flood magnitude estimates made with paleoflood hydrologic data. We collected two sediment cores 500 m apart from the same elevation on a natural levee along a bank of the Tennessee River near Guntersville, Alabama. We measured grain size from each core at a 1-cm resolution using a Malvern 3000 laser granulometer. Optically stimulated luminescence dating of flood deposits revealed approximate age ranges of 50 – 6500 years calibrated before present (yrs. B.P.) for the downstream 3.5 m core (i.e., BO1) and 190 – 8500 yrs. B.P. for the upstream 4.18 m core (i.e., EL2). First, a sensitivity analysis revealed adjusted floodplain elevations (AFE) for each paleoflood cross-sectional geometry to reflect floodplain aggradation over time enabled the detection of paleoflood magnitude differences, suggesting floodplain aggradation should be considered in paleoflood reconstruction within alluvial settings. Minimum paleodischarge intervals were estimated in a 1D HEC-RAS step backwater model by calculating the minimum paleoflood stage needed to transport the d90 of each paleoflood deposit. Big Oak and East Levee 2 sediment cores each contained 15 high magnitude, identifiable paleofloods. The majority of the BO1 paleofloods occurred in the last 2,000 years, while most EL2 paleofloods occurred between 2,000 and 5,000 yrs. B.P., suggesting localized geomorphic complexity produced distinct paleoflood records for the same hydrogeomorphic surface. Four paleoflood events correlate across the two cores based on their ages and grain size distributions, and we combined these four floods to create a ‘harmonized’ flood chronology for the site. The timing of these four floods corresponds with paleofloods that occurred in the last 2000 years in the middle section of the Tennessee River identified by prior paleoflood hydrologic studies. Flood frequency analysis (Bayesian Markov Chain Monte Carlo method) scenarios with ten configurations of paleoflood hydrologic data revealed differences in the number and timing of paleofloods in three paleoflood chronologies (BO1, EL2, and the harmonized) resulting from hydrogeomorphic complexities affected model distribution parameters, goodness-of-fit, and the estimated discharges of annual exceedance probabilities used to inform the design of critical flood infrastructure. As a consequence of longer records containing smaller floods, the estimated discharge of the 0.01 (100-yr), 0.001 (1000-yr), and 0.0001 (10,000-years) AEPs for EL2 were 16%, 11%, and 5% smaller, respectively, than BO1 estimates. Alluvial rivers present more challenges for reconstructing paleoflood records than bedrock, confined channel settings. The strategies presented in this paper can help integrate paleoflood hydrologic data into flood frequency analyses for alluvial rivers identifying and minimizing error stemming from hydrogeomorphic complexity. These strategies offer opportunities to expand the use of paleoflood hydrologic data in flood frequency analyses to include more rivers located in temperate environments where climate change is intensifying precipitation and a compelling need exists to anticipate and plan for changes in extreme flood occurrence.

极端洪水在流量记录中的代表性不足。过去洪水（古洪水）的沉积学证据产生了更长的记录，允许在几个全新世气候时期检查极端洪水。本研究考察了水文地貌复杂性（泛滥平原加积和空间可变洪水沉积）对古洪水记录“完整性”的影响及其对利用古洪水水文数据进行的洪水强度估计的影响。我们在阿拉巴马州甘特斯维尔附近的田纳西河沿岸的天然堤坝上收集了距同一海拔 500 m 的两个沉积物岩心。我们使用 Malvern 3000 激光粒度仪以 1 厘米的分辨率测量了每个核心的粒度。洪水沉积物的光学激发发光测年显示，下游 3.5 m 岩心（即 BO1）和 190 - 8500 年前校准的年龄范围约为 50 - 6500 年（BP 年）。BP 为上游 4.18 m 核心（即 EL2）。首先，敏感性分析揭示了每个古洪水横截面几何形状的调整洪泛区高程 (AFE)，以反映洪泛区随时间的加积情况，从而能够检测古洪水幅度差异，这表明在冲积环境内的古洪水重建中应考虑洪泛区加积。在一维 HEC-RAS 阶跃回水模型中，通过计算输送每个古洪水沉积物 d90 所需的最小古洪水阶段来估计最小古放电间隔。Big Oak 和 East Levee 2 个沉积岩芯各包含 15 个高震级，可识别的古洪水。大多数 BO1 古洪水发生在过去 2000 年，而大多数 EL2 古洪水发生在 2000 到 5000 年之间。BP，表明局部地貌复杂性为同一水文地貌表面产生了不同的古洪水记录。四个古洪水事件根据它们的年龄和粒度分布在两个岩心中相互关联，我们将这四个洪水结合起来为该地点创建了一个“协调的”洪水年表。这四次洪水的发生时间与过去 2000 年发生在田纳西河中段的古洪水相一致，这些洪水由先前的古洪水水文研究确定。具有十种古洪水水文数据配置的洪水频率分析（贝叶斯马尔可夫链蒙特卡罗方法）情景揭示了由于水文地貌复杂性影响模型分布参数而导致的三个古洪水年表（BO1、EL2 和协调）中古洪水数量和时间的差异，拟合优度，以及用于为关键洪水基础设施设计提供信息的年度超标概率的估计排放量。由于包含较小洪水的较长记录，EL2 的 0.01（100 年）、0.001（1000 年）和 0.0001（10,000 年）AEP 的估计流量分别小 16%、11% 和 5% ，分别高于 BO1 的估计。与基岩、受限河道环境相比，冲积河流对重建古洪水记录提出了更多挑战。本文提出的策略可以帮助将古洪水水文数据整合到冲积河流的洪水频率分析中，识别和最小化因水文地貌复杂性引起的误差。这些策略提供了扩大在洪水频率分析中使用古洪水水文数据的机会，以包括更多位于气候变化正在加剧降水的温带环境的河流，并且迫切需要预测和计划极端洪水发生的变化。