Geomorphic change on the Missouri River during the flood of 2011: Chapter I in 2011 Floods of the Central United States
Geomorphic change on the Missouri River during the flood of 2011: Chapter I in 2011 Floods of the Central United States
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2011年洪水期间密苏里河地貌变化：2011年美国中部洪水第一章
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发表时间：
2014
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通讯作者：
C. Hupp
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作者：
E. Schenk;K. Skalak;A. Benthem;Benjamin J. Dietsch;B. Woodward;G. J. Wiche;J. Galloway;R. Nustad;C. Hupp
2011 年密苏里河洪水是自 20 世纪中叶一系列高坝对河流进行调节以来最严重的洪水之一（高峰期每秒超过 150,000 立方英尺）。洪水持续了整个夏季的大部分时间，侵蚀了河岸，增加了沙洲的沙子，并移动了许多地方的河道海堤。美国地质调查局监测和评估了密苏里河两个河段的变化：北达科他州的驻军河段，以驻军大坝和奥赫湖水库为界，以及沿着南达科他州和内布拉斯加州边界的休闲河段，以加文斯角大坝为界上游，并从内布拉斯加州庞卡向下游延伸。从加里森大坝关闭到洪水爆发之前的历史横截面数据表明，大坝附近河流的上游在大坝关闭后经历了快速侵蚀、河道切割和岛屿/沙洲损失。随着时间的推移，侵蚀、切割和土地损失逐渐减少。相反，随着到达水库的水流减慢，奥赫湖水库附近的下游河段滞流水逐渐沉积，河道充填，沙洲生长。加里森河段洪水后的初步结果表明，主航道的大部分横截面都加深了，而沙洲和岛屿则垂直生长。沙洲和海潮在休闲河段内迁移，但净冲刷和沉积作用极小。沙洲和岛屿二维面积的变化仍在使用高分辨率卫星图像进行评估。可以使用横截面、测深数据、风吹材料的沙坑、准三维数值模型和沉积物岩心测年来为驻军河段构建沉积物平衡。目前（2014 年）正在进行范围内沉积物平衡的数据收集和分析，以及河岸和岛屿植被对沉积物沉积影响的同步分析。简介 2011 年密苏里河发生的洪水是自 20 世纪中叶一系列高坝对河流进行调节以来最严重的洪水之一。由于冬季大量积雪融化以及源头发生大雨，洪水从春季开始，一直持续到夏季（Vining 等，2013 年）。本章是美国地质调查局 (USGS) 一系列报告的一部分，该报告记录了 2011 年整个密西西比河流域（包括密苏里河）大范围洪水的影响。本章的目的是详细描述洪水期间和之后密苏里河两个河段的地貌研究。密苏里河是美国最长的河流，长度超过 2,500 英里（Kammerer，1990），流经美国大陆近六分之一（Elliot 和 Jacobson，2006）。雅克·马凯特 (Jacques Marquette) 神父于 1673 年记录了欧洲人对这条河的最早观察，当时他描述了密苏里河水的凶猛和沉积物丰富的性质（与相对田园诗般的密西西比河相比；Gillespie，2000 年，第 3 页）：“我没有见过更可怕的东西，一大片长满树枝的大树，真正的漂浮岛屿来自 Pekitanoui（美洲原住民对密苏里河]……搅动太大了，水都浑浊了，无法澄清。”北达科他州俾斯麦-曼丹下游河岸侵蚀导致停车场受损。摄影：Edward Schenk，美国地质调查局。 2 2011 年洪水期间密苏里河的地貌变化 这条河很快获得了“大泥泞”的绰号。在频繁移动的沙洲以及高度集中的木塞和水下障碍物中航行是危险的（图 1）。尽管存在这些危险，这条河仍然是刘易斯和克拉克探险队首次使用的一条重要运输路线，用于进入西北地区，并在短时间内通过河明轮船将货物和客运运输至上游至蒙大拿州本顿堡（Gillespie，2000）。正如“汽船比尔”赫克曼所说，“我们在密苏里河口将男人和男孩分开。男孩们沿着密西西比河而上，男人们则沿着大泥泞河而上[Gillespie，2000，封底]。”河流上游的货运和客运交通随着铁路的兴起而减慢，并最终随着河流的筑坝而结束（Gillespie，2000）。河流在流量、沙洲位置和障碍方面的不可预测性在 20 世纪中叶大部分得到了控制，一系列大型水坝的修建为大平原各州提供水源，改善航运并提供防洪功能（表 1）。这一系列大型水坝通过持续释放流量来维持干旱期间的高低流量，并通过截留洪水来减轻洪峰流量的严重程度。尽管它们具有防洪等用途，但在过度潮湿时期，大坝的泄水量必须超过常规泄水量，因为大坝后面的水库将达到满负荷。 2011 年春季的暴雨和异常大的积雪导致密苏里河上游发生大坝调节时代最大规模的洪水（Holmes 等，2013；Vining 等，2013）。北达科他州加里森大坝下方的流量超过 155,000 立方英尺/秒 (ft3/s)，持续两周多，高流量持续整个夏季（Galloway 等人，2013 年）。直到 9 月和 10 月，即源头暴雨发生近 6 个月后，流经驻军河段的流量才降至正常水平。相比之下，下一次最大的洪水（1975 年）大约是 2011 年洪水流量的一半。 图 01 图 1. 历史上（1832 年）的密苏里河河道，包括大型木材障碍、河岸侵蚀以及植被岛屿和河岸。 “密苏里河景观，冲积河岸倾泻而下，圣路易斯上方 600 英里”（乔治·卡特林，1832 年，布面油画，史密森尼美国艺术博物馆 1985.66.363。）
The 2011 flood on the Missouri River was one of the largest floods since the river became regulated by a series of high dams in the mid-20th century (greater than 150,000 cubic feet per second during the peak). The flood persisted through most of the summer, eroding river banks, adding sand to sandbars, and moving the thalweg of the channel in many places. The U.S. Geological Survey monitored and assessed the changes in two reaches of the Missouri River: the Garrison Reach in North Dakota, bounded by the Garrison Dam and the Lake Oahe Reservoir, and the Recreational Reach along the boundary of South Dakota and Nebraska bounded upstream by the Gavins Point Dam and extending downstream from Ponca, Nebraska. Historical cross-section data from the Garrison Dam closure until immediately before the flood indicate that the upper reaches of the river near the dam experienced rapid erosion, channel incision, and island/sandbar loss following the dam closure. The erosion, incision, and land loss lessened with time. Conversely, the lower reach near the Lake Oahe Reservoir slackwaters became depositional with channel in-filling and sandbar growth through time as the flow slowed upon reaching the reservoir. Preliminary post-flood results in the Garrison Reach indicate that the main channel has deepened at most crosssections whereas sandbars and islands have grown vertically. Sandbars and the thalweg migrated within the Recreational Reach, however net scouring and aggradation was minimal. Changes in the two-dimensional area of sandbars and islands are still being assessed using high-resolution satellite imagery. A sediment balance can be constructed for the Garrison Reach using cross-sections, bathymetric data, sand traps for wind-blown material, a quasi-three-dimensional numerical model, and dating of sediment cores. Data collection and analysis for a reach-scale sediment balance and a concurrent analysis of the effects of riparian and island vegetation on sediment deposition currently (2014) is ongoing. Introduction The flood of 2011 on the Missouri River was one of the largest floods since the river became regulated by a series of high dams in the mid-20th century. The flood began in the spring and continued well into the summer because of the melt of a large snow accumulation during the winter and a large rain event in the headwaters (Vining and others, 2013). This chapter is a part of a larger series of U.S. Geological Survey (USGS) reports documenting the effects of widespread flooding in 2011 throughout the Mississippi River watershed including the Missouri River. The purpose of this chapter is to describe, in detail, geomorphic studies on two reaches of the Missouri River during and following the flood. The Missouri River is the longest river in the United States, with a length of more than 2,500 miles (Kammerer, 1990) and drains nearly one-sixth of the continental United States (Elliot and Jacobson, 2006). Father Jacques Marquette recorded the earliest European observations of the river in 1673 when he described the ferocity and sediment laden nature of the water of the Missouri River (compared to the relatively idyllic Mississippi River; Gillespie, 2000, pg. 3): “I have seen nothing more frightful, a mass of large trees entire with branches, real floating islands came from the Pekitanoui [a Native American name for the Missouri River].... The agitation was so great that the water was all muddy, and could not get clear.” A damaged parking lot caused by bank erosion downstream of Bismarck-Mandan, North Dakota. Photograph by Edward Schenk, U.S. Geological Survey. 2 Geomorphic Change on the Missouri River During the Flood of 2011 The river quickly gained the nickname “The Big Muddy.” It was hazardous to navigate with frequently shifting sandbars and a high concentration of log jams and submerged snags (fig. 1). Despite these hazards, the river was a vital transportation route first used by the Lewis and Clark expedition to access the Northwest Territories and, for a short time, to transport by river paddleboat freight and passenger transport as far upstream as Fort Benton, Montana (Gillespie, 2000). As stated by “Steamboat Bill” Heckmann, “We separated the men from the boys at the mouth of the Missouri. The boys went up the Mississippi and the men went up the Big Muddy [Gillespie, 2000, back cover].” Freight and passenger traffic on the upper reaches of the river slowed with the rise of the railroad and eventually ended with the damming of the river (Gillespie, 2000). The unpredictable nature of the river, in terms of flow, sandbar locations, and snags, mostly was tamed in the mid-20th century with the creation of a series of large dams to provide water for the Great Plains states, improve navigation, and provide flood control (table 1). The series of large dams sustain high low flows during droughts by releasing a consistent discharge and reduce the severity of flood peak flows by retaining floodwaters. Despite their function for flood protection, among other uses, dam releases during excessively wet periods must exceed routine discharge because the reservoir behind the dam will reach full capacity. Heavy rainstorms and abnormally large snow pack in the spring of 2011 led to releases that produced the largest flood of the dam-regulated era on the upper Missouri River (Holmes and others, 2013; Vining and others, 2013). The discharge below the Garrison Dam in North Dakota exceeded 155,000 cubic feet per second (ft3/s) for more than 2 weeks with high discharges continuing through the summer (Galloway and others, 2013). The flow through the Garrison Reach did not recede to normal levels until September and October, nearly 6 months after the rainstorms in the headwaters. For comparison, the next largest flood (1975) was approximately one-half the discharge of the flood of 2011. figure 01 Figure 1. The historical (1832) Missouri River channel including large wood snags, bank erosion, and vegetated islands and banks. “View on the Missouri, Alluvial Banks Falling in, 600 Miles above St. Louis” (George Catlin, 1832, oil on canvas, Smithsonian American Art Museum 1985.66.363.)