A Review of “The Illustrated History of Natural Disasters”

Zonal patterns of salt marsh plants and physical conditions have been addressed primarily across the elevation gradient from inland to coastline rather than across tidal creeks in relation to their hydro-geomorphic processes such as bar formation and bank erosion. We found at a Danish marsh that by shaping major geomorphic features and providing sediments to the adjacent sites, fluvial-geomorphic processes of tidal creeks exert fundamental controls on the cross-channel distribution of abiotic and biotic factors. These results point to a need for biogeomorphic and landscape ecological perspectives to fully understand the underlying structure and geographic variability in salt marshes.     

黄河三角洲潮滩潮沟近期变化遥感监测

作者选用1986年和2001年两个时相的Landsat TM/ETM+影像,结合遥感、地理信息系统 (GIS) 和分形理论研究了黄河三角洲典型河口岸段潮滩潮沟的发育演变。结果表明：近15年来,挑河口~106站和现行水河口(1976~1996年黄河入海口,下同)~甜水沟岸段分维值D增大,呈顺向演替;其余岸段分维值均减小,表现为逆向演替。人工围垦挤占了大面积潮滩,人工岸线不断向海扩展,其最大推进速率达0.8 km/yr;同时,潮沟长度缩短,最严重的岸段每年减少14.9 km。通过分析表明,人类活动已成为影响三角洲潮滩发育的重要因子,其导致整个三角洲的潮滩体系严重退化,自然形成的潮沟时空谱系不复存在;另外,发育时间长短、入海泥沙、海浪和潮流对潮滩发育演变亦非常重要。目前,黄河三角洲潮滩可分为河口滩、潮控滩、浪控滩和人控滩4大类型。     

影响海岸带盐沼土壤有机质、TN和TP含量时空变化的关键因子：潮水和植被

为了更好地理解在潮水和植被交互作用梯度上有机质、全氮和全磷分布的特殊性,分别于生长季和非生长季在苏北盐沼沿近潮沟带和远潮沟带2条样线布设9个固定采样区,采集土壤样品,测定土壤有机质、全氮和全磷的含量。结果表明：植被和潮水的交互作用是决定土壤有机质、全氮和全磷时空变化的关键因素。相对于潮水,植被的影响更为稳定和持久;盐沼土壤有机质和全氮含量具有显著的空间和月份变化,空间变化主要包括植被带（潮水）梯度上和样带间（潮流）的差异,在植被带（潮水）梯度上植被带高于光滩,而植被带间呈现由海向陆的递减趋势,即互花米草(Spartina alterniflora)>盐地碱蓬(Suaeda salsa)>獐茅(Aeluropus littoralis)>芦苇( Phragmites australis);有机质样带间差异表现为在低位盐沼远潮沟带大于近潮沟带,而中位盐沼却是近潮沟带大于远潮沟带,全磷含量时间和空间变化上均不具显著性;pH、土壤含水量和盐分含量等与潮水作用直接相关的因子与距海距离呈负相关关系,土壤有机质、全氮和全磷与距海距离也表现为负相关关系,而土壤有机质、全氮和全磷含量与pH、土壤含水量及盐分含量等因子为正相关关系,一定程度上诠释了潮水对土壤有机质、全氮和全磷含量变化的影响。     

黄河三角洲潮沟网络形态特征的时空分异规律及其发育过程

潮沟是陆海生态系统交互作用的主要通道,具有高度的时空动态性。以黄河三角洲新旧河道为界,划分黄河北岸（Ⅰ区）、南岸东（Ⅱ区）、南岸西（Ⅲ区）为对比研究区,选择1998—2018年5个关键时间节点的遥感影像为数据源,利用GIS空间分析功能,定量表征了潮沟发育程度的典型形态特征参数,剖析了黄河改道、湿地恢复工程和外来物种入侵等因子对潮沟发育过程的影响,挖掘潮沟网络的时空演变规律。结果表明：① 潮沟的形态特征具有明显的时空异质性。从1998年黄河改道初期至2004年湿地恢复初期,Ⅰ区各等级潮沟数量大幅减少;由于潮滩淤进和互花米草固滩作用,Ⅱ区潮沟数量、密度、分汊率呈现增长趋势,在2013年互花米草的快速扩张期,潮沟曲率、分汊率明显增大;Ⅲ区潮沟发育的等级、长度、密度最大,等级间的转化频率最高,湿地恢复工程导致低等级潮沟数量、长度明显减少,较强的海洋动力抑制了潮沟曲流的发育。② 黄河改道和侧弯丁坝建设导致平均归槽长度（OPL）增大,潮沟网络的排水效率减少。互花米草的快速扩张使排水效率增大,OPL不断降低;潮沟发育程度Ⅲ区>Ⅱ区>Ⅰ区。③ 潮沟系统由快速变化青年期整体进入缓慢变化中年期,发育状态趋于动态平衡。研究成果有助于了解大范围潮沟的发育过程及演变机制,为沿岸潮滩开发与利用提供科学的决策支撑。     

江苏中部海岸潮沟的形态变化特征

作为一种典型的潮滩地貌单元,潮沟(tidal creeks) 在江苏中部海岸潮间带广泛发育,并表现出复杂的摆动性。本文根据1981-2010 年遥感影像和野外观测数据,对该区潮沟的平面形态变化进行了分析。结果显示,江苏中部海岸潮沟(根据其长度差异) 一般分为4 级,1~3 级潮沟的个数分别为21、97 和688 (1:5:33),其平均长度分别为7587±1544 m、1921±916 m和327±200 m;潮沟在中潮带比较发育,其密度和分汊率明显大于高潮带和低潮带;主潮沟曲率从高潮带向低潮带呈减小趋势;外来引种互花米草扩展和大规模滩涂围垦活动对潮沟产生了较大影响,其中后者明显地抑制了潮沟的发育。研究区潮沟的摆动性具有明显的区域和级别差异。与研究区北部的潮滩区域相比,海岸潮差较大的南部潮滩区潮沟的摆动性较大;同一个潮沟系统其1 级潮沟的摆动性>2 级潮沟>3 级潮沟;同一条潮沟其下段的摆动性>中段>上段;同一区段弯曲岸段的摆动性>顺直岸段;1 级潮沟上段、2 级潮沟中上段、3 级潮沟或位于盐沼内的潮沟其摆动性不明显。     

近期长江北支口门圆陀角附近潮滩地貌动态变化

圆陀角位于长江北支岸线与江苏海岸线的交会处,独特的互花米草潮滩、淤泥质光滩环境和复杂的河海沉积动力,决定了潮滩地貌对海洋环境变化的响应具有敏感性。由于大规模的围垦,圆陀角附近过去40年来海岸线向东推进了6km。根据2006年以来多次的野外调查和室内粒度与钻孔岩芯的¹³⁷Cs分析, 2006年以来圆陀角附近潮滩淤积明显加强,由¹³⁷Cs时标估算的互花米草滩多年平均沉积速率为2.3cm/a, 2006~2008年观测到的互花米草滩淤积速率>4cm/a,粉砂淤泥质光滩的淤积速率更高;圆陀角风景区内互花米草滩前缘陡坎在风暴潮影响下侵蚀后退,并因粉砂淤泥质光滩的快速淤长而消亡,圆陀角附近潮滩地貌动态是对人类围垦活动、风暴潮与潮汐海洋动力的综合响应,互花米草与光滩快速淤积是近期圆陀角附近潮滩地貌演化的主要特点。     

A potential mechanism for disturbance-mediated channel migration in a southeastern United States salt marsh

Coastal salt marsh tidal creeks are thought to show less channel adjustment/movement relative to their terrestrial fluvial counterparts. We propose a mechanism for disturbance-mediated bank failure that may allow/initiate channel migration in these otherwise stable systems. The stability of tidal creeks is promoted by the extensive vegetation root structure along the banks. However, wrack mats (i.e., dead vegetation) deposited on creek banks can cause the death of below-ground vegetation leading to bare, unstable banks that may slump into the channel. We measured the frequency of bank failures associated with wrack-disturbed sites along three creeks on Sapelo Island, Georgia, USA to determine whether these sites were vulnerable to erosion. Approximately 81% of the disturbed sites showed signs of bank failure. Therefore, wrack-induced bank failure may potentially lead to channel migration in creeks previously believed to be static landscape features.     

Spatiotemporal differentiation and development process of tidal creek network morphological characteristics in the Yellow River Delta

Tidal creeks are the main channels of land-sea ecosystem interactions, and their high dynamics are an important factor affecting the hydrological connectivity of tidal flats.Taking the Yellow River Delta as the research area, we selected remote sensing images obtained during five periods from 1998 to 2018 as the data sources. Based on the spatial analysis function in GIS, the typical morphological characteristics of tidal creeks, such as the level,length, density, curvature, bifurcation ratio, and overmarsh path length(OPL), were extracted to characterize the degree of development of the tidal creeks in the Yellow River Delta wetlands. The spatio-temporal evolution of the tidal creeks was studied, and the development process and the characteristics of the tidal creeks during the different stages of development were investigated. The results revealed that(1) The number, density, and bifurcation ratio of tidal creeks exhibit an increasing trend, but the growth of the trend is slowing. The number of tidal creeks increased by 44.9% from the initial stage of the Yellow River diversion to the late stage of the wetland restoration, but it only increased by 26.2% from the late stage of the wetland restoration to the slow expansion of the Spartina alterniflora.(2) The curvature of the tidal creeks on the landward side is greater than that on the seaward side.(3) The development degree of tidal creek has spatial heterogenetiy, which is Area Ⅲ Area Ⅱ Area Ⅰ.(4)The drainage efficiency is significantly correlated with the tidal creak density and bifurcation ratio. Based on the analysis of the various morphological parameters and the drainage efficiency, it was found that after the rapid change in the tidal creek system in the early stage, the tidal creeks entered a state of slow change, and the development state of the tidal creeks tends to be in dynamic balance. The results of this study are expected to provide scientific support for the sustainable development and utilization of coastal tidal flats.     

互花米草盐沼潮沟地貌特征

互花米草是我国从国外引进的能在淤泥质潮滩的高潮带下部和中潮带上部生长的禾木科植物,由于其植株高大、群落盖度高、根系发达等生物学特性,其对潮流动力特征有显著的影响。本文通过对江苏省东台市笆斗垦区外互花米草盐沼地的潮沟形态调查,发现互花米草盐沼潮沟具有以下一些特征:潮沟密度大,可达50km/km²以上;潮沟的宽深比小,大多在8以下;沿岸堤发育,且多呈不对称状;一级主潮沟与主潮沟接头处普遍呈逆弧形;盐沼外侧边缘发育有明显的陡坎冲沟。分析认为这些特征主要与互花米草发达的根系和致密的植株等生物学特性有关,发达的根系增加了盐沼滩面的稳定性,而互花米草高大的植株增加了对潮水运动的阻力,提高了滩面落潮水的归槽水位,改变了潮流的泥沙沉积地点和涨、落潮流的流向,从而形成比较特别的互花米草盐沼潮沟特征。     

Retrieval of small-relief marsh morphology from Terrestrial Laser Scanner, optimal spatial filtering, and laser return intensity

Marshes are ubiquitous landforms in estuaries and lagoons, where important hydrological, morphological and ecological processes take place. These areas attenuate sea action on the coast and act as sediment trapping zones. Due to their ecosystem functions and effects on coastal stabilization, marshes are crucial structures in tidal environments, both biologically and geomorphologically, and are fundamental elements in wetland restoration and coastal realignment schemes. The spatially-distributed study of the geomorphology of intertidal areas using remotely-sensed digital terrain models remains problematic, owing to their small relief, often of the order of a few tens of centimetres, and to the presence of short and dense vegetation, which strongly reduces the number of resolvable ground returns. Here, we use high-resolution Terrestrial Laser Scanning ( 200 returns/m²) to retrieve a high-resolution and high-accuracy Digital Terrain Model within a tidal marsh in the Venice lagoon. To this aim we apply a new filtering scheme to Terrestrial Laser Scanner data which selects the lowest values within moving windows, whose optimal size is determined with the aid of a limited number of ancillary Differential GPS data in order to maximize resolution while ensuring the identification of true ground returns. The accuracy of the filtered data is further refined using classifications of the intensity of the returns to extract additional information on the surface (ground or canopy) originating the returning laser beam. Validations against about 200 reference Differential GPS ground elevation observations indicates that the best separation of canopy and ground signals is obtained using a low-pass filter with window size of the order of 1 m and the maximum likelihood classifier to further refine the detection of ground returns. In this case the average estimation error is about 1 cm (slight overestimation of ground elevation), while its standard deviation is about 3 cm. Our approach allows the separation of laser returns coming from the low marsh vegetation from those coming from the marsh surface. The overall result is a new observation technique producing Digital Terrain and Digital Surface Models in areas with very small relief, which is shown to provide unprecedented high-resolution and high-accuracy characterizations of marsh morphology.     

Statistical analysis of modern seed assemblages from the San Francisco Bay: applications for the reconstruction of paleo-salinity and paleo-tidal inundation

The feasibility of developing a seed calibration data set to infer environmental constraints was examined by studying modern seed and vegetation assemblages from 53 stations at three distinct tidal marshes within the San Francisco Bay (salt, brackish and freshwater). Pearson's correlation indicates autochthonous derivation of seeds at the marsh sites. Multivariate statistical analysis of the vegetation and seed data indicates discrete assemblages, which can be used as a proxy for modern salinity and tidal inundation within the estuary. Canonical discriminant analysis indicates that the vegetation and seed assemblages of the three marsh types are statistically distinct. Cluster analysis suggests that marsh vegetation is zoned at the mean higher high water mark at all the sites. The calibration set was then applied to stratigraphic seed data from the estuary to infer changes in paleo-salinity and -tidal inundation during the Holocene. The results of the calibration are compared to earlier reconstructions within the estuary. While the calibrated reconstruction broadly compares to previous reconstructions, a period of inferred lower salinity within the estuary between 3800–2000 cal yr B.P. is calibrated as brackish and not fresh by the calibration model.     

Sediment delivery and accumulation in a developed area during coastal floods

The study area is a portion of a coastal community that is adjacent to a salt marsh and tidal creek. The developed area was built upon wetlands, and like the marsh, it floods during spring tides and strong coastal storms. Following floods, little visual evidence of sedimentation is in the built area, prompting the hypothesis that physical characteristics of coastal development limit sediment availability during floods and reduce deposition. Because the area floods from a tidal creek during the same events that lead to inundation of the adjacent high marsh, salt marshes are used as an analogue system for planning this research. Although salt marsh geomorphology is a starting point, people are endogenous actors at this site who influence geomorphic evolution by changing the flow of naturally occurring energy. Suspended sediment levels or deposition were measured during 10 flooding events. Water samples were collected from the tidal creek, at a catchbasin, at another location in the street, and in the Spartina patens marsh. Sediment is found to be delivered to the street in the same quantities and for the same duration as in the salt marsh. Suspended sediment levels are alike throughout the research area. The amount of sediment that accumulates following coastal floods was measured by placing samplers within the street and the marsh. Highly significant differences in sediment accumulation exist between the environments. After a flood event, much less sediment is deposited in the street than in the marsh.     

长江口盐沼土壤有机质更新特征的滩面趋势

Characteristics and tidal flat trends of soil organic matter （SOM） turnover were studied for the Chongmingdongtan Salt Marsh in the Yangtze River estuary, based on analyses of stable carbon isotope composition （δ^13C）, grain sizes and contents of particulate organic carbon （POC）, total nitrogen （TN） and inorganic carbon （TIC） for three cores excavated from high tidal flat, middle tidal flat and bare flat. Results demonstrate that correlations between soil POC contents and δ^13C values of the salt marsh cores were similar to those between soil organic carbon （SOC） contents and δ^13C values of the upper soil layers of mountainous soil profiles with different altitudes. SOM of salt marsh was generally younger than 100 years, and originated mainly from topsoil erosions in catchments of the Yangtze River. Correlations of TN content with C/N ratio, POC content with TIC content and POC content with δ^13C values for the cores suggest that turnover degrees of SOM from the salt marsh are overall low, and trends of SOM turnover are clear from the bare flat to the high tidal flat. Bare flat samples show characteristics of original sediments, with minor SOM turnover. Turnover processes of SOM have occurred and are discernable in the high and middle tidal flats, and the mixing degrees of SOM compartments with different turnover rates increase with evolution of the muddy tidal flat. The exclusive strata structure of alternate muddy laminae and silty laminae originated from dynamic depositional processes on muddy tidal flat was a great obstacle to vertical migration of dissolved materials, and SOM turnover was then constrained. The muddy tidal flat processes exerted direct influences on sequestration and turnover of SOM in the salt marsh, and had great constraints on the spatial and temporal characteristics of SOM turnover of the Chongmingdongtan Salt Marsh in the Yangtze River estuary.     

Beach morphology and change along the mixed grain-size delta of the dammed Elwha River, Washington

Sediment supply provides a fundamental control on the morphology of river deltas, and humans have significantly modified these supplies for centuries. Here we examine the effects of almost a century of sediment supply reduction from the damming of the Elwha River in Washington on shoreline position and beach morphology of its wave-dominated delta. The mean rate of shoreline erosion during 1939–2006 is ~ 0.6 m/yr, which is equivalent to ~ 24,000 m³/yr of sediment divergence in the littoral cell, a rate approximately equal to 25–50% of the littoral-grade sediment trapped by the dams. Semi-annual surveys between 2004 and 2007 show that most erosion occurs during the winter with lower rates of change in the summer. Shoreline change and morphology also differ spatially. Negligible shoreline change has occurred updrift (west) of the river mouth, where the beach is mixed sand to cobble, cuspate, and reflective. The beach downdrift (east) of the river mouth has had significant and persistent erosion, but this beach differs in that it has a reflective foreshore with a dissipative low-tide terrace. Downdrift beach erosion results from foreshore retreat, which broadens the low-tide terrace with time, and the rate of this kind of erosion has increased significantly from ~ 0.8 m/yr during 1939–1990 to ~ 1.4 m/yr during 1990–2006. Erosion rates for the downdrift beach derived from the 2004–2007 topographic surveys vary between 0 and 13 m/yr, with an average of 3.8 m/yr. We note that the low-tide terrace is significantly coarser (mean grain size ~ 100 mm) than the foreshore (mean grain size ~ 30 mm), a pattern contrary to the typical observation of fining low-tide terraces in the region and worldwide. Because this cobble low-tide terrace is created by foreshore erosion, has been steady over intervals of at least years, is predicted to have negligible longshore transport compared to the foreshore portion of the beach, and is inconsistent with oral history of abundant shellfish collections from the low-tide beach, we suggest that it is an armored layer of cobble clasts that are not generally competent in the physical setting of the delta. Thus, the cobble low-tide terrace is very likely a geomorphological feature caused by coastal erosion of a coastal plain and delta, which in turn is related to the impacts of the dams on the Elwha River to sediment fluxes to the coast.     

Effects of sample size on the accuracy of geomorphological models

Commonly, the most costly part of geomorphological distribution modelling studies is gathering the data. Thus, guidance for researchers concerning the quantity of field data needed would be extremely practical. This paper scrutinises the relationship between the sample size (the number of observations varied from 20 to 600) and the predictive ability of the generalized linear model (GLM), generalized additive model (GAM), generalized boosting method (GBM) and artificial neural network (ANN) in two data settings, i.e., independent and split-sample approaches. The study was performed using empirical data of periglacial processes from an area of 600 km² in northernmost Finland at grid resolutions of 1 ha (100 × 100 m) and 25 ha (500 × 500 m). A rather sharp increase in the predictive ability of the models was observed when the number of observations increased from 20 to 100, and the level of robust predictions was reached with 200 observations. The result indicates that no more than a few hundred observations are needed in geomorphological distribution modelling at a medium scale resolution (ca. 0.01–1 km²).     

Geomorphological evidence for the Late Holocene evolution and the Holocene lake level maximum of the Aral Sea

During the Holocene the Aral Sea underwent various transgressive and regressive phases of different magnitudes. However, previous work has not yet fully clarified the evolution and chronology of the individual phases. Research presented here throws light on the evolution of the Aral Sea during the past  2000 years. It includes field surveys, tachymetric and DGPS-derived altitude measurements, analysis of sediments from two areas of the northern and southern Aral Basin (Tastubek Bay and Karaumbet Bay), and their correlation with GIS-based lake area models. Geomorphological and sedimentological evidence from the study areas shows a transgression of the Aral Sea around 200 AD, ending at a lake level maximum of 54 to 55 m. After a medieval regression, the lake reached this level again between the late 16th and early 19th century AD. The digital elevation model SRTM-3 was used to estimate a lake size of 72,400 km² for the lake level maximum.Elevated palaeoshorelines, specifically at 72–73 m, are completely absent in the study areas. Local remains of escarpments at elevations of 66 m and 73 m are due to resistant Miocene caprock and are therefore not interpretable as shoreline features.     

Relating accretion and erosion at an exposed tidal wetland to the bottom shear stress of combined current-wave action

Sediment dynamics have an important influence on the morphological evolution of tidal wetlands, which consist of mudflats and salt marshes. To understand the nature of sediment behavior under combined current-wave action at an exposed tidal wetland, we measured the waves, currents, water depths, bed-level changes, and sediment properties at a mudflat-salt marsh transition on the Yangtze Delta, China, during five consecutive tides under onshore winds of ~ 8 m/s, and calculated the bed shear stresses due to currents (τ_c), waves (τ_w), combined current-wave action (τ_cw), and the critical shear stress for erosion of the bottom sediment (τ_ce). The bed shear stresses under combined current-wave action (τ_cw) were approximately five times higher on the mudflat (up to 1.11 N/m²; average 0.27 N/m²) than on the salt marsh (up to 0.14 N/m²; average, 0.06 N/m²). On the mudflat, τ_cw was larger than the critical erosion shear stress (τ_ce = 0.103 N/m²) for 70% of the period of submergence, whereas τ_cw was always lower than τ_ce at the salt marsh site (τ_ce = 0.116 N/m²). This result indicates that the sediment dynamics on the mudflat were dominated by erosion, whereas at the salt marsh they were governed by deposition, which is in agreement with the observed bed-level change during the study period (− 3.3 mm/tide on the mudflat and 3.0 mm/tide on the salt marsh). A comparison of τ_cw values calculated using the [van Rijn, 1993] and [Soulsby, 1995] models for bed shear stresses under combined current-wave action indicates that both models are applicable to the present case and effectively predict the bottom shear stress under combined current-wave action. Overall, we conclude that τ_cw in combination with τ_ce is useful in assessing the hydrodynamic mechanisms that underlie the morphological evolution of exposed tidal wetlands.     

Salt-marsh foraminifera in Ho Bugt,western Denmark,and their use as sea-level indicators

Twenty-seven surface samples were collected from the Kjelst Enge salt marsh in Ho Bugt, Jutland, to investigate the potential use of salt-marsh foraminifera as sea-level indicators in Holocene intertidal deposits. Previous work from many coastlines around the world has shown that the vertical distribution of salt-marsh foraminifera is strongly controlled by the duration of tidal flooding (or height relative to tide level). Concentrations of foraminifera in Kjelst Enge salt marsh are low, especially in the high marsh, but a strong correlation was found between relative abundances of foraminiferal species and tidal elevation for 16 samples containing more than 30 specimens per 5 cc of sediment (r² = 0.80, root mean square error = 0.16 m). Sediment pH also has a strong influence on the distribution of the foraminifera, but salinity appears to be insignificant.     

Late Pleistocene–Holocene landscape evolution in Fossa Bradanica, Basilicata (southern Italy)

Studies in the middle Basento river basin supported by reliable chronological data (tephra layers and a number of absolute datings) have allowed the reconstruction of Late Pleistocene–Holocene geomorphological evolution of the middle to low Fossa Bradanica area (Basilicata, southern Italy). The original Upper Pleistocene hillslope has been dissected by deep gullies leaving relict slope pediments. Holocene filling of the Basento river valley and gullies occurred as a succession of downcut and fill episodes. A first phase of accumulation occurred in the Late Neolithic, which was followed by a downcutting between 4500 and 3700 cal. yr BP. A second deposition phase took place in the Greek–Roman period between 2800 and 1620 cal. yr BP, which was interrupted at around 2500 cal. yr BP. Another downcutting phase took place between 1620 and 1500 cal. yr BP, followed by a deposition phase between 1440 and 1000 cal. yr BP. After 1000 cal. yr BP a deep downcutting took place. Evidence collected with this study, coupled with climate data recorded in other Italian and European locations, suggests that filling and downcutting episodes in Fossa Bradanica were predominantly climate-driven. Anthropogenic impact only intensified or weakened these processes.     

Acoustic Doppler current profiler surveys along the Yangtze River

An acoustic Doppler current profiler is used to characterize the river velocity against the morphology of the Yangtze River from Chonqing to the sea. High flow velocities occur in the Three Gorges section and lower velocities in the middle and lower reaches of the river. This is largely due to the change in river pattern from a high gradient deeply-cut valley to a flat fluvial plain. Flow velocities fluctuate in the middle Yangtze due to the presence of meander bends of different length. There are numerous smaller velocity fluctuations in the lower Yangtze channel that reflect multichannel pattern with numerous sand bars and a river morphology affected by bedrock outcrops. Water depths of 40–100 m occur in the Three Gorges valley but decrease to 15–40 m in the middle and lower Yangtze. At the Gezhou Reservoir, 30 km downstream of the Three Gorges damsite velocity drops to low (< 1.0 m s^− 1) 20 km reach. A second low velocity (< 0.5 m s^− 1) zone, about 20 km in length, is located in the lower Yangtze near the coast probably due to the tidal influence. The results from this research will serve as a datum for evaluating changes to the river once the Three Gorges dam is completed in 2009.