Streamflow regulation and multi-level flood plain formation: channel narrowing on the aggrading Green River in the eastern Uinta Mountains, Colorado and Utah

The style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river, the magnitude of streamflow regulation, and the post-dam sediment transport regime. We measured different magnitudes of channel adjustment on the Green River downstream from Flaming Gorge Dam, UT, USA, that are related to these three factors. Bankfull channel width decreased by an average of about 20% in the study area. In reaches with abundant debris fans and eddy deposited sand bars, the amount of channel narrowing was proportional to the decrease in specific stream power. The fan–eddy-dominated reach with the greatest decrease in stream power narrowed by 22% while the reach with the least decrease in stream power narrowed by 11%. In reaches with the same magnitude of peak flow reduction, meandering reaches narrowed by 15% to 22% and fan–eddy-dominated reaches narrowed by 11% to 12%. Specific stream power was not significantly affected by flow regulation in the meandering reaches.In the diverse array of reach characteristics and deposit types found in the study area, all pre- and post-dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre-dam floods, an intermediate bench that was inundated by rare post-dam floods, and a post-dam floodplain that was inundated by the post-dam mean annual flood. Analysis of historical photographs and tree-ring dating of Tamarix sp. shows that the intermediate bench and post-dam floodplain are post-dam landforms in each reach type. Although these two surfaces occur at different levels, they are forming simultaneously during flows of different magnitude. And while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies, both reach types contain deposits at all of these topographic levels.The process of channel narrowing varied between fan–eddy-dominated and meandering reaches. In the meandering reaches, where stream power has not changed, narrowing was accomplished by essentially the same depositional processes that operated prior to regulation. In fan–eddy-dominated reaches, where significant reductions in stream power have occurred, channel narrowing has been accompanied by a change in dominant depositional processes. Mid-channel sand deposits are aggrading on deposits that, in the pre-dam era, were active gravel bars. These deposits are creating new islands and decreasing the presence of open-framework gravel bars. In eddies, bare sand bars are replaced with vegetated bars that have a simpler topography than the pre-dam deposits.     

Exploring the causes of an extreme flood event in Central New York,USA

This study examined the causes of an extreme flood event on 28 June 2013 in Central New York, USA by comparing its hydrological, hydrometeorological, and rainfall-runoff transformation characteristics with those of a typical flood event. Flood frequency analyses showed that the maximum rainfall intensity and the peak discharge of the extreme event had recurrence intervals (RIs) of 8 and 86 years, respectively, while RIs for the typical event were 42 and 11 years, respectively. Their severity diagrams and quantification of their rainfall spatial variations illustrated that the extreme event was spatially localized with high intensities, whereas the typical event was spatially uniform and prolonged. Watershed modeling indicated that the rainfall-runoff transformation was dominated by the infiltration excess process for the extreme flood, while controlled by both infiltration and saturation excess processes for the typical event. These analyses revealed that the upgrade-magnitude conversion pattern of the extreme flood event was induced by the spatial pattern of the rainfall and the “lubricant” effect of the watershed, and emphasized the need for better understanding of such type of extreme events.     

Suspended sediment and Cs fluxes during the exceptional December 2003 flood in the Rhone River, southeast France

At the beginning of December 2003, one of the biggest floods for at least 150 yr was recorded on the Rhone River. In the lower part of the river, the peak flood reached 11,000 m³ s⁻¹. The geomorphological and radioecological consequences of such an event were investigated downstream all the nuclear installations by using measured and calculated fluxes and the total export of suspended sediment and associated ¹³⁷Cs. Results pointed out the major role played by large floods in the annual suspended sediment load, as 3.70 × 10⁶ tons of silts, 0.85 × 10⁶ tons of sands, and 0.84 × 10⁶ tons of clays were transferred towards the coastal environment. Nevertheless, these solid loads were found to be lower than those expected as regards the liquid discharge reached during this event and suggested that previous floods that occurred on the river and on its main tributaries during the last decade have probably led to the removal of available sediment from the channels and their banks. Besides, the ¹³⁷Cs activity measured within the suspended load was estimated at 14.9 ± 0.4 Bq kg⁻¹, which is a level characteristic of the suspended sediments from the Rhone catchment area and demonstrated that nuclear installations located along the Rhone valley did not significantly contribute to any increase in ¹³⁷Cs activity in the water during the flood. The total ¹³⁷Cs particulate export amounted to 77 ± 17 GBq and was mainly associated with the silt fraction that contributes to around 70% of the total ¹³⁷Cs export.     

云南哈尼梯田景观格局指数筛选研究（英文）

本文旨在筛选出一系列适合描述哈尼梯田景观格局的指数。我们从面积与边缘、形状、核心面积、隔离度/临近度、聚集度、多样性6个方面选取了47个景观指数,基于ArcGIS平台,将土地利用图矢量数据转化成栅格数据,栅格大小由30m×30m,以5m为间隔,增加至175m×175m,共计30幅土地利用图。运用SPSS软件,通过Pearson相关分析,47个景观格局指数分为21个小组,组内指数的两两相关性都在90%以上。每组选取代表性的指数,进行主成分分析,得出前5个主成分的方差变量能够占总方差变量的93%。选取旋转后各分量中具有最大相关性的5个景观结构指数来表达主成分的5个因子分别是分散指数(Splitting index[SPLIT]),斑块面积分布方差指数(Patch area distribution[AREA_CV]),香农多样性指数(Shannon’s diversity index[SHDI]),几何最近邻距离加权平均值指数(Euclidean nearest neighbor distance distribution[ENN_AM]),总核心面积指数(Total core area[TCA])。考虑到哈尼梯田的实际景观特征并结合前人的研究成果,我们增加平均分维数(Patch fractal dimension distribution[FRAC_MN])作为第6个指数。由于景观指数具有尺度效应,我们通过景观格局指数与尺度变化关系图,计算得出红河哈尼梯田景观格局指数的尺度域为40–45m之间。通过分维分析法,识别出哈尼梯田的特征格局在40–45m之间,即研究哈尼梯田的景观格局的最适宜栅格大小为40–45m之间。通过研究,我们认为以上6个景观指数能够反映哈尼梯田的景观格局特征,同时也可以运用这6个指数来描述类似哈尼梯田的景观格局。     

Spatial analysis of boil water advisories issued during an extreme weather event in the Hudson River Watershed,USA

Water infrastructure in the United States is aging and vulnerable to extreme weather. In August 2011, Tropical Storm Irene hit the eastern part of New York and surrounding states, causing great damage to public drinking water systems. Several water supply districts issued boil water advisories (BWAs) to their customers as a result of the storm. This study seeks to identify the major factors that lead water supply systems to issue BWAs by assessing watershed characteristics, water supply system characteristics and treatment plant parameters of water districts in the Mohawk-Hudson River watershed in New York. Logistic regression model suggests that the probability of a BWA being issued by a water supply district is enhanced by higher precipitation during the storm, high density of septic systems, lack of recent maintenance and low population density. Interviews with water treatment plant operators suggested physical damage to water distribution systems were the main causes of boil water advisories during storms. BWAs result in additional costs to residents and communities, and the public compliance of the advisory instructions is low, so efforts must be made to minimize their occurrence. Prior investments in infrastructure management can proactively address municipal water supply and quality issues.     

The influence of valley aggradation and listric normal faulting on styles of river avulsion: A case study of the Brazos River, Texas, USA

The Holocene avulsion history of the lower Brazos alluvial valley of east Texas, USA, was studied using 10 drill cores, 26 radiocarbon dates, aerial photos, and a digital elevation model. This study shows that two long-term processes, aggradation and localized valley tilting (along a normal listric fault), are responsible for generating two styles of avulsion. The first process precedes avulsion-by-progradation, while the second process precedes avulsion-by-annexation. As valley aggradation migrated updip over the last 7.5 ka, three regional backstepping avulsions occurred along the lower 140 km of the valley and each generated sizable deposits. A pattern emerges of landward stepping progradational avulsions tracking the locus of valley aggradation and of valley aggradation migrating inland even after the rate of sea level rise diminishes. At the same time, several local nodal avulsions occurred between 50 and 55 km updip of the current highstand shoreline but generated no observable deposits. Geomorphic evidence indicates that, since the late Pleistocene, active movement along a previously undocumented normal listric fault has occurred at the location of the nodal avulsion. These two long-term processes do not operate mutually exclusive of each other to promote avulsions; rather, they operate concurrently. Only aggradation promotes avulsions that affect floodplain alluviation, although the total volume of these deposits comprises a small portion of the valley fill.     

Spatial and temporal variations in the grain-size characteristics of historical flood plain deposits, Blue River, Wisconsin, USA

This study examined vertical, lateral, and downstream variations in the grain-size characteristics of historical (post-1830) overbank deposits in a watershed that has experienced high rates of accelerated flood plain sedimentation. More than 800 samples were collected from 53 cores along nine flood plain transects. Overbank deposits exhibit a coarsening-upward sequence attributed to historical changes in the sand content of source materials. The erosion of loess-capped soils increased the exposure, erosion, and transport of sandy parent materials. The average sand content of near-channel cores increases moderately downstream along two of the reaches because sandy source materials are increasingly exposed in larger main valleys in the northern part of the watershed. The two northernmost reaches are coarser overall, but do not display significant downstream trends. The sand content of surface and early historical overbank deposits generally decreases laterally as an exponential function of distance from the channel, suggesting transport by turbulent diffusion. The presence of sand throughout the transects and lateral coarsening at two of the transects, however, suggests that sediment transport by convection is also important.     

Natural levee deposition during the 2005 flood of the Saskatchewan River

In summer 2005, a controlled flood of the Saskatchewan River (east-central Saskatchewan, Canada) resulted in general floodplain inundation and extensive natural levee deposition along a 60-km reach extending from 40 km below the E.B. Campbell dam to Cumberland Lake. Levee crests along channel banks were inundated for up to 7 weeks in some areas of the floodplain. New deposits on levee crests varied from 0 to 70 cm in thickness, displaying large variations both along reach and in opposing sites across channels. Mean grain sizes, mainly silt and very fine sand, likewise varied considerably among sample sites.Pre- and post-flood surveys of channel cross sections along the flooded reach permitted assessments of relationships between channel-area changes and patterns of levee sedimentation in this system in which virtually all new flood sediment was derived by channel scour. Results show that both net deposition and net erosion occurred within the channel cross sections, but that on average, net channel enlargement of 4.2% prevailed over the entire survey reach when weighted by cross-section size. Over the 60-km flooded reach, zones of thick levee deposition occur at or just downstream of two areas of major channel enlargement, and an intermediate zone of thin levee deposits is associated with an intermediate area of net channel aggradation. This bimodal distribution of flood-deposit thickness is inferred to have resulted from differences in sediment supply produced locally by the different extents of channel-perimeter erosion. Two other factors—(1) position of interfacial zones between clear floodbasin water and turbid channel water, and (2) difference in pre-flood levee heights—contributed to the poor correspondence in thickness and grain size between opposing levees at some sites. Additional features of the new levee deposits, including increases in transverse slopes, abrupt basinward fining, and paucity of deposition in distal areas due to clear floodbasin waters, are characteristic of strong front loading that results when suspended sediment production is restricted to channel erosion processes.     

Response of the St. Joseph River to lake level changes during the last 12,000 years in the Lake Michigan basin

The water level of the Lake Michigan basin is currently 177 m above sea level. Around 9,800 ¹⁴C years B.P., the lake level in the Lake Michigan basin had dropped to its lowest level in prehistory, about 70 m above sea level. This low level (Lake Chippewa) had profound effects on the rivers flowing directly into the basin. Recent studies of the St. Joseph River indicate that the extreme low lake level rejuvenated the river, causing massive incision of up to 43 m in a valley no more than 1.6 km wide. The incision is seen 25 km upstream of the present shoreline. As lake level rose from the Chippewa low, the St. Joseph River lost competence and its estuary migrated back upstream. Floodplain and channel sediments partially refilled the recently excavated valley leaving a distinctly non-classical morphology of steep sides with a broad, flat bottom. The valley walls of the lower St. Joseph River are 12–18 m tall and borings reveal up to 30 m of infill sediment below the modern floodplain. About 3 × 10⁸ m³ of sediment was removed from the St. Joseph River valley during the Chippewa phase lowstand, a massive volume, some of which likely resides in a lowstand delta approximately 30 km off-shore in Lake Michigan. The active floodplain below Niles, Michigan, is inset into an upper terrace and delta graded to the Calumet level (189 m) of Lake Chicago. In the lower portion of the terrace stratigraphy a 1.5–2.0 m thick section of clast-supported gravel marks the entry of the main St. Joseph River drainage above South Bend, Indiana, into the Lake Michigan basin. This gravel layer represents the consolidation of drainage that probably occurred during final melting out of ice-marginal kettle chains allowing stream piracy to proceed between Niles and South Bend. It is unlikely that the St. Joseph River is palimpsest upon a bedrock valley. The landform it cuts across is a glaciofluvial-deltaic feature rather than a classic unsorted moraine that would drape over pre-glacial topography. This is the fifth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue.     

River mouth bar formation, riverbed aggradation and channel migration in the modern Huanghe (Yellow) River delta, China

This paper addresses the recent (1970s–1990s) processes of river mouth bar formation, riverbed aggradation and distributary migration in the Huanghe River mouth area, in the light of station-based monitoring, field measurements and remote sensing interpretation. The results show that the morphological changes of the river mouth bar have been closely associated with the largely reduced fluvial discharge and sediment load. Landform development such as bar progradation occurred in two phases, i.e. before and after 1989, which correspond to faster and lower bar growth rates, respectively. Fast riverbed aggradation in the mouth channel was strongly related to river mouth bar progradation. During 1976–1996, about 2.8% of the total sediment loads were deposited in the river channel on the upper to middle delta. Therefore, the river water level rose by a few meters from 1984 to 1996. The frequent distributary channel migration, which switched the radial channel pattern into the SE-directed pattern in the mid-1980s, was linked with mouth bar formation. Marine conditions also constrain seaward bar progradation. Furthermore, the history of river mouth bar formation reflects human impacts, such as dredging and dyking in order to stabilize the coastal area.     

1978-2015年喀喇昆仑山克勒青河流域冰川变化的遥感监测

本文采用1978、1991、2001和2015年的Landsat MSS、TM、ETM+和OLI遥感影像,通过遥感图像计算机辅助分类和目视解译等方法提取冰川边界,分析喀喇昆仑山克勒青河流域冰川在1978-2015年间的进退变化。结果表明：1978-2015年间研究区冰川面积由1821.70 km2减少至1675.92 km2,减少145.78 km2,占1978年冰川总面积的8.00%;冰川消融率较低,在气候变暖的背景下反而呈现出退缩速率由快变慢的趋势。研究区东南向冰川退缩率明显高于西北向,冰川退缩率随冰川规模的增大而减小。研究区内有27处冰川在1978-2015年间发生过特殊的前进现象,面积与长度显著增加。其中,木斯塔冰川西侧冰川末端在1996-1998年间前进速度为904 m/a,乔戈里冰川东侧冰川末端在2007-2009年间前进速度为446 m/a,5Y654D0097冰川末端在1978-1990年间前进速度为238 m/a,初步判定这三条冰川为跃动冰川。以10 a为滞后期分析研究区周边气象站点资料发现：研究区气温持续升高,降水量以1981年为分界点呈现“先减后增”趋势是冰川退缩速率减慢的原因之一;此外,亚大陆型冰川性质、巨大山势条件和高山冷储作用,也可能是冰川退缩幅度较小的原因。     

江汉平原钟桥遗址地层揭示的史前洪水事件

通过野外考古调查研究,利用对钟桥遗址疑似古洪水层和研究区现代洪水沉积物的锆石微形态、粒度、磁化率、Rb/Sr等地球化学指标的比较、AMS¹⁴C技术和考古器物断代,发现钟桥遗址在4800~4597 cal. a BP、4479~4367 cal. a BP和4168~3850 cal. a BP分别经历了三次古洪水事件并相应堆积了古洪水沉积层;结合江汉平原及其周边地区众多遗址的古洪水沉积层时代对比证据,揭示了屈家岭文化中晚期（4900~4600 cal. a BP）和石家河文化末期至夏代（4100~3800 cal. a BP）两次大洪水事件在江汉平原地区非常普遍。对史前洪水发生环境背景的进一步分析,反映江汉平原在5000~4500 a BP及4000 a BP前后的时段气候表现得不稳定,古洪水事件与气候环境变化驱动的江汉平原湖群扩张存在一定的联系,并影响区域新石器文化兴衰过程。同时,其它证据也表明该区社会发展过程和环境变化过程特别是古水文过程的矛盾在石家河文化末期已特别突出,发现具有全球意义的4000 a BP前后气候异常引起的大洪水事件是江汉平原地区石家河文化消亡的重要环境因素;而石家河文化末期该区内部或同中原以及其它地区间的冲突,都加速了石家河文化的崩溃。这些研究成果,提供了可靠的大禹时代史前洪水证据来说明其对新石器文化兴衰的社会影响,对于揭示4000 a BP气候事件中区域气候水文变化的响应规律,亦具有重要的科学意义。     

渭河三阳川盆地最高级阶地的年代厘定及其对河谷发育的指示意义

青藏高原东缘水系的演化历史长期存在着重大争议,鉴于任一水系的形成演化都是通过主要河谷的发育及其不断延展与整合完成的,因此确定河谷发育的起始时代是研究水系演化的关键。本文针对渭河上游三阳川盆地最高级阶地形成时代的研究,发现李家小湾河流阶地砾石层的ESR年代为1.26±0.15 Ma和1.32±0.19 Ma,26Al/10Be埋藏年代为1.45±0.70 Ma和1.04±0.43 Ma,说明该段河谷形成于早更新世晚期。综合青藏高原东缘夷平面、剥蚀面与河流阶地的研究成果,推断该区现代河谷系列主要形成于1.2 Ma以后,河流平均下切速率较高,为0.1~0.32 m/ka,指示了中更新世以来该区快速的地表抬升与河谷发育过程;而其前少数地段的先成河谷下切速率介于0.04~0.29 m/ka之间,说明区域地势总体低平,地表过程以剥蚀夷平为主,即高原东缘的现今水系格局主要是第四纪期间构造和气候共同作用下河流侵蚀的产物。     

Glacial Lake Musselshell: Late Wisconsin slackwater on the Laurentide ice margin in central Montana, USA

Cosmogenic surface exposure ages of glacial boulders deposited in ice-marginal Lake Musselshell suggest that the lake existed between 20 and 11.5 ka during the Late Wisconsin glacial stage (MIS 2), rather than during the Late Illinoian stage (MIS 6) as traditionally thought. The altitude of the highest ice-rafted boulders and the lowest passes on the modern divide indicate that glacial lake water in the Musselshell River basin reached at least 920–930 m above sea level and generally remained below 940 m. Exposures of rhythmically bedded silt and fine sand indicate that Lake Musselshell is best described as a slackwater system, in which the ice-dammed Missouri and Musselshell Rivers rose and fell progressively throughout the existence of the lake rather than establishing a lake surface with a stable elevation. The absence of varves, deltas and shorelines also implies an unstable lake. The changing volume of the lake implies that the Laurentide ice sheet was not stable at its southernmost position in central Montana. A continuous sequence of alternating slackwater lake sediment and lacustrine sheetflood deposits indicates that at least three advances of the Laurentide ice sheet occurred in central Montana between 20 and 11.5 ka. Between each advance, it appears that Lake Musselshell drained to the north and formed two outlet channels that are now occupied by extremely underfit streams. A third outlet formed when the water in Lake Musselshell fully breached the Larb Hills, resulting in the final drainage of the lake. The channel through the Larb Hills is now occupied by the Missouri River, implying that the present Missouri River channel east of the Musselshell River confluence was not created until the Late Wisconsin, possibly as late as 11.5 ka.     

Temporal river channel changes in the Gila Box Riparian National Conservation Area,Arizona, USA

Human activities have led to the serious decline and degradation of riparian areas in the Southwestern United States. Areas in Arizona, such as the Gila Box Riparian National Conservation Area, are protected to conserve and restore these unique ecosystems. This study investigated temporal change in the channel form and sinuosity of the Gila River in the Gila Box Riparian National Conservation Area. Specifically, cross sections were measured in 1994, 2001, and 2008 to evaluate changes in channel form. Aerial photographs between 1935 and 2007 were also compared to assess changes in channel sinuosity. Based on cross-sectional measurements, the thalweg was significantly deeper in 1994 than in 2001 and 2008, while width-depth ratios were significantly lower in 2008 than in 1994 and 2001. These measurements indicate that the channel is stabilizing. Sinuosity changed only slightly. Overall, climate patterns that cause large flood events appear to have been the major influence on the channel form. Still, exclusion of livestock and all-terrain vehicles, which increased vegetation in the riparian areas, also appeared to mitigate flood impacts. The stream channel appears to have been more resilient to the impacts of the flood in 2005, when vegetation was better established, than to the flood of 1993, when past management influences were still lingering.     

Hydro-geomorphic hazards and impact of man-made structures during the catastrophic flood of June 2000 in the Upper Guil catchment (Queyras, Southern French Alps)

The Guil River Valley (Queyras, Southern French Alps) is prone to catastrophic floods, as the long historical archives and Holocene sedimentary records demonstrate. In June 2000, the upper part of this valley was affected by a “30-year” recurrence interval (R.I.) flood. Although of lower magnitude and somewhat different nature from that of 1957 (>100-year R.I. flood), the 2000 event induced serious damage to infrastructure and buildings on the valley floor. Use of methods including high-resolution aerial photography, multi-date mapping, hydraulic calculations and field observations made possible the characterisation of the geomorphic impacts on the Guil River and its tributaries. The total rainfall (260 mm in four days) and maximum hourly intensity (17.3 mm h⁻¹), aggravated by pre-existing saturated soils, explain the immediate response of the fluvial system and the subsequent destabilisation of slopes. Abundant water and sediment supply (landsliding, bank erosion), particularly from small catchment basins cut into slaty, schist bedrock, resulted in destructive pulses of debris flow and hyperconcentrated flows. The specific stream power of the Guil and its tributaries was greater than the critical stream power, thus explaining the abundant sediment transport. The Guil discharge was estimated as 180 m³ s⁻¹ at Aiguilles, compared to the annual mean discharge of 6 m³ s⁻¹ and a June mean discharge of 18 m³ s⁻¹. The impacts on the Guil valley floor (flooding, aggradation, generalised bank erosion and changes in the river pattern) were widespread and locally influenced by variations in the floodplain slope and/or channel geometry. The stream partially reoccupied former channels abandoned or modified in their geometry by various structures built during the last four decades, as exemplified by the Aiguilles case study, where the worst damage took place. A comparative study of the geomorphic consequences of both the 1957 and 2000 floods shows that, despite their poor maintenance, the flood control structures built after the 1957 event were relatively efficient, in contrast to unprotected places. The comparison also demonstrates the role of land-use changes (conversion from traditional agro-pastoral life to a ski/hiking-based economy, construction of various structures) in reducing the Guil channel capacity and, more generally, in increasing the vulnerability of the human installations. The efficiency of the measures taken after the 2000 flood (narrowing and digging out of the channel) is also assessed. Final evaluation suggests that, in such high mountainous environments, there is a need to keep most of the 1957 flooded zone clear of buildings and other structures (aside from the existing villages and structures of particular economic interest), in order to enable the river to migrate freely and to adjust to exceptional hydro-geomorphic conditions without causing major damage.     

The braided Milk River, northern Montana, fails the Leopold–Wolman discharge-gradient test

The Milk River, the northernmost tributary to the Missouri–Mississippi River system, exhibits an anomalous sand-bed braiding reach in an otherwise meandering system. Shortly after leaving Alberta and entering Montana the river suddenly changes to braiding and maintains this pattern for 47 km before entering Fresno Reservoir. Measured stream gradient and bankfull discharge in the braiding reach severely fail the Leopold and Wolman [U.S. Geol. Surv. Prof. Pap. 282B (1957) 39] slope–discharge test for differentiating channel patterns. While channel slope has long been regarded as one of the primary variables associated with braiding, our data from the sand-bed Milk River do not support this relationship. Instead, the data show that the braiding reach has a lower channel slope (0.00047) than the meandering reach (0.00055). Coupled with a constant discharge the unit length stream power is comparable between the two reaches. At the morphologic transition between meandering and braiding, a dramatic reduction in channel bank strength occurs where the sampled silt–clay content declines from 65% in the meandering reach to 18% in the braiding. This enables channel widening which is reflected in a 60% reduction in unit area stream power in the braiding reach. Thus, sediment transport capacity declines and channel bars are deposited. During waning flows, these bars are dissected, producing a braiding morphology. We suggest that for sand-bed braiding rivers the silt–clay percentage in the channel banks may be more important than slope. A review of the original Leopold and Wolman [U.S. Geol. Surv. Prof. Pap. 282B (1957) 39] dataset, and many subsequent analyses, reveals that most braided rivers studied were gravel-bed. As a result, causal variables associated with braiding in sand-bed environments may need a thorough evaluation.     

Riparian plant species richness along lateral and longitudinal gradients of water stress and flood disturbance, San Pedro River, Arizona, USA

Diversity theory predicts that species numbers should be highest at intermediate levels of both disturbance and environmental stress. We examined woody and herbaceous plant species richness and cover in the San Pedro River flood plain, along lateral gradients of water availability (ground-water depth), flood disturbance (inundation frequency), and distance from and elevation above the channel, and along longitudinal gradients of water availability (ground-water depth, surface flow permanence, and rainfall) and flood disturbance (total stream power). Herbaceous species were recorded during four sampling periods, and spatial patterns for this group were time-dependent, reflecting temporal variation in limiting factors. During the summer dry season of a dry year, when overall richness was low, richness and cover of herbaceous species declined laterally from the stream channel with increasing ground-water depth, consistent with the idea that low resource levels can limit species richness. Following the summer monsoon rains and floods, when water was less limiting and annuals were seasonally abundant, lateral patterns shifted such that herbaceous species richness and cover increased with increasing plot location above or from the channel. The relationship of herbaceous species richness with tree canopy cover also varied seasonally, shifting from positive (greater richness under canopy) in dry seasons to negative (lesser richness under canopy) in wet seasons. Longitudinally, herbaceous species richness and cover were limited primarily by stream flow and/or ground-water availability during the summer dry season of a dry year. Following the summer monsoon rains and floods, patterns were weighted by the seasonally abundant annuals, and richness increased among sites primarily with distance upstream (and related rainfall gradients). Richness and cover patterns also varied between years with different flood conditions. During the two sampling seasons in the year following a large flood, herbaceous species richness increased with flood disturbance intensity but declined at the few most intensely disturbed sites, consistent with intermediate disturbance theory.For woody species, richness within plant functional groups varied in opposing ways along the lateral gradients: hydromesic pioneer species decreased and hydromesic and xeric competitors increased with distance from or above the channel, with no overall change in species richness. Among sites, woody species richness patterns were related to water availability, but not to flood disturbance. However, richness of woody hydromesic pioneer species increased with both increasing site moisture and flood disturbance. Woody and herbaceous species richness both increased among sites as a function of increasing flood-plain width, likely due to species–area effects. Overall, results indicate that flood disturbance and water availability both influence species richness of riparian plants in the flood plain of this semi-arid region river, with the relative influence of each factor varying among plant groups and over time.     

Effectiveness of monsoon floods on the Tapi River, India: role of channel geometry and hydrologic regime

Geomorphic effects of floods are a function of several controlling factors, such as magnitude, frequency, rate of sediment movement, flood power, duration of effective flows, sequence of events and the channel geometry. In this paper, these measures of effectiveness have been evaluated for the monsoon-dominated, flood-controlled and incised Tapi River, India by defining four flow categories: low flows, moderate flows, floods and large floods. Ratios between effectiveness parameters of moderate flows on one hand and the floods, large floods and maximum floods on the other, were computed to understand the relative importance of moderate and large flows. In addition to this, stream-power graphs for large floods were constructed, and the changes in channel form were analyzed by using multi-date cross-sections. The results of the study indicate that the morphological characteristics of the bedrock as well as the alluvial channels of the monsoonal and incised Tapi River are maintained by large-magnitude, but low frequency floods that occur at long intervals. Because the channel is incised the effectiveness of large flows is accentuated. The incised channel form enhances the role of large floods by reducing the width–depth ratio, and by increasing the velocity as well as the energy per unit area. The low and moderate flows are superior to high-magnitude flows, only in terms of suspended sediment transport and frequency of occurrence. Another conclusion is that the suspended sediment carried by flows may not be the most appropriate criterion for measuring the geomorphic effectiveness of flows, particularly for monsoonal rivers.     

1644-2009年黄河中游旱涝序列重建与特征诊断

据清代各县级政区的历史沿革,及现存历史文献资料的详细程度,选取黄河中游18个代表站点,在提高空间分辨率的基础上,采用旱涝等级法与面积加权法重建了1644-2009年各站点旱涝等级序列。利用小波分析、累积距平、滑动t-检验等方法,检测了全区过去366年旱涝发生的周期、阶段性和突变点。结果显示:旱涝序列存在21年、70年、114年左右的多年代际尺度周期信号;1644-1683年、1737-1775年、1885-1921年为多雨期,1684-1736年、1776-1814年、1922-2001年为少雨期;目前黄河中游开始进入雨涝多发期;1815-1895年旱涝波动频繁,气候进入不稳定期;过去366年存在2个由干旱转为雨涝期的气候突变点,分别位于1723-1726和1814-1816年。