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1.
Understanding the relative contributions of climatic and anthropogenic drivers of channel change are important to inform river management, especially in the context of environmental change. This global debate is especially pertinent in Australia as catchments have been severely altered since recent European settlement, and there is also strong evidence of cyclical climate variability controlling environmental systems. Corryong/Nariel Creek is an ideal setting to further study the interaction between climate and anthropogenic changes on channel evolution as it has experienced both significant periods of flood and drought, controlled by the El Niño Southern Oscillation (ENSO), and extensive anthropogenic changes. Since European settlement the floodplain has been completely cleared, the riparian zone almost entirely invaded by willows, and every reach of the channel has experienced some form of direct channel modification. Through the combined analysis of channel evolution, climate changes and anthropogenic history of the river it was found that both the ENSO-driven climate and anthropogenic drivers are significant, although at different scales of channel change. Significant straightening in response to land clearing in the early twentieth century occurred before any records of direct channel modifications. Following this, most river management works were in response to instabilities created in the clearing period, or to instabilities created by flooding triggering a new phase of instability in reaches which had already undergone stabilisation works. Overall, human activities triggered channel instability via land clearing, and management works since then generally exacerbated erosion during high flows that are driven by climate fluctuations. This research raises the interesting question of whether rivers in Australia have become more responsive to the ENSO cycle since the clearing of catchment and riparian vegetation, or whether the past response to climate variability was different.  相似文献   

2.
Channel planform change was analysed using a variety of data-sources for the medium-term (>25 years and <250 years) and short-term (<25 years) on a reach of the Rivers Tay and Tummel, Scotland. Map data were input into a Geographic Information System (GIS) and used to determine planform characteristics and changes in width, braiding index and sinuosity for the study reach between 1755 and 1976. Aerial photographs were utilised to determine the more recent changes that had taken place between 1971 and 1994. The analysis showed that significant changes had occurred over the medium term with a mean reduction in channel width of 34% for this period. These changes are comparable to those found in studies of similar European rivers for this period. Changes determined for the short-term displayed a continuance of this trend at a comparable rate of change. An analysis of flood frequency and magnitude, precipitation and discharge records for both periods does not show an associated decrease and therefore does not reflect the changes in channel planform. Evidence points towards flood embankment construction in the mid-1800s as the initial cause of channel change for the study reach which was later exacerbated by flow regulation. Incision and the subsequent stabilisation of lateral and mid-channel gravel bars by vegetation succession has resulted in an overall increase in the stability of the study reach which has persisted even where the embankments have fallen into disrepair.  相似文献   

3.
Intensive hydro technical works were conducted in the middle course of the Obra River (Poland) at the beginning of the nineteenth century. The ‘natural’ river course (functioning before the major construction works) was transformed into three artificial canals. Ground‐penetrating radar investigations, ground‐truthed with coring and remote sensing surveys, were conducted to reconstruct the course taken by the river prior to the hydro technical works. This work demonstrated that the Obra formerly had an anastomosing planform. Radiocarbon dating indicated that the earliest of the retraced channels were active before 9000 bp . The retraced river system was formed in a proglacial stream valley perpendicularly cut by remains of subglacial tunnels, now partly filled with lakes. The planform consisted of one or two major channels and a number of secondary channels formed by avulsions. During the last 2000 years, some of the avulsions may have been caused by anthropogenic interventions. The sequences of channel fill deposits indicate that particular channels changed from major to secondary ones. Sand deposits filling the bottom and middle parts of the channels point to an important role of upstream sediment supply causing in‐channel aggradation triggering the avulsions. Moreover, spatial variability in river patterns was found within the anastomosing system. Valley sections with meandering anabranches, anastomosing patterns with traces of a lateral migration and traces of a transition from meandering to anastomosing planform were distinguished.  相似文献   

4.
Since European settlement, the Cann River in East Gippsland, Victoria has experienced a 700% increase in channel capacity, a 150-fold increase in the rate of lateral channel migration, a 45-fold increase in bankfull discharge and a 860-fold increase in annual sediment load. Over the last century, and primarily the last 40 years, channel incision has removed the equivalent of around 1500 years of floodplain deposition. A numerical floodplain evolution model is presented which suggests that under a best case scenario, infilling the incised channel trench will take 31,000 years and this is predicated on the full recovery of the immediate riparian vegetation and the in-channel loading of woody debris. The asymmetry in the recovery time following rapid channel change, compared with the original deposition of the material, is explained by a combination of the sediment-starved character of the catchment and the altered hydraulic conditions within the channel, principally associated with the role of woody debris. These factors have major implications for geomorphic recovery potential, constraining what can be realistically achieved in river rehabilitation.  相似文献   

5.
In February/March 2007 extreme rainfall occurred over a four‐day period in the 7000 km2 East Alligator River catchment in Arnhem Land, northern Australia. The resultant large flood caused extensive bank erosion, channel widening, stripping of point bars and floodplain, resulting in large amounts of sand transport. This sand was largely deposited in the downstream river channel as a sand slug, and as deep overbank sand splays where the valley abruptly widened immediately downstream of an island anabranching, bedrock‐confined reach. Interpretation of a time series of aerial photographs and satellite images from 1950 to 2012 showed that there have been considerable channel changes along the study reach. The aerial photographs show that extensive sedimentation in the same reach as in 2007 also occurred in 1975, 1981 and 1984. Each time, the sand slug was reworked rapidly over succeeding years by subsequent smaller floods, and the channel deepened naturally as sand supply declined due to revegetation of the upstream riparian zone and the reformation and stabilisation of point and lateral bars. Sand slug formation at an intermediate floodout is an episodic process dependent on the supply of large volumes of sand by extensive channel erosion during extreme floods. A conceptual geomorphic model was developed to highlight the differential effectiveness of extreme versus moderate floods.  相似文献   

6.
Northwestern California is prone to regional, high magnitude winter rainstorms, which repeatedly produce catastrophic floods in the basins of the northern Coast Ranges. Major floods on the Eel River in 1955 and 1964 resulted in substantial geomorphic changes to the channel, adjacent terraces, and tributaries. This study evaluated the changes and the effects of a moderate flood in 1997 through field observations and examination of aerial photographs that spanned from 1954 to 1996. The purpose was to document the nature and magnitude of geomorphic responses to these three floods and assess the rates and controls on the recovery of the Eel River and its tributaries. Channel widening from extensive bank erosion was the dominant geomorphic change along the lower Eel River during major floods. As a result of the 1964 flood, the largest amount of widening was 195 m and represented an 80% change in channel width. Channel narrowing characterized the periods after the 1955 and 1964 floods. More than 30 years after the 1964 flood, however, the river had not returned to pre-flood width, which suggests that channel recovery required decades to complete. A long recovery time is unusual given that the Eel River is located in an area with a “superhumid” climate and has an exceptionally high sediment yield. This long recovery time may reflect highly seasonal precipitation and runoff, which are concentrated in 3–5 months each winter. In contrast to the main stem of the Eel River, the dominant effects of floods on the tributaries of the Eel River were rapid aggradation of channel bed and valley floor followed by immediate downcutting. Dendrogeomorphic data, aerial photographs, and field observations indicate that thick wedges of gravel, derived largely from hillslope failures in upper reaches of the tributaries, are deposited at and immediately upstream of the mouths of tributaries as the stage of the Eel River exceeded that of the tributaries during major floods. In the waning stages of the flood, the tributaries cut through the gravel at a rate equal to the lowering of the Eel and generated unpaired terraces and nickpoints. The complete process of deposition and incision can occur within a few days of peak discharge. Although reworking of some sediment on the valley floor may continue for years after large floods, channel morphology in the tributaries appears to be a product of infrequent, high magnitude events. The morphology of the tributary channel also appears to be greatly influenced by the frequency and magnitude of mass wasting in headwater areas of small basins.  相似文献   

7.
The Kent River flows from semi-arid headwaters in the agricultural (wheatbelt) region of Western Australia to a wetter and forested lower-catchment. It is set in an atypical fluvial environment, with rainfall decreasing inland towards a low-relief upper catchment. Replacement of native deep-rooted perennial vegetation with shallow-rooted seasonal crops has altered the hydrology of the upper catchment. Clearing for agriculture has also increased recharge of regional groundwater systems causing groundwater to rise and mobilise salt stores. This has increased stream salinity which has degradation riparian vegetation and decreased flow resistance. Elevated groundwater has also affected streamflow, increasing flow duration and annual discharge. The altered hydrological regime has affected geomorphic stability, resulting in channel responses that include incision and removal of uncohesive material. Channel response is variable, showing a high dependence on channel morphotype, channel boundary material and severity of salinity (degree of vegetation degradation). Response in confined reaches bounded by sandy material has been characterised by minor lateral bank erosion. In the fine-grained, wider, low-gradient reaches, mid-channel islands have been stripped of sandy sediment where vegetation has degraded. Following an initial period of high erosion rates in these reaches, the channel is now slowly adjusting to a new set of boundary conditions. The variable response has significant implications for management of salt affected rivers in southwestern Australia.  相似文献   

8.
l llltroductionIn North Che ~ of lack of water resources, moSt reservoir detain high poisons of both sediment andWater, so the oncoming Water in the POSt~ channel is severely reduced. Chalmel adjUStlnellt takes Placeunder the conditions of attenuated flow and sediment load. and the capedty Of the find conveyance Of thectal is ctrisot accodegh. hi the ~ or the ~o her, the ea~ty orfind convm ho bornerelatively lower air many you of chalmel adjUStment. Hence, the small fled, Which had been …  相似文献   

9.
Woody vegetation affects channel morphogenesis in Ozark streams of Missouri and Arkansas by increasing local roughness, increasing bank strength, providing sedimentation sites, and creating obstructions to flow. Variations in physiographic controls on channel morphology result in systematic changes in vegetation patterns and geomorphic functions with increasing drainage basin area. In upstream reaches, streams have abundant bedrock control and bank heights that typically are less than or equal to the rooting depth of trees. In downstream reaches where valleys are wider and alluvial banks are higher vegetation has different geomorphic functions. At drainage areas of greater than 100–200 kM2, Ozarks streams are characterized by longitudinally juxtaposed reaches of high and low lateral channel migration rates, referred to as disturbance reaches and stable reaches, respectively. Whereas stable reaches can develop stable forested floodplains (if they are not farmed), disturbance reaches are characterized by dynamic vegetation communities that interact with erosion and deposition processes.Disturbance reaches can be subdivided into low-gradient and high-gradient longitudinal zones. Low-energy zones are characterized by incremental, unidirectional lateral channel migration and deposition of gravel and sand bars. The bars are characterized by prominent bands of woody vegetation and ridge and swale topography. Channel monitoring data indicate that densely vegetated bands of woody vegetation formed depositional sites during bedload-transporting events. The same floods caused up to 20 m of erosion of adjacent cutbanks, scoured non-vegetated areas between vegetation bands, and increased thalweg depth and definition. In high-energy (or riffle) zones, channel movement is dominantly by avulsion. In these zones, vegetation creates areas of erosional resistance that become temporary islands as the channel avulses around or through them. Woody vegetation on islands creates steep, root-defended banks that contribute to narrow channels with high velocities.Calculation of hydraulic roughness from density and average diameter of woody vegetation groups of different ages indicates that flow resistance provided by vegetation decreases systematically with group age, mainly through decreasing stem density. If all other factors remain constant, the stabilizing effect of a group of woody vegetation on a gravel bar decreases with vegetation age.  相似文献   

10.
J.A. Moody  R.H. Meade 《Geomorphology》2008,99(1-4):387-403
Flood processes no longer actively increase the planform area of terraces. Instead, lateral erosion decreases the area. However, infrequent extreme floods continue episodic aggradation of terraces surfaces. We quantify this type of evolution of terraces by an extreme flood in May 1978 on Powder River in southeastern Montana. Within an 89-km study reach of the river, we (1) determine a sediment budget for each geomorphic feature, (2) interpret the stratigraphy of the newly deposited sediment, and (3) discuss the essential role of vegetation in the depositional processes.Peak flood discharge was about 930 m3 s− 1, which lasted about eight days. During this time, the flood transported 8.2 million tons of sediment into and 4.5 million tons out of the study reach. The masses of sediment transferred between features or eroded from one feature and redeposited on the same feature exceeded the mass transported out of the reach. The flood inundated the floodplain and some of the remnants of two terraces along the river. Lateral erosion decreased the planform area of the lower of the two terraces (~ 2.7 m above the riverbed) by 3.2% and that of the higher terrace (~ 3.5 m above the riverbed) by 4.1%. However, overbank aggradation, on average, raised the lower terrace by 0.16 m and the higher terrace by 0.063 m.Vegetation controlled the type, thickness, and stratigraphy of the aggradation on terrace surfaces. Two characteristic overbank deposits were common: coarsening-upward sequences and lee dunes. Grass caused the deposition of the coarsening-upward sequences, which had 0.02 to 0.07 m of mud at the base, and in some cases, the deposits coarsened upwards to coarse sand on the top. Lee dunes, composed of fine and very fine sand, were deposited in the wake zone downstream from the trees. The characteristic morphology of the dunes can be used to estimate some flood variables such as suspended-sediment particle size, minimum depth, and critical shear velocity. Information about depositional processes during extreme floods is rare, and therefore, the results from this study aid in interpreting the record of terrace stratigraphy along other rivers.  相似文献   

11.
A large number of rivers in Tuscany have channel planforms, which are neither straight nor what is usually understood as meandering. In the typical case, they consist of an almost straight, slightly incised main channel fringed with large lateral bars and lunate-shaped embayments eroded into the former flood plain. In the past, these rivers have not been recognised as an individual category and have often been considered to be either braided or meandering. It is suggested here that this type of river planform be termed pseudomeandering.A typical pseudomeandering river (the Cecina River) is described and analysed to investigate the main factors responsible for producing this channel pattern. A study reach (100×300 m) was surveyed in detail and related to data on discharge, channel changes after floods and grain-size distribution of bed sediments. During 18 months of topographic monitoring, the inner lateral bar in the study reach expanded and migrated towards the concave outer bank which, concurrently, retreated by as much as 25 m. A sediment balance was constructed to analyse bar growth and bank retreat in relation to sediment supply and channel morphology. The conditions necessary to maintain the pseudomeandering morphology of these rivers by preventing them from developing a meandering planform, are discussed and interpreted as a combination of a few main factors such as the flashy character of floods, sediment supply (influenced by both natural processes and human impact), the morphological effects of discharges with contrasting return intervals and the short duration of flood events. Finally, the channel response to floods with variable sediment transport capacity (represented by bed shear stress) is analysed using a simple model. It is demonstrated that bend migration is associated with moderate floods while major floods are responsible for the development of chute channels, which act to suppress bend growth and maintain the low sinuosity configuration of the river.  相似文献   

12.
The purpose of this paper is to describe and explain channel metamorphosis of the Ain River in east-central France and the effects of this metamorphosis on floodplain disturbance and vegetation development. The Ain River is a 195 km long stream originating in the Jura Mountains which flows into the Rhône River between Lyon, France, and Geneva, Switzerland. The lower 40 km of the Ain River, beyond the mountain front, are situated in a valley of outwash deposits where the floodplain is 0.2 to 1.2 km wide. A complex mosaic of floodplain landscape units has developed. Maps dating back to 1766 and six sets of aerial photographs dated between 1945 and 1991 were used to document changes in channel pattern. Aerial photos and field surveys were used to compile maps of landscape units based on dominant vegetation life-forms, species, and substrate. Six maps dated between 1945 and 1991 were digitized in ARC/INFO and an overlay was generated to determine the changes in landscape units as related to channel disturbance. Change from a braided to a single-thread meandering channel probably took place in the period 1930–1950. The process of river entrenchment has occurred throughout the Holocene but has accelerated in the present century due to shortening of the river course, construction of lateral embankments, and vegetation encroachment following reservoir construction and cessation of wood-cutting and grazing. The increase in horizontal channel stability coupled with channel entrenchment have decreased floodplain disturbance and lowered the water table by approximately one meter. Pioneer and disturbance-dependent landscape units have experienced a more terrestrial-like succession to an alluvial forest. Abandoned channels have also been replaced by alluvial forests. On poorly drained soils, shrub-swamp communities of willow and hydrophytic herbaceous plants have been replaced by mixed forests of ash, alder, black poplar, and oak. On well drained alluvial soils, ash and oak dominated hardwood forests have declined in favor of mesophytic stands of black poplar. All types of vegetation, but particularly dry grasslands-shrublands, have been cleared for mines, campgrounds, agriculture, and other types of development. Using several measures, landscape diversity decreased between 1945 and 1991.  相似文献   

13.
While many studies have documented pathways of river degradation, few studies have assessed the character, capacity, and stages of river recovery. In this paper, a generic procedure to measure river recovery is developed and applied in Bega catchment, on the south coast of New South Wales (NSW), Australia. The approach is based on analysis of geomorphic units and ergodic reasoning. Historical data and field analyses are used to identify stages of river evolution throughout Bega catchment. From this, stages of river condition and pathways of adjustment are assessed for three river styles at different positions within the catchment. Five categories of river condition are identified. Intact reaches operate in a self-adjusting manner, whereby processes maintain the pre-disturbance geomorphic character of the reach. The processes occurring in restoration reaches maintain and enhance the geomorphic structure of the reach. These reaches are moderately resilient to change. The river has experienced degradation, but has recovered to a condition approximating its pre-disturbance character and behavior. Degraded reaches are still adjusting to disturbance and the processes of recovery have not yet begun. The river is experiencing progressive deterioration away from the structure and function of the pre-disturbance condition. Turning-point reaches are at the transitional stage where they can either recover or revert to a degraded state. Finally, a creation reach has a self-adjusting character and behavior but operates under altered catchment boundary conditions. The character and behavior of the river do not equate to pre-disturbance conditions; rather, the river is well adjusted to the prevailing catchment boundary conditions of water and sediment transfer, and vegetation cover and composition (among many factors). Once these conditions have been identified for each river style, all reaches in a catchment are placed on pathways of degradation and recovery, and predictions made about their direction of change. The three river styles analyzed in Bega catchment demonstrate differing recovery pathways. Some reaches are adjusting toward a restored condition, while others are adjusting toward a new (or creation) condition. The geomorphic recovery potential of each reach is determined by assessing the connectivity of reaches throughout the catchment and interpreting limiting factors to recovery (e.g., water and sediment transfer, vegetation and coarse woody debris [CWD] character and distribution). [Key words: geomorphic river condition, river recovery, river degradation, recovery potential, fluvial geomorphology, river management.]  相似文献   

14.
Martin W. Doyle   《Geomorphology》2006,77(3-4):235
Whereas certain linkages between stream channel morphology and stream ecology are fairly well-understood, how geomorphology influences trophic interactions remains largely unknown. As a first step, a simple, heuristic model is developed that couples reach-scale geomorphic morphology with trophic dynamics between vegetation, detritus, herbivores, and predators. Predation is assumed to increase with depth beyond a threshold depth, and herbivory is assumed to decrease with velocity beyond a threshold velocity. Results show that the modeled food chain is sensitive to channel geometry, particularly around the threshold conditions for predators and herbivores. Importantly, geomorphic influences are not isolated to a particular trophic level, but rather are transferred through the food chain via top-down and bottom-up effects. The modeled system is particularly sensitive to changes in the end-members of the food chain: vegetation and predators. Results illustrate that geomorphic disturbances, known to affect a single trophic level (e.g., fish), likely impact multiple trophic levels in the stream ecosystem via trophic interactions. Such impacts at the multiple trophic level are poorly understood. While limited by the lack of empirical long-term data for testing and calibration, this simple model provides a structure for generating hypotheses, collecting targeted data, and assessing the potential impacts of stream disturbance or restoration on entire stream ecosystems. Further, the model illustrates the potential for future coupled stream models to explore spatial and temporal linkages.  相似文献   

15.
高海拔或高纬度山区（尤其高山冰川及冻土急剧消退区）常孕育适宜泥石流发育的地形和物源条件。气候变化（如升温、强降雨事件增多等）影响下,高山区潜在孕灾环境更易于成灾,泥石流成为主要的灾害类型和物质输移方式,也是高山区地貌变化的重要驱动力。由于野外监测困难,数据资料匮乏,鲜有针对高山区泥石流过程地貌效应的分析报道。以中国藏东南高山区泥石流多发的帕隆藏布流域为研究区,以古乡沟、天摩沟和扎木弄沟为典型小流域,结合遥感影像、DEM数据、无人机航拍、高精度RTK测量和野外踏勘调查,分析泥石流沟道地貌发育特征（冲淤变化、平面摆动）及其对主河河流地貌的影响,并探讨大规模泥石流事件影响下河谷地貌的长期演变趋势。高山区泥石流过程强烈塑造沟道自身地貌,上游物源区深切展宽和溯源蚀退,沟口堆积扇冲淤变化受控于泥石流事件规模和水流强度。泥石流过程显著影响主河道河流地貌,造成主河道横向冲淤和摆动,并影响堰塞体上游河段平面形态发育。长时间尺度上,河谷地貌在平面上发育形成宽窄相间的藕节状而在纵剖面上形成台阶状形态。  相似文献   

16.
提高黄河下游游荡段的输沙能力是河道治理的主要任务,而河道输沙效率（排沙比）受到来水来沙条件和河床边界条件的共同影响。本文基于1971—2016年花园口—高村河段（简称花高段）的实测水沙及地形资料,计算了花高段的平均河相系数及水沙条件（来沙系数和水流冲刷强度）,从汛期和场次洪水2个时间尺度,定量分析了排沙比与水沙条件及前一年汛后主槽形态之间的响应关系。分析结果表明：① 汛期和场次洪水排沙比与来沙系数呈负相关,与水流冲刷强度呈正相关,临界的汛期不淤来沙系数为0.012 kg?s/m 6,场次洪水排沙比与来沙系数及水量比的决定系数为0.76;② 游荡段排沙比与河相系数呈负相关,当河相系数大于15 /m 0.5时,河段排沙比基本小于1;③ 以来沙系数与河相系数为自变量的汛期排沙比计算式的决定系数为0.82,计算精度较高,对于场次洪水排沙比而言,断面形态的影响权重大于来沙系数。这些排沙比计算公式能够反映游荡段的输沙特点,有助于定量掌握断面形态及水沙条件对河道输沙能力的影响。  相似文献   

17.
河流地貌系统的过程响应模型实验   总被引：6，自引：0，他引：6  
金德生 《地理研究》1990,9(2):20-28
本文讨论了河流地貌系统的过程响应模型的基本原理、设计步骤、模型砂料配制以及实验数据的系统分析,并给出了设计实例和某些分析成果。  相似文献   

18.
Historical and modern scientific contexts are provided for the 2006 Binghamton Geomorphology Symposium on the Human Role in Changing Fluvial Systems. The 2006 symposium provides a synthesis of research concerned with human impacts on fluvial systems — including hydrologic and geomorphic changes to watersheds — while also commemorating the 50th anniversary of the 1955 Man's Role in Changing the Face of the Earth Symposium [Thomas, Jr., W. L. (Ed.), 1956a. Man's Role in Changing the Face of the Earth. Univ. Chicago Press, Chicago. 1193 pp]. This paper examines the 1955 symposium from the perspective of human impacts on rivers, reviews current inquiry on anthropogenic interactions in fluvial systems, and anticipates future directions in this field.Although the 1955 symposium did not have an explicit geomorphic focus, it set the stage for many subsequent anthropogeomorphic studies. The 1955 conference provided guidance to geomorphologists by recommending and practicing interdisciplinary scholarship, through the use of diverse methodologies applied at extensive temporal and geographical scales, and through its insistence on an integrated understanding of human interactions with nature. Since 1956, research on human impacts to fluvial systems has been influenced by fundamental changes in why the research is done, what is studied, how river studies are conducted, and who does the research. Rationales for river research are now driven to a greater degree by institutional needs, environmental regulations, and aquatic restoration. New techniques include a host of dating, spatial imaging, and ground measurement methods that can be coupled with analytical functions and digital models. These new methods have led to a greater understanding of channel change, variations across multiple temporal and spatial scales, and integrated watershed perspectives; all changes that are reflected by the papers in this volume. These new methods also bring a set of technical demands for the training of geomorphologists. The 2006 Binghamton Geomorphology Symposium complements the 1956 symposium by providing a more specific and updated view of river systems coupled with human interactions. The symposium focuses on linkages between human land use, structures, and channel modification with geomorphology, hydrology, and ecology. The emergence of sustainability as a central policy guideline in environmental management should generate greater interest in geomorphic perspectives, especially as they pertain to human activities. The lack of theories of anthropogeomorphic change, however, presents a challenge for the next generation of geomorphologists in this rapidly growing subfield.  相似文献   

19.
The distribution of riparian vegetation in relation to channel morphology is poorly understood in canyon rivers, which are characterized by in-channel fluvial sediment deposits rather than flood plains. This study focuses on vegetation and sandbar characteristics in two reaches of the lower Little Colorado River canyon in Arizona–one reach with ephemeral flow from the watershed, and another with perennial baseflow from a spring. Both reaches have been colonized by the exotic Tamarix chinensis, a riparian species known for its geomorphic influence on river channels. On the basis of a sampling of 18 bars, results show that vegetation frequency and density is significantly greater in the perennial study reach. However, sandbar morphology variables do not differ between reaches, despite a significantly narrower and deeper ephemeral channel. Hydraulic calculations of flood depths and Pearson correlations between bar and vegetation variables indicate reach-specific biogeomorphic relationships. In the ephemeral reach, higher bars are less affected by flood inundation, support older vegetation, and may be more stable habitat for vegetation. In the wider perennial reach where bars are lower and more expansive, vegetation patterns relate to bar size, Tamarix being most common on the largest bars. Overall results suggest that (1) vegetation variation relates to baseflow hydrology, (2) bar formation relates to high discharge events, and (3) vegetation patterns respond to, rather than influence, sandbar form in this canyon riparian system.  相似文献   

20.
The accumulation of sediment in river channels is a phenomenon that is not only influenced by the channel morphology, but also by the physical and geographical characteristics and the endogenous and exogenous processes taking place in the catchment. This paper presents an analysis of the impact the changes in lithological conditions have on the morphological and morphometric parameters of the Udava River channel and their relation to the channel accumulation forms representative of the river’s longitudinal profile as well as of its planform. Results document when accumulation forms occur and what is their spatial distribution within the longitudinal and cross-sectional river profiles. More resistant structures created sections with a lower degree of sedimentation, while in depression segments the degree of sedimentation was higher. With the increase in longitudinal slope, the impact of channel width on the average channel bar size increased. Also a difference in the accumulation was observed between the left and right bank which could be possibly explained by the impact of Coriolis force.  相似文献   

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