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1.
We quantified how rates of stream channel migration in a montane meadow vary as a function of the riparian vegetation community. The South Fork of the Kern River at Monache Meadow, located in California's southern Sierra Nevada range, supports two distinct types of vegetation: a dry meadow community dominated by sagebrush and non‐native grasses (xeric scrub and meadow), and a wet meadow community dominated by rushes and sedges (hydric graminoids). We measured rates of lateral stream migration for dry versus wet meadow reaches from aerial photographs spanning a 40‐year period (1955–1995). While stream migration rates averaged only 0·24 ± 0·02 m a?1 in the wet meadow, the dry meadow channel migrated an average of 1·4 ± 0·3 m a?1. We used a linear model of meander migration to calculate coefficients that characterize bank migration potential, or bank erodibility, independent of channel curvature. These calculations demonstrate that, at Monache Meadow, banks without wet meadow vegetation are roughly ten times more susceptible to erosion than banks with wet meadow vegetation. Where stream bank heights consistently exceed 1 m, low water availability creates riparian habitats dominated by dry meadow vegetation. Thus, channel incision may reduce bank stability not only by increasing bank height, but also by converting banks from wet meadow to dry meadow vegetation. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
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Bank erosion rates and processes across a range of spatial scales are poorly understood in most environments, especially in the seasonally wet tropics of northern Australia where sediment yields are among global minima. A total of 177 erosion pins was installed at 45 sites on four sand‐bed streams (Tributaries North and Central, East Tributary and Ngarradj) in the Ngarradj catchment in the Alligator Rivers Region. Bank erosion was measured for up to 3·5 years (start of 1998/99 wet season to end of 2001/02 wet season) at three spatial scales, namely a discontinuous gully (0·6 km2) that was initiated by erosion of a grass swale between 1975 and 1981, a small continuous channel (2·5 km2) on an alluvial fan that was formed by incision of a formerly discontinuous channel between 1964 and 1978, and three medium‐sized, continuous channels (8·5–43·6 km2) with riparian vegetation. The bank erosion measurements during a period of average to above‐average rainfall established that substantial bank erosion occurred during the wet season on the two smaller channels by rapid lateral migration (Tributary Central) and by erosion of gully sidewalls due to a combination of within‐gully flows and overland flow plunging over the sidewalls (Tributary North). Minor bank erosion also occurred during the dry season by faunal activity, by desiccation and loss of cohesion of the sandy bank sediments and by dry flow processes. The larger channels with riparian vegetation (East Tributary and Ngarradj) did not generate significant amounts of sediment by bank erosion. Deposition (i.e. negative pin values) was locally significant at all scales. Bank profile form and channel planform exert a strong control on erosion rates during the wet season but not during the dry season. Copyright © 2006 Commonwealth Government of Australia. 相似文献
4.
Streambank sediment loading rates at the watershed scale and the benefit of riparian protection 
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下载免费PDF全文 Streambank erosion is a pathway for sediment and nutrient loading to streams, but insufficient data exist on the magnitude of this source. Riparian protection can significantly decrease streambank erosion in some locations, but estimates of actual sediment load reductions are limited. The objective of this research was to quantify watershed‐scale streambank erosion and estimate the benefits of riparian protection. The research focused on Spavinaw Creek within the Eucha‐Spavinaw watershed in eastern Oklahoma, where composite streambanks consist of a small cohesive topsoil layer underlain by non‐cohesive gravel. Fine sediment erosion from 2003 to 2013 was derived using aerial photography and processed in ArcMap to quantify eroded area. ArcMap was also utilized in determining the bank retreat rate at various locations in relation to the riparian vegetation buffer width. Box and whisker plots clearly showed that sites with riparian vegetation had on average three times less bank retreat than unprotected banks, statistically significant based on non‐parametric t‐tests. The total soil mass eroded from 2003 to 2013 was estimated at 7.27 × 107 kg yr.?1, and the average bank retreat was 2.5 m yr.?1. Many current erosion models assume that fluvial erosion is the dominant stream erosion process. Bank retreat was positively correlated with stream discharge and/or stream power, but with considerable variability, suggesting that mass wasting plays an important role in streambank erosion within this watershed. Finally, watershed monitoring programs commonly characterize erosion at only a few sites and may scale results to the entire watershed. Selection of random sites and scaling to the watershed scale greatly underestimated the actual erosion and loading rates. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
5.
River banks are important sources of sediment and phosphorus to fluvial systems, and the erosion processes operating on the banks are complex and change over time. This study explores the magnitude of bank erosion on a cohesive streambank within a small channelized stream and studies the various types of erosion processes taking place. Repeat field surveys of erosion pin plots were carried out during a 4‐year period and observations were supplemented by continuous monitoring of volumetric soil water content, soil temperature, ground water level and exposure of a PEEP sensor. Bank erosion rates (17·6–30·1 mm year?1) and total P content on the banks were relatively high, which makes the bank an important source of sediment and phosphorus to the stream, and it was estimated that 0·27 kg Ptot year?1 ha?1 may potentially be supplied to the stream from the banks. Yearly pin erosion rates exceeding 5 cm year?1 were mainly found at the lower parts of the bank and were associated with fluvial erosion. Negative erosion pin readings were widespread with a net advance of the bank during the monitoring period mainly attributed to subaerial processes and bank failure. It was found that dry periods characterized by low soil water content and freeze–thaw cycles during winter triggered bank failures. The great spatial variability, in combination with the temporal interaction of processes operating at different scales, requires new tools such as 3‐D topographical surveying to better capture bank erosion rates. An understanding of the processes governing bank erosion is required for riparian management using vegetational measures as root size and structure play different roles when it comes to controlling bank erosion processes. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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This paper focuses on one aspect of riparian zone change associated with channel incision. It presents information on sedimentation within the riparian zone of the River Garonne, France, derived from both historical and contemporary sources. The riparian zone investigated is morphologically complex, containing a remnant island and cut‐off channel. Information drawn from historical and contemporary sources permits the reconstruction of channel planform and cross‐profile change, as well as an assessment of changes in riparian zone sedimentation rates and patterns. Very significant changes are identified over the last 50 years: a change from a multiple thread to single thread channel planform; high rates of riparian zone sedimentation of the order of 0·5 to 2·5 cm a−1; clear spatial patterns in over‐bank sedimentation, reflecting topographic and vegetational variations and flood event magnitude; and a reduction in morphological and vegetational diversity through the period, which is expected to continue as the zone is transformed into an increasingly homogeneous sediment sink. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
7.
Restoration of the upper Strawberry River included bank stabilization techniques because it was assumed that excessive bank erosion was degrading spawning habitat for Bonneville cutthroat trout (BCT). Using a long‐term aerial photograph record, the historical range of variability in bank erosion rates and channel geometry was determined, and this information was used to assess present‐day conditions and the rationale for restoration. Relative to historical variability, the channel planform was relatively stable and bank erosion rates were the lowest recorded in the post‐disturbance era. Although a historical loss of riparian vegetation coincided with a shift to a wider and more sinuous channel, lateral migration rates declined and the channel narrowed as riparian cover increased in the decades before restoration, indicating a process of natural recovery. Furthermore, it was found that the percentage of fine sediment in the streambed before restoration was insufficient to affect BCT spawning success. Together these results suggest that bank erosion and fine sediment did not affect the quality of spawning habitat or the abundance of BCT on the upper Strawberry River. The results highlight how a historical analysis can be used to identify the sources of habitat degradation and inform the selection of restoration goals and strategies. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
8.
Although there is much evidence of intense soil erosion in cultivated areas of Navarre (Spain), information on it is currently scarce. Rill and ephemeral gully volumes can be used as a guide to minimum erosion rates. With the main purpose of determining the annual soil loss rates in cultivated areas of central Navarre, a detailed assessment of rainfall and of rill and gully erosion was made in 19 small catchments from October 1999 to September 2001. Seventeen of them were randomly selected, and were cultivated with winter cereals, vineyards or sunflowers. The other two catchments were selected to represent partially uncultivated lands abandoned for ten years. Channel cross‐sections were measured by using a 1‐m‐wide micro‐topographic profile meter, describing 632 cross‐sections and processing information from 31 600 pins. Erosive events happened every year in the three study areas. For cereal catchments, soil losses occurred in only one or two rainfall events each year, usually at the end of autumn and in some summers, with high erosion rates (0·20–11·50 kg m?2 a?1). In vineyards, soil losses occurred several times per year, and in any season. This is attributed to the small percentage of surface covered by the crop throughout the year. Again, high erosion rates were found (0·33–16·19 kg m?2 a?1), with ephemeral gully erosion causing more loss than rill erosion. No‐till is proposed as an effective conservation measure. From this large data set, it can be stated that rill erosion and ephemeral gully erosion are widespread in Mediterranean regions, and that much more attention should be paid to the problem. Abandoned fields showed very high erosion rates (16·19 kg m?2 a?1 on average), suggesting that the abandonment of marginal lands without implementing any erosion control can lead to severe erosion rates. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
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The hydrology and nitrogen biogeochemistry of a riparian zone were compared before and after the construction of beaver dams along an agricultural stream in southern Ontario, Canada. The beaver dams increased surface flooding and raised the riparian water table by up to 1·0 m. Increased hydraulic gradients inland from the stream limited the entry of oxic nitrate‐rich subsurface water from adjacent cropland. Permeable riparian sediments overlying dense till remained saturated during the summer and autumn months, whereas before dam construction a large area of the riparian zone was unsaturated in these seasons each year. Beaver dam construction produced significant changes in riparian groundwater chemistry. Median dissolved oxygen concentrations were lower in riparian groundwater after dam construction (0·9–2·1 mg L?1) than in the pre‐dam period (2·3–3·9 mg L?1). Median NO3‐N concentrations in autumn and spring were also lower in the post‐dam (0·03–0·07 mg L?1) versus the pre‐dam period (0·1–0·3 mg L?1). In contrast, median NH4‐N concentrations in autumn and spring months were higher after dam construction (0·3–0·4 mg L?1) than before construction (0·13–0·14 mg L?1). Results suggest that beaver dams can increase stream inflow to riparian areas that limit water table declines and increase depths of saturated riparian soils which become more anaerobic. These changes in subsurface hydrology and chemistry have the potential to affect the transport and transformation of nitrate fluxes from adjacent cropland in agricultural landscapes. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
10.
Tomoko Kyuka Satomi Yamaguchi Yusuke Inoue Kattia Rubi Arnez Ferrel Hideto Kon Yasuyuki Shimizu 《地球表面变化过程与地形》2021,46(7):1225-1237
Previous studies have demonstrated that riparian vegetation leads to channel transformation from a multi-bar to a single-thread channel planform. However, it still remains unclear how the presence of pioneer and mature vegetation affects the morphodynamics of single-thread meandering rivers. In this study, we therefore investigated the effects of vegetation strength on the morphodynamic evolution of an experimental meandering channel. Three physical laboratory experiments were conducted using alfalfa sprouts in different life stages – no vegetation, immature vegetation, and mature vegetation – to simulate different floodplain vegetation strengths. Our results demonstrate that vegetation plays a key role in mediating bank erosion and point-bar accretion, and that this is reflected in both the evolution of the channel bed as well as the sediment flux. The presence of mature vegetation maintained a deep, single-thread channel by reducing bank erosion, thereby limiting both channel widening and sediment storage capacity. Conversely, an unvegetated floodplain led to channel widening and high sediment storage capacity. Channel evolution in the unvegetated scenario showed that the active sediment supply from outer bank erosion led to slightly delayed point-bar accretion on the inner banks due to helical flow, deflecting the surface flow toward the outer banks and causing further erosion. In contrast, in the immature vegetation scenario, the outer banks were also initially eroded, but point-bar accretion did not clearly progress. This led to a greater width-to-depth ratio, resulting in a transition from a single- to a multi-thread channel with minor flow paths on the floodplain. The experimental results suggest that the eco-morphodynamic effects of young (low-strength) and mature (high-strength) vegetation are different. Notably, low-strength, early-stage vegetation increases channel complexity by accelerating both channel widening and branching, and therefore might promote the coexistence of multi-bars and pioneer vegetation. 相似文献
11.
Vegetation turnover in a braided river: frequency and effectiveness of floods of different magnitude 下载免费PDF全文
下载免费PDF全文 Nicola Surian Matteo Barban Luca Ziliani Giovanni Monegato Walter Bertoldi Francesco Comiti 《地球表面变化过程与地形》2015,40(4):542-558
This work addresses the temporal dynamics of riparian vegetation in large braided rivers, exploring the relationship between vegetation erosion and flood magnitude. In particular, it investigates the existence of a threshold discharge, or a range of discharges, above which erosion of vegetated patches within the channel occurs. The research was conducted on a 14 km long reach of the Tagliamento River, a braided river in north‐eastern Italy. Ten sets of aerial photographs were used to investigate vegetation dynamics in the period 1954–2011. By using different geographic information system (GIS) procedures, three aspects of geomorphic‐vegetation dynamics and interactions were addressed: (i) long‐term (1954–2011) channel evolution and vegetation dynamics; (ii) the relationship between vegetation erosion/establishment and flow regime; (iii) vegetation turnover, in the period 1986–2011. Results show that vegetation turnover is remarkably rapid in the study reach with 50% of in‐channel vegetation persisting for less than 5–6 years and only 10% of vegetation persisting for more than 18–19 years. The analysis shows that significant vegetation erosion is determined by relatively frequent floods, i.e. floods with a recurrence interval of c. 1–2.5 years, although some differences exist between sub‐reaches with different densities of vegetation cover. These findings suggest that the erosion of riparian vegetation in braided rivers may not be controlled solely by very large floods, as is the case for lower energy gravel‐bed rivers. Besides flow regime, other factors seem to play a significant role for in‐channel vegetation cover over long time spans. In particular, erosion of marginal vegetation, which supplies large wood elements to the channel, increased notably over the study period and was an important factor for in‐channel vegetation trends. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
12.
Maeve McBride W. Cully Hession Donna M. Rizzo Douglas M. Thompson 《地球表面变化过程与地形》2007,32(13):2019-2037
Measurements from a fixed‐bed, Froude‐scaled hydraulic model of a stream in northeastern Vermont demonstrate the importance of forested riparian vegetation effects on near‐bank turbulence during overbank flows. Sections of the prototype stream, a tributary to Sleepers River, have increased in channel width within the last 40 years in response to passive reforestation of its riparian zone. Previous research found that reaches of small streams with forested riparian zones are commonly wider than adjacent reaches with non‐forested, or grassy, vegetation; however, driving mechanisms for this morphologic difference are not fully explained. Flume experiments were performed with a 1:5 scale, simplified model of half a channel and its floodplain, mimicking the typical non‐forested channel size. Two types of riparian vegetation were placed on the constructed floodplain: non‐forested, with synthetic grass carpeting; and forested, where rigid, randomly distributed, wooden dowels were added. Three‐dimensional velocities were measured with an acoustic Doppler velocimeter at 41 locations within the channel and floodplain at near‐bed and 0·6‐depth elevations. Observations of velocity components and calculations of turbulent kinetic energy (TKE), Reynolds shear stress and boundary shear stress showed significant differences between forested and non‐forested runs. Generally, forested runs exhibited a narrow band of high turbulence between the floodplain and main channel, where TKE was roughly two times greater than TKE in non‐forested runs. Compared to non‐forested runs, the hydraulic characteristics of forested runs appear to create an environment with higher erosion potential. Given that sediment entrainment and transport can be amplified in flows with high turbulence intensity and given that mature forested stream reaches are wider than comparable non‐forested reaches, our results demonstrated a possible driving mechanism for channel widening during overbank flow events in stream reaches with recently reforested riparian zones. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
13.
Colin R. O'Farrell Arjun M. Heimsath Daniel E. Lawson Laura M. Jorgensen Edward B. Evenson Grahame Larson Jon Denner 《地球表面变化过程与地形》2009,34(15):2008-2022
Glacial erosion rates are estimated to be among the highest in the world. Few studies have attempted, however, to quantify the flux of sediment from the periglacial landscape to a glacier. Here, erosion rates from the nonglacial landscape above the Matanuska Glacier, Alaska are presented and compare with an 8‐yr record of proglacial suspended sediment yield. Non‐glacial lowering rates range from 1·8 ± 0·5 mm yr?1 to 8·5 ± 3·4 mm yr?1 from estimates of rock fall and debris‐flow fan volumes. An average erosion rate of 0·08 ± 0·04 mm yr?1 from eight convex‐up ridge crests was determined using in situ produced cosmogenic 10Be. Extrapolating these rates, based on landscape morphometry, to the Matanuska basin (58% ice‐cover), it was found that nonglacial processes account for an annual sediment flux of 2·3 ± 1·0 × 106 t. Suspended sediment data for 8 years and an assumed bedload to estimate the annual sediment yield at the Matanuska terminus to be 2·9 ± 1·0 × 106 t, corresponding to an erosion rate of 1·8 ± 0·6 mm yr?1: nonglacial sources therefore account for 80 ± 45% of the proglacial yield. A similar set of analyses were used for a small tributary sub‐basin (32% ice‐cover) to determine an erosion rate of 12·1 ± 6·9 mm yr?1, based on proglacial sediment yield, with the nonglacial sediment flux equal to 10 ± 7% of the proglacial yield. It is suggested that erosion rates by nonglacial processes are similar to inferred subglacial rates, such that the ice‐free regions of a glaciated landscape contribute significantly to the glacial sediment budget. The similar magnitude of nonglacial and glacial rates implies that partially glaciated landscapes will respond rapidly to changes in climate and base level through a rapid nonglacial response to glacially driven incision. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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Artificially straight river channels tend to be unstable, and ultimately develop into river meanders through bank erosion and point‐bar deposition. In this paper account is taken of the effects of riparian and floodplain vegetation on bank strength, floodplain flow resistance, shear stress partitioning, and bedload transport. This is incorporated into an existing 2D hydrodynamic‐morphological model. By applying the new model to an initially straight and single‐threaded channel, the way that its planform and cross‐sectional geometry evolve for different hydraulic and floodplain vegetation conditions is demonstrated. The results show the formation and upstream migration of gravel bars, confluence scouring and the development of meandering and braiding channel patterns. In cases where the channel becomes unstable, the instability grows out of bar formation. The resulting braiding patterns are similar to analytical results. The formation of a transition configuration requires a strong influence from vegetation. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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Tim Stott 《地球表面变化过程与地形》2005,30(3):349-357
Erosion rates surveyed using 230 erosion pins on 24 occasions over eight years (1994–2001) on forested stream banks, tributaries and forest ditches in the 0·89 km2 Nant Tanllwyth catchment, part of the Hafren Forest on Plynlimon, mid‐Wales, showed statistically significant increases of up to 40 mm a?1 in mean erosion rates during the two‐year period in which environmentally sensitive plot‐scale timber harvesting operations took place (1996–97). In the four years following timber harvesting mean erosion rates at all sites recovered to levels that were lower than before the harvesting operations began. This is attributed to increased light levels, following canopy removal, allowing vegetation to colonize exposed banks. There was a statistically significant relationship (p < 0·05) between mean erosion rate in 2000–01 (four years after harvesting) and percentage vegetation cover at erosion monitoring sites in the clearfelled (south tributaries) area though the same relationship did not hold for sites on the mainstream banks or for sites on the north (mature forest) ditch sites. The implications of natural vegetation colonization for management of such streams are discussed. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
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This study investigates how medium‐term gully‐development data differ from short‐term data, and which factors influence their spatial and temporal variability at nine selected actively retreating bank gullies situated in four Spanish basin landscapes. Small‐format aerial photographs using unmanned, remote‐controlled platforms were taken at the gully sites in short‐term intervals of one to two years over medium‐term periods of seven to 13 years and gully change during each period was determined using stereophotogrammetry and a geographic information system. Results show a high variability of annual gully retreat rates both between gullies and between observation periods. The mean linear headcut retreat rates range between 0·02 and 0·26 m a–1. Gully area loss was between 0·8 and 22 m² a–1 and gully volume loss between 0·5 to 100 m³ a–1, of which sidewall erosion may play a considerable part. A non‐linear relationship between catchment area and medium‐term gully headcut volume change was found for these gullies. The short‐term changes observed at the individual gullies show very high variability: on average, the maximum headcut volume change observed in 7–13 years was 14·3 times larger than the minimum change. Dependency on precipitation varies but is clearly higher for headcuts than sidewalls, especially in smaller and less disturbed catchments. The varying influences of land use and human activities with their positive or negative effects on runoff production and connectivity play a dominant role in these study areas, both for short‐term variability and medium‐term difference in gully development. The study proves the value of capturing spatially continuous, high‐resolution three‐dimensional data using small‐format aerial photography for detailed gully monitoring. Results confirm that short‐term data are not representative of longer‐term gully development and demonstrate the necessity for medium‐ to long‐term monitoring. However, short‐term data are still required to understand the processes – particularly human activity at varying time scales – causing fluctuations in gully erosion rates. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
17.
River channel dynamics following extirpation of wolves in northwestern Yellowstone National Park,USA
Historical records indicate that gray wolves (Canis lupus) were extirpated from the upper Gallatin River Basin in the early 1900s. Following the removal of these large carnivores, elk (Cervis elaphus) began to increasingly browse streamside vegetation in the winter range, causing widespread loss of formerly extensive willow (Salix spp.) communities. Historical aerial photographs and chronosequences of ground photographs were used to characterize general changes in vegetation and channel morphology over time. In August of 2004, riparian vegetation and channel cross‐sections were surveyed along three reaches of the upper Gallatin River. Reach A was located upstream of the elk winter range (control reach) whereas reaches B and C (treatment reaches) were located within the elk winter range. Willow cover on floodplains averaged 85 per cent for reach A, but only 26 per cent and 5 per cent for reaches B and C, respectively. The average return period of calculated bankfull discharges was 3·1 yrs for reach A but increased to 32·4 yrs and 10·6 yrs for reaches B and C, respectively. The long‐term loss of streamside vegetation allowed channels to generally increase in hydraulic capacity (via increases in width, incision or both) and decrease their hydrologic connectivity with adjacent floodplains. This study documents, perhaps for the first time, the resultant impacts to riparian vegetation functions and stream channel characteristics following the extirpation of a large mammalian carnivore. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
18.
Co‐evolution of riparian vegetation and channel dynamics in an aggrading braided river system,Mount Pinatubo,Philippines 下载免费PDF全文
下载免费PDF全文 Increased bank stability by riparian vegetation can have profound impacts on channel morphology and dynamics in low‐energy systems, but the effects are less clear in high‐energy environments. Here we investigate the role of vegetation in active, aggrading braided systems at Mount Pinatubo, Philippines, and compare results with numerical modeling results. Gradual reductions in post‐eruption sediment loads have reduced bed reworking rates, allowing vegetation to finally persist year‐round on the Pasig‐Potrero and Sacobia Rivers. From 2009–2011 we collected data detailing vegetation extent, type, density, and root strength. Incorporating these data into the RipRoot model and BSTEM (Bank Stability and Toe Erosion Model) shows cohesion due to roots increases from zero in unvegetated conditions to > 10·2 kPa in densely‐growing grasses. Field‐based parameters were incorporated into a cellular model comparing vegetation strength and sediment mobility effects on braided channel dynamics. The model shows both low sediment mobility and high vegetation strength lead to less active systems, reflecting trends observed in the field. The competing influence of vegetation strength versus channel dynamics is a concept encapsulated in a dimensionless ratio between timescales for vegetation growth and channel reworking known as T*. An estimated T* between 1·5 and 2·3 for the Pasig‐Potrero River suggests channels are still very mobile and likely to remain braided until aggradation rates decline further. Vegetation does have an important effect on channel dynamics, however, by focusing flow and thus aggradation into the unvegetated fraction of braidplain, leading to an aggradational imbalance and transition to a more avulsive state. The future trajectory of channel–vegetation interactions as sedimentation rates decline is complicated by strong seasonal variability in precipitation and sediment loads, driving incision and armoring in the dry season. By 2011, incision during the dry season was substantial enough to lower the water‐table, weaken existing vegetation, and allow for vegetation removal in future avulsions. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
19.
Flow regulation and water diversion for irrigation have considerably impacted the exchange of surface water between the Murray River and its floodplains. However, the way in which river regulation has impacted groundwater–surface water interactions is not completely understood, especially in regards to the salinization and accompanying vegetation dieback currently occurring in many of the floodplains. Groundwater–surface water interactions were studied over a 2 year period in the riparian area of a large floodplain (Hattah–Kulkyne, Victoria) using a combination of piezometric surface monitoring and environmental tracers (Cl−, δ2H, and δ18O). Despite being located in a local and regional groundwater discharge zone, the Murray River is a losing stream under low flow conditions at Hattah–Kulkyne. The discharge zone for local groundwater, regional groundwater and bank recharge is in the floodplain within ∼1 km of the river and is probably driven by high rates of transpiration by the riparian Eucalyptus camaldulensis woodland. Environmental tracers data suggest that the origin of groundwater is principally bank recharge in the riparian zone and a combination of diffuse rainfall recharge and localized floodwater recharge elsewhere in the floodplain. Although the Murray River was losing under low flows, bank discharge occurred during some flood recession periods. The way in which the water table responded to changes in river level was a function of the type of stream bank present, with point bars providing a better connection to the alluvial aquifer than the more common clay‐lined banks. Understanding the spatial variability in the hydraulic connection with the river channel and in vertical recharge following inundations will be critical to design effective salinity remediation strategies for large semi‐arid floodplains. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
20.
Processes and forms of an unstable alluvial system with resistant,cohesive streambeds 总被引：1，自引：0，他引：1
As a response to channelization projects undertaken near the turn of the 20th century and in the late 1960s, upstream reaches and tributaries of the Yalobusha River, Mississippi, USA, have been rejuvenated by upstream‐migrating knickpoints. Sediment and woody vegetation delivered to the channels by mass failure of streambanks has been transported downstream to form a large sediment/debris plug where the downstream end of the channelized reach joins an unmodified sinuous reach. Classification within a model of channel evolution and analysis of thalweg elevations and channel slopes indicates that downstream reaches have equilibrated but that upstream reaches are actively degrading. The beds of degrading reaches are characterized by firm, cohesive clays of two formations of Palaeocene age. The erodibility of these clay beds was determined with a jet‐test device and related to critical shear stresses and erosion rates. Repeated surveys indicated that knickpoint migration rates in these clays varied from 0·7 to 12 m a?1, and that these rates and migration processes are highly dependent upon the bed substrate. Resistant clay beds of the Porters Creek Clay formation have restricted advancement of knickpoints in certain reaches and have caused a shift in channel adjustment processes towards bank failures and channel widening. Channel bank material accounts for at least 85 per cent of the material derived from the channel boundaries of the Yalobusha River system. Strategies to reduce downstream flooding problems while preventing upstream erosion and land loss are being contemplated by action agencies. One such proposal involves removal of the sediment/debris plug. Bank stability analyses that account for pore‐water and confining pressures have been conducted for a range of hydrologic conditions to aid in predicting future channel response. If the sediment/debris plug is removed to improve downstream drainage, care should be taken to provide sufficient time for drainage of groundwater from the channel banks so as not to induce accelerated bank failures. Published in 2002 John Wiley & Sons, Ltd. 相似文献