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1.
This research evaluates the impact of rural-to-urban land use conversion on channel morphology and riparian vegetation for three streams in the Central Redbed Plains geomorphic province (central Great Plains ecoregion) of Oklahoma. The Deep Fork Creek watershed is largely urbanized; the Skeleton Creek watershed is largely rural; and the Stillwater Creek watershed is experiencing a rapid transition from rural to urban land cover. Each channel was divided into reaches based on tributary junctions, sinuosity, and slope. Field surveys were conducted at transects in a total of 90 reaches, including measurements of channel units, channel cross-section at bankfull stage, and riparian vegetation. Historical aerial photographs were available for only Stillwater Creek watershed, which were used to document land cover in this watershed, especially changes in the extent of urban areas (impervious cover).The three streams have very low gradients (< 0.001), width-to-depth ratios < 10, and cohesive channel banks, but have incised into red Permian shales and sandstone. The riparian vegetation is dominated by cottonwoods, ash, and elm trees that provide a dense root mat on stream banks where the riparian vegetation is intact. Channels increased in width and depth in the downstream direction as is normally expected, but the substrate materials and channel units remained unchanged. Statistical analyses demonstrated that urbanization did not explain spatial patterns of changes in any variables. These three channels in the central Redbed Plains are responding as flumes during peak flows, funneling runoff and the wash-load sediment downstream in major runoff events without any effect on channel dimensions. Therefore, local geological conditions (similar bedrock, cohesive substrates and similar riparian vegetation) are mitigating the effects of urbanization.  相似文献   

2.
Stream channel changes are often assumed to propagate downstream of the disturbed or urbanized reach, especially where the main controlling variables of discharge and sediment load vary regularly in the downstream direction. A survey of bankfull cross-sectional sizes at 10 stations along a tropical stream shows that downstream propagation of the disturbance may not always occur. Serial regression analysis suggests that a reduction in channel size induced by urbanization did not extend beyond the urbanized reach. Furthermore, local channel change did not alter the downstream log linear trend between channel size and basin area. Apparently, the storage of floodwaters and sediments on floodplains reduced the downstream effects of urbanization.  相似文献   

3.
Bankfull channel width is a fundamental measure of stream size and a key parameter of interest for many applications in hydrology, fluvial geomorphology, and stream ecology. We developed downstream hydraulic geometry relationships for bankfull channel width w as a function of drainage area A, w = α Aβ, (DHGwA) for nine aggregate ecoregions comprising the conterminous United States using 1588 sites from the U.S. Environmental Protection Agency's National Wadeable Streams Assessment (WSA), including 1152 sites from a randomized probability survey sample. Sampled stream reaches ranged from 1 to 75 m in bankfull width and 1 to 10,000 km2 in drainage area. The DHGwA exponent β, which expresses the rate at which bankfull stream width scales with drainage area, fell into three distinct clusters ranging from 0.22 to 0.38. Width increases more rapidly with basin area in the humid Eastern Highlands (encompassing the Northern and Southern Appalachians and the Ozark Mountains) and the Upper Midwest (Great Lakes region) than for the West (both mountainous and xeric areas), the southeastern Coastal Plain, and the Northern Plains (the Dakotas and Montana). Stream width increases least rapidly with basin area in the Temperate Plains (cornbelt) and Southern Plains (Great Prairies) in the heartland. The coefficient of determination (r2) was least in the noncoastal plains (0.36–0.41) and greatest in the Appalachians and Upper Midwest (0.68–0.77). DHGwA equations differed between streams with dominantly fine bed material (silt/sand) and those with dominantly coarse bed material (gravel/cobble/boulder) in six of the nine analysis regions. Where DHGwA equations varied by sediment size, fine-bedded streams were consistently narrower than coarse-bedded streams. Within the Western Mountains ecoregion, where there were sufficient sites to develop DHGwA relationships at a finer spatial scale, α and β ranged from 1.23 to 3.79 and 0.23 to 0.40, respectively, with r2 > 0.50 for 10 of 13 subregions (range: 0.36 to 0.92). Enhanced DHG equations incorporating additional data for three landscape variables that can be derived from GIS—mean annual precipitation, elevation, and mean reach slope—significantly improved equation fit and predictive value in several regions, most notably the Western Mountains and the Temperate Plains. Channel width was also related to human disturbance. We examined the influence of human disturbance on channel width using several indices of local and basinwide disturbance. Contrary to our expectations, the data suggest that the dominant response of channel width to human disturbance in the United States is a reduction in bankfull width in streams with greater disturbance, particularly in the Western Mountains (where population density, road density, agricultural land use, and local riparian disturbance were all negatively related to channel width) and in the Appalachians and New England (where urban and agricultural land cover and riparian disturbance were all negatively associated with channel width).  相似文献   

4.
The River Continuum Concept (RCC) is an existing theory regarding the distribution of plant communities along riverbanks. This study is concerned with the pattern of riparian vegetation along the Lower Ogun River (LOR) in relation to the observed downstream variation of various environmental factors (bank height, width, gradient, distance, cross‐sectional area and particle size). The LOR transits between undisturbed and disturbed sections. Forty‐eight cross sections were established at bankfull stage along the river channel stretch of about 90 km. Environmental variables distribution were determined using standard field procedures. Gradient was estimated from a 1:50 000 topographical map. At each corridor along the study sites, a 0.1 ha sampling site was chosen, in which 10 transects of 100 m2 were demarcated. In all, 332 individuals stratified under 80 species were recorded across the transects. The study revealed that total species richness (TSR) downstream decreases exponentially. TSR is significantly correlated with channel width, suggesting that hydraulic variable contributes more to the variation in TSR. The downstream total environmental factors revealed that width is inversely significant with downstream distance, suggesting that the river channel is narrower downstream. This affects plant life form distribution downstream and confirms why there is appearance of anthropogenic induced species downstream as against those in the upper course.  相似文献   

5.
Mediterranean environments have been subject to major land cover change since the end of the second world war. Housing, agricultural activities, forests, green spaces and other land uses have shifted due to urbanisation and tourism. These changes influence runoff, and municipal authorities often cannot estimate the net impact of complex land cover transitions. During this period, elected representatives have become increasingly sensitive to the risks of flooding and have implemented a number of channel management strategies. The main objective of this case study was to analyse the impact of land cover change on total storm runoff between 1950 and 2003 in a Mediterranean catchment near St Tropez, France. A secondary objective was to compare these changes to the impacts of channel management on bankfull discharge. Aerial photographs were used to classify land cover in 3 urban categories, vineyards and bare soil, forests, and green spaces. Stream discharge was estimated using a distributed event based total runoff approach. After validating the model for a large winter event (114 mm) for 1982, runoff was calculated for the same event for 1950 and 2003. Land cover changes occurred mainly in the alluvial plain area. Total gauge catchment urban area increased from 30.1 ha to 393.8 between 1950 and 2003 at the expense mainly of agricultural land, but this was compensated in part by an increase in grassed area. Some of the loss in vineyards was replaced by clearing forested land on the first hills close to the plain. Bank stabilisation and channel maintenance since the 1980’s reduced surface roughness and increased channel area, thereby greatly increasing bankfull discharge. While the impact of urbanisation on runoff was small, channel management effects increased bankfull discharge substantially. Flood damage from extreme events was not studied here.  相似文献   

6.
Channel responses to flow depletions in the lower Duchesne River over the past 100 years have been highly complex and variable in space and time. In general, sand-bed reaches adjusted to all perturbations with bed-level changes, whereas the gravel-bed reaches adjusted primarily through width changes. Gravel-bed reaches aggraded only when gravel was supplied to the channel through local bank erosion and degraded only during extreme flood events.A 50% reduction in stream flow and an increase in fine sediment supply to the study area occurred in the first third of the 20th century. The gravel-bed reach responded primarily with channel narrowing, whereas bed aggradation and four large-scale avulsions occurred in the sand-bed reaches. These avulsions almost completely replaced a section of sinuous channel about 14 km long with a straighter section about 7 km long. The most upstream avulsion, located near a break in valley slope and the transition from a gravel bed upstream and a sand bed downstream, transformed a sinuous sand-bed reach into a braided gravel-bed reach and eventually into a meandering gravel-bed reach over a 30-year period. Later, an increase in flood magnitudes and durations caused widening and secondary bed aggradation in the gravel-bed reaches, whereas the sand-bed reaches incised and narrowed. Water diversions since the 1950s have progressively eliminated moderate flood events, whereas larger floods have been less affected. The loss of frequent flooding has increased the duration and severity of drought periods during which riparian vegetation can establish along the channel margins. As a result, the channel has gradually narrowed throughout the study area since the late 1960s, despite the occasional occurrence of large floods. No tributaries enter the Duchesne River within the study area, so all reaches have experienced identical changes in stream flow and upstream sediment supply.  相似文献   

7.
Little is known about the downstream propagation of stream channel alterations initiated by urban flow regimes. In this study, freely available Light Detection and Ranging (LiDAR) data were combined with ground survey data to derive information on changes in stream cross sections in a North Carolina watershed, moving downstream across an urban-to-rural land-use boundary. Regression analyses relating LiDAR model and ground survey data exhibit poor relationships for all channel morphometric measurements except reach-average channel capacity. Application of the channel capacity regression reveals a negative power function pattern of downstream decline in channel enlargement. The largest declines occur before streams enter the rural landscape, with more gradual declines afterward. The largest declines in channel size might be attributable to reservoir effects along reaches with wide floodplains. Model results indicate no discernible enlargement once rural land covers and percentage impervious areas of about 60 percent and 16 percent of watershed area, respectively, are reached.  相似文献   

8.
Human impacts to mountain streams   总被引：5，自引：2，他引：5  
Ellen Wohl   《Geomorphology》2006,79(3-4):217
Mountain streams are here defined as channel networks within mountainous regions of the world. This definition encompasses tremendous diversity of physical and biological conditions, as well as history of land use. Human effects on mountain streams may result from activities undertaken within the stream channel that directly alter channel geometry, the dynamics of water and sediment movement, contaminants in the stream, or aquatic and riparian communities. Examples include channelization, construction of grade-control structures or check dams, removal of beavers, and placer mining. Human effects can also result from activities within the watershed that indirectly affect streams by altering the movement of water, sediment, and contaminants into the channel. Deforestation, cropping, grazing, land drainage, and urbanization are among the land uses that indirectly alter stream processes. An overview of the relative intensity of human impacts to mountain streams is provided by a table summarizing human effects on each of the major mountainous regions with respect to five categories: flow regulation, biotic integrity, water pollution, channel alteration, and land use. This table indicates that very few mountains have streams not at least moderately affected by land use. The least affected mountainous regions are those at very high or very low latitudes, although our scientific ignorance of conditions in low-latitude mountains in particular means that streams in these mountains might be more altered than is widely recognized. Four case studies from northern Sweden (arctic region), Colorado Front Range (semiarid temperate region), Swiss Alps (humid temperate region), and Papua New Guinea (humid tropics) are also used to explore in detail the history and effects on rivers of human activities in mountainous regions. The overview and case studies indicate that mountain streams must be managed with particular attention to upstream/downstream connections, hillslope/channel connections, process domains, physical and ecological roles of disturbance, and stream resilience.  相似文献   

9.
汶川地震后唐家山、小岗剑等堰寒湖的应急处置中都采用了开挖泄流槽这一措施,但出现了溃决流量超过了下游城镇防洪标准的危险情景.为控制堰塞湖排泄洪水超过下游区域的防护能力,排泄后期在泄流槽中加入人工结构体可能是一种有效的措施.通过室内模型实验,在泄流槽排泄后期抛投不同型式的人工结构体进行流量调控,包括四面体实体结构、四面体实体结构串、四面体实心框架结构和四面体实心框架结构串4种型式.通过抛投人工结构体与未抛投人工结构体的对比实验,发现:1.前者比后者溃决洪峰流量减小了26.6%～61.7%,说明人工结构体在控制溃决洪峰流量方面有显著效果;2.通过流量综合分析,提出人工结构式优选次序为:四面体实心框架结构串、四面体实体结构串、四面体实心框架结构、四面体实体结构;3.从残留体高度上看,未抛投人工结构体的坝体残留高度为原始坝高的1/3,堰塞体后部基本上是冲刷到底,而抛投了人工结构体的坝体残留高度为原始坝高的1/2,说明人工结构体控制堰塞坝下切侵蚀方面有较显著的效果.且从制作工序、制作时间和制作成本三方面,探讨了四面体实体和四面体实心框架结构的适用性,在施工条件许可的情况下,使用四面体实心框架结构更为理想.  相似文献   

10.
In this study, data measured from 1955-2016 were analysed to study the relationship between the water level and river channel geometry adjustment in the downstream of the Three Gorges Dam (TGD) after the impoundment of the dam. The results highlight the following facts: (1) for the same flow, the low water level decreased, flood water level changed little, lowest water level increased, and highest water level decreased at the hydrological stations in the downstream of the dam; (2) the distribution of erosion and deposition along the river channel changed from “erosion at channels and deposition at bankfulls” to “erosion at both channels and bankfulls;” the ratio of low-water channel erosion to bankfull channel erosion was 95.5% from October 2002 to October 2015, with variations between different impoundment stages; (3) the low water level decrease slowed down during the channel erosion in the Upper Jingjiang reach and reaches upstream but sped up in the Lower Jingjiang reach and reaches downstream; measures should be taken to prevent the decrease in the channel water level; (4) erosion was the basis for channel dimension upscaling in the middle reaches of the Yangtze River; the low water level decrease was smaller than the thalweg decline; both channel water depth and width increased under the combined effects of channel and waterway regulations; and (5) the geometry of the channels above bankfulls did not significantly change; however, the comprehensive channel resistance increased under the combined effects of riverbed coarsening, beach vegetation, and human activities; as a result, the flood water level increased markedly and moderate flood to high water level phenomena occurred, which should be considered. The Three Gorges Reservoir effectively enhances the flood defense capacity of the middle and lower reaches of the Yangtze River; however, the superposition effect of tributary floods cannot be ruled out.  相似文献   

11.
Ecoregions are generally homogenous areas that comprise similar associations of geology, climate, soils, land use, and potential natural vegetation. However, a certain amount of physical variability exists in ecoregions which makes them spatially diverse in their characteristics. Euclidean Sum of Squares was used to calculate the amount of stream reach dissimilarity within and between three prominent eastern Oklahoma ecoregions: Boston Mountains, Ozark Highlands, and Ouachita Mountains. Dissimilarity proximity coefficients were used to cluster stream reaches and create exemplars for each cluster. Streams were classified using the Rosgen classification to evaluate whether dissimilarity values could be applied to the classification. Metrics used to calculate stream reach dissimilarity were median particle size, bankfull width, width–depth ratio, gradient, and sinuosity. Maximum stream reach dissimilarity between two sites within the same ecoregion is highest in the Ouachita Mountains (18.59) and followed by the Ozark Highlands (13.12) and Boston Mountains (9.58). Removal of ecoregion boundaries increased maximum proximity coefficient dissimilarity between two sites in the Ozark Highlands and Ouachita Mountains to 34.26. Utilizing the dissimilarity of channel morphology for the purpose of regional classification helps in the understanding of how broad-scale processes influence the fluvial system.  相似文献   

12.
Historical planform changes in a 14.7 km reach of the lower Pages River were determined to assess whether they were autogenic (inherent in the river regime) or allogenic (driven by external changes) in nature so as to better focus river management activities and river restoration works. A pattern metamorphosis or complete change in river morphology occurred during the February 1955 flood. The peak discharge of this event exceeded the slope and grain size (intrinsic) threshold for braiding, converting the narrow, slightly sinuous stream to a wide, braided-like river. Five subsequent intrinsic threshold-exceeding floods did not cause further bar development because an over-widened channel already existed. Autogenic channel planform changes included sinuosity variations due to lateral migration and pattern metamorphosis due to the exceedance of a discharge–slope–grain size geomorphic threshold. Allogenic channel planform changes included: (1) realignment/channel straightening and artificial cutoffs by river training works; (2) lateral migration by increased bank erodibility due to riparian vegetation clearing; (3) lateral migration by the operation of a transitive geomorphic threshold involving the onset of a flood-dominated regime after 1946 and increased catchment runoff after 1830 due to large-scale clearing of catchment vegetation; and (4) the occurrence of a large flood in February 1955. Multiple forcing factors have clearly caused historical channel planform changes of the lower Pages River, making the design of river management and restoration works a complex matter outside the scope of simple formulaic protocols.  相似文献   

13.
This research examines variations in bankfull cross-sections along a steep stream of the Dolomites (Cordevole stream, Belluno, Northern Italy). Field measurements were conducted to determine variations in the channel top-width at bankfull stage in relation to the drainage area and to the length of the flow path. After grouping the bed morphologies according to the Montgomery and Buffington [Montgomery, D.R., Buffington, J.M., 1997. Channel-reach morphology in mountain drainage basins. Geol. Soc. of Am. Bull. 109 (5) 596–611.] classification, we analyzed the increase in bankfull width for the dominant stream units (cascades, step pools, isolated pools and colluvial reaches at the head of the basin). We observed that the morphologies more related to the drainage area are colluvial reaches and pools; the less adaptable are steps and cascades. These differences likely result from the absence of lateral constriction in the colluvial reaches and pools, whereas the presence of coarser sediments in the bed can affect the transverse adjustments in steps and reaches dominated by cascades. Linkages between cross-section geometry and parameters related to flow (i.e. drainage area and stream power) have been analyzed together with the distribution of surface grain sizes and its coarsening pattern. The existence of distinctive bankfull widths between different morphological units points out the degree of susceptibility to be modeled according to the channel slope, reference diameter (D90), and contributing area.  相似文献   

14.
三峡大坝下游水位变化与河道形态调整关系研究   总被引：8，自引：2，他引：6  
三峡水库蓄水利用已有13年,对坝下游洪、枯水位和河道形态调整的影响已初步显现,通过对1955-2016年长江中游水位、河道地形等资料的分析,结果表明：① 坝下游各水文站同流量枯水位下降、洪水位变化不大,最低水位上升,最高水位下降趋势;② 2002年10月-2015年10月枯水河槽冲刷量占平滩河槽冲刷量的95.5%,冲淤分布由蓄水前“冲槽淤滩”转为“滩槽均冲”,不同蓄水阶段存在差异;③ 河槽冲刷过程中,上荆江及以上河段枯水位下降趋势趋缓,下荆江及以下河段下降速率增加,应采取防控措施遏制河道水位下降趋势;④ 枯水河槽冲刷是长江中下游航道水深提升的基础,枯水位降幅小于深槽下切深度,在河道和航道整治工程综合作用下航道尺度提升,提前5年实现了2020年航道尺度规划目标;⑤ 平滩水位以上河槽形态调整不大,在河床粗化、岸滩植被、人类活动等综合作用下河道综合阻力增加,出现了中洪水流量—高水位现象,应引起足够重视。三峡水库汛期调蓄作用可有效提升中下游洪水防御能力,但不排除遭遇支流洪水叠加效应,中下游洪水压力仍然较大。  相似文献   

15.
Measurements of two small streams in northeastern Vermont, collected in 1966 and 2004–2005, document considerable change in channel width following a period of passive reforestation. Channel widths of several tributaries to Sleepers River in Danville, VT, USA, were previously measured in 1966 when the area had a diverse patchwork of forested and nonforested riparian vegetation. Nearly 40 years later, we remeasured bed widths and surveyed large woody debris (LWD) in two of these tributaries, along 500 m of upper Pope Brook and along nearly the entire length (3 km) of an unnamed tributary (W12). Following the longitudinal survey, we collected detailed channel and riparian information for nine reaches along the same two streams. Four reaches had reforested since 1966; two reaches remained nonforested. The other three reaches have been forested since at least the 1940s. Results show that reforested reaches were significantly wider than as measured in 1966, and they are more incised than all other forested and nonforested reaches. Visual observations, cross-sectional surveys, and LWD characteristics indicate that reforested reaches continue to change in response to riparian reforestation. The three reaches with the oldest forest were widest for a given drainage area, and the nonforested reaches were substantially narrower. Our observations culminated in a conceptual model that describes a multiphase process of incision, widening, and recovery following riparian reforestation of nonforested areas. Results from this case study may help inform stream restoration efforts by providing insight into potentially unanticipated changes in channel size associated with the replanting of forested riparian buffers adjacent to small streams.  相似文献   

16.
河岸湿地研究的理论与应用技术   总被引：2，自引：0，他引：2  
作为水陆之间的过渡带与生态交错区,河岸湿地具有一系列独特的性质和多种生态服务功能。鉴于河岸湿地在流域规划、河流保护与管理中的重要地位和作用,河岸湿地生态学研究已经开始得到重视。中国学者对河岸湿地生态学研究历经十余载,取得了众多研究成果,但是到目前为止,还没有形成一套比较系统和完备的学科体系,因而迫切需要加强该领域的系统研究工作。简要介绍了国内外河岸湿地基础理论,包括河岸湿地的定义、边界、结构和功能及其机理的研究;以及河岸湿地应用技术,主要包括河岸湿地生态模型与GIS技术的运用、岸坡稳定的生物工程技术和河岸湿地生态恢复的理论与实践等,提出了适应学科发展需要,综合已有的研究成果,构建河岸湿地生态学理论框架和学科体系的设想,以期推动河岸湿地生态学研究的发展。  相似文献   

17.
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.  相似文献   

18.
The spatial distribution of riparian vegetation is closely allied to abiotic processes along streams and rivers. There are dynamic relations between physical process, fluvial forms, and biotic structures. Explanation of these associations is critical to scientific understanding and practical management of riverine environments. Therefore, this study determines what geophysical parameters lead to the spatial patterns found in species of warm interior and cold montane riparian deciduous forests in central Arizona. Five riparian vegetation populations were examined along five perennial streams in the transition zone of central Arizona. The populations included Populus angustifolia (narrowleaf cottonwood), two commonly associated species of willow Salix lasiandra (western black willow) and Salix lasiolepis (arroyo willow), Alnus oblongifolia (Arizona alder), and Platanus wrightii (Arizona sycamore). Canonical correspondence analysis (CCA) with a forward selection was used to assess quantitatively the role of stream power in riparian vegetation patterns. Results indicated 40% of the spatial variability in the riparian populations was explained by channel morphology and several other variables related to changing channel geometry. Although floods are linked to the formation of geomorphic surfaces and the regeneration of riparian vegetation, changing fluvial landforms and channel patterns were closely related to the riparian species patterns in central Arizona. [Key words: Mogollon Rim, channel morphology, multivariate statistics.]  相似文献   

19.
Land use practices in Colorado during the last two centuries altered the supply of sediment and water to many channels in the upper South Platte Basin. As a result of increased supply of sediment and mobility and reduced peak flows, the characteristics of pools associated with channel constrictions, referred to as forced pools, may have been altered. Increased supply of sediment and reduced transport capacity of high flows could lead to aggradation in forced pools. Channel confined by road corridors could lead to high velocities at normal flows, increased energy dissipation from riprap, or even increased pool frequency resulting from failed riprap. To assess potential alterations, four hypotheses were tested: (1) impacted streams will show significantly different mean volume of pools than a control stream; (2) mean volume of pools on streams where land-use activities increased the supply of sediment will be significantly different from streams solely affected by flow regulation; (3) the strongest change in characteristics of pools of impacted streams will be a reduced volume of pools; (4) streams affected by road corridors will show statistically lower spacing of pools than streams unaffected by roads. The downstream spacing and residual volume of twenty consecutive forced pools were surveyed on five streams in the Colorado Front Range that varied from no contemporary impact to multiple historical and contemporary impacts. ANCOVA with stepwise model selection indicated that degree of land-use (categorical), bankfull spacing of pools, upstream riffle slope and expansion ratio were all significant (α = 0.1) predictors of volume of pools (R2 = 0.73). Simple linear regression of mean volume of pools and stream specific variables (gradient, drainage area and discharge) and least square means comparison of mean volume of pools indicated a need to standardize volumes of pools by slope and discharge so that the volumes of pools could be compared among different levels of land-use. Significant correlations between drainage area and volume of pools allowed volume of pools to be standardized by drainage area and thereby discharge. This dimensionless variable was also significantly correlated with channel slope, which permitted the construction of a new variable, PVQS (volume of pools standardized by discharge and slope). Least square means comparison of mean PVQS revealed that the control reach was significantly different from road-impacted reaches. Mean volume of pools was significantly larger in the control reach compared to all but one road-impacted stream. This was likely a function of higher wood loading in the control reach and the competence of high flows in the road-impacted reach. Streams affected by road corridors did not have significantly different bankfull spacing of pools from streams not impacted by roads. The multiple interactions among control and response variables explored in this study indicate the need to identify the most constrained and sensitive response variables when attempting to assess channel response to land use.  相似文献   

20.
Natural resource management may change local and regional ecosystems, especially in drought-prone environments. Livestock are commonly kept as a source of capital in agriculturally dominated communities in Southern Africa, but the mis-management of available forage and water resources has led to significant land and water degradation. In Northwest Zimbabwe, to reverse trends in environmental degradation a community-based conservation program was established that uses intensive Holistic Management Planned Grazing (HMPG) to restore lost habitat and re-establish natural vegetation. We examined riparian ecosystem structure and water quality to compare the environmental impact of this management to nearby communal lands during a drought. The results demonstrate that concentrating livestock on ephemeral stream standing pools results in reduced water quality and altered riparian ecosystem structure. These results were not significantly different from what was observed when wildlife utilized similar water resources without livestock influence. When water is scarce, as during extreme droughts, livestock usage of surface water resources must be weighted against community water needs. The long-term regional benefits of HMPG may prevail over short-term reductions in local water quality but more research is needed to assess all the consequences of such management.  相似文献   

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