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1.
Thad A. Wasklewicz 《自然地理学》2013,34(5):361-375
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
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Channel change is an important aspect of geomorphological evolution and habitat dynamics in large alluvial rivers. Planimetric maps of channel locations were used to investigate spatio-temporal alluvial channel changes in a geomorphic context along the Lower Mississippi River (LMR). Analyses were conducted with the aid of a time-weighted locational probability map. The locational probability map was constructed in ArcGIS and covered a period of 205 years. An examination of the pixel data from the probability maps indicates a high occurrence of low probability pixels along the Lower Mississippi River, which is in accordance with the dynamism of alluvial rivers. The northern section of the Lower Mississippi River (Columbus, KY to Memphis, TN) has been much more stable than the southern river segments (Helena, AR to Natchez, MS). Areas of high channel probability (channel stability) were often associated with alluvial channel confinement from a combination of flood-plain deposits, geologic structures and large stable islands. Low channel probability locations were found along sections exhibiting the following geomorphic characteristics: changes in meander amplitude, meander neck and chute cutoffs, meander extensional processes and islands lost in channel migrational processes. The results provide a strong foundation for understanding channel change on the Lower Mississippi River and serves as a valuable instrument for future management and restoration schemes. 相似文献
3.
This study explores the surface variability of alluvial fans from digital elevations model (DEM) derivatives generated from 1-m planimetric resolution airborne laser swath mapping data. Channel and interfluve dimensions of debris flow (DF) fans and fans generated from predominantly fluvial flows and some older debris flows (mixed flow [MF]) are extracted with the aid of a planimetric curvature classification. Significant differences are identified between the fan surface topography of DF and MF fans. MF fans tend to have smaller channel and interfluve widths, have smaller elevation differences between the crest of the interfluve and channel, and are more dissected than DF fans. The morphometric differences between the two fan classes can be explained by differences in the primary processes that develop the surficial features, but also the preponderance for secondary erosional processes acting on the MF fans. 相似文献
4.
Relict nebkhas (pimple mounds) record prolonged late Holocene drought in the forested region of south-central United States 总被引：2，自引：0，他引：2
Christopher L. Seifert Randel Tom Cox Steven L. Forman Tom L. Foti Thad A. Wasklewicz Andrew T. McColgan 《Quaternary Research》2009,71(3):329-339
The origin and significance of pimple mounds (low, elliptical to circular dune-like features found across much of the south-central United States) have been debated for nearly two centuries. We cored pimple mounds at four sites spanning the Ozark Plateau, Arkansas River Valley, and Gulf of Mexico Coastal Plain and found that these mounds have a regionally consistent textural asymmetry such that there is a significant excess of coarse-grained sediment within their northwest flanks. We interpret this asymmetry as evidence of an eolian depositional origin of these mounds and conclude they are relict nebkhas (coppice dunes) deposited during protracted middle to late Holocene droughts. These four mounds yield optically stimulated luminescence ages between 2400 and 700 yr that correlate with well-documented periods of eolian activity and droughts on the southern Great Plains, including the Medieval Climate Anomaly. We conclude vegetation loss during extended droughts led to local eolian deflation and pimple mound deposition. These mounds reflect landscape response to multi-decadal droughts for the south-central U.S. The spatial extent of pimple mounds across this region further underscores the severity and duration of late Holocene droughts, which were significantly greater than historic droughts. 相似文献
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Coupled hillslope and channel processes in headwater streams (HWS) lead to rapid changes in channel dimensions. Changes in channel size and shape caused by a debris flow event along the length of a headwater stream in the Ashio Mountains, Japan, were captured with the aid of repeat high-definition surveys using terrestrial laser scanning (TLS) techniques. The HWS was classified into three distinct reaches below the debris flow initiation zone. A large knickpoint separated an upper bedrock reach from a colluvial reach along the midsection of the drainage. The colluvial reach transitioned to a lower bedrock reach that terminated at the master stream. Cross-sectional and morphometric analyses revealed no statistically significant changes in channel size or shape along the upper bedrock reach. Debris flow erosion generated significant differences in channel size and shape along a colluvial reach. Sediment bulking associated with erosion along the colluvial reach led to increases in channel size along the lower bedrock reach, but no statistical differences in channel shape. Morphometric analyses from the TLS point cloud revealed that debris flow erosion produced a distinct nonlinear change in channel dimensions in the downstream direction within the HWS. Variations in channel substrate along the length of HWS contributed directly to this nonlinear response. The episodic nature and nonlinearity of erosion associated with the current debris flow event highlights the importance of debris flows in general in understanding the transport of sediment, coarse to fine particulate organic material, and large woody debris, which are critical to the long-term management of riverine environments. TLS sampling methods show promise as one component of a multianalytical approach needed to continuously monitor and manage the dynamics of HWS. 相似文献
7.
Rapid urbanization around Palm Springs, California has encroached upon the sand habitats of the endangered Coachella Valley fringetoed lizard (Uma inornata). The Nature Conservancy and the Coachella Valley Preserve System are concerned about maintaining an adequate sediment supply to the dune environments that urban growth might interrupt. Determining the source of the existing aeolian sediments is a critical first step toward ensuring a perpetual sediment supply. Geochemical links between the aeolian sediment sources and the Coachella Valley Preserve dunes were discovered by mapping the aeolian sand distribution and analyzing trace elements using multivariate statistical techniques. 相似文献
8.
Thad A. Wasklewicz 《地球表面变化过程与地形》1994,19(8):715-734
Examination of weathering rinds from lava flows on Hawaii with backscatter electron microscopy and electron micro-probe analysis reveals that olivine weathers more slowly than adjacent clinopyroxene and plagioclase in environments with a paucity of organic acids. Yet when weathering rinds are in contact with abundant organic acids secreted by lichens, olivine weathers before clinopyroxene and plagioclase weathers last. The exception to Goldich's widely accepted mineral stability series in organic-poor environments runs counter to a thermodynamics explanation for the order of mineral weathering and illustrates the importance of the biogeochemical environment. 相似文献
9.
Surficial patterns of debris flow deposition on alluvial fans in Death Valley, CA using airborne laser swath mapping data 总被引：2，自引：4，他引：2
Debris flows are a common event in mountainous environments. They often possess the greatest potential for destruction of property and loss of lives in these regions. Delimiting the spatial extent of potential damage from debris flows relies on detailed studies of the location of depositional zones. Current research indicates debris flow fans have two distinct depositional zones. However, the two zones were derived from studies containing detailed analyses of only a few fans. High resolution airborne laser swath mapping (ALSM) data is used to calculate profile curvature and surface gradient on 19 debris flow fans on the eastern side of Death Valley. The relationship between these parameters is assessed to 1) identify if debris flow fans are accurately represented by two depositional zones, and 2) to assess how these terrain parameters relate to one another at the individual fan scale. The results show at least three zones of deposition exist within the sampled fans. These zones do not hold consistent when individual fan morphometry is analyzed in conjunction with localized fan surface gradients. Fans with consistently shallower gradients exhibit numerous depositional zones with more subtle changes in profile curvature. Steeper gradient fans exhibit significantly fewer zones with more pronounced local changes in profile curvature. The surface complexity of debris flow fans is evident from these analyses and must be accounted for in any type of hazard studies related to these features. 相似文献
10.
Thomas A.G. Smyth Patrick A. Hesp Ian J. Walker Thad Wasklewicz Paul A. Gares Alexander B. Smith 《地球表面变化过程与地形》2019,44(10):1988-1999
A number of studies have measured and numerically modelled near surface wind velocity over a range of aeolian landforms and made suppositions about topographic change and landform evolution. However, the precise measurement and correlation of flow dynamics and resulting topographic change have not yet been fully realized. Here, using repeated high-resolution terrestrial laser scanning and numerical flow modelling within a bowl blowout, we statistically analyse the relationship between wind speed, vertical wind velocity, turbulent kinetic energy and topographic change over a 33-day period. Topographic results showed that erosion and deposition occurred in distinct regions within the blowout. Deposition occurred in the upwind third of the deflation basin, where wind flow became separated and velocity and turbulent kinetic energy decreased, and erosion occurred in the downwind third of the deflation basin, where wind flow reattached and aligned with incident wind direction. Statistical analysis of wind flow and topographic change indicated that wind speed had a strong correlation with overall topographic change and that vertical wind velocity (including both positive and negative) displayed a strong correlation with negative topographic change (erosion). Only weak or very weak correlations exist for wind flow parameters and positive topographic change (accretion). This study demonstrates that wind flow modelling using average incident wind conditions can be utilized successfully to identify regions of overall change and erosion for a complex aeolian landform, but not to identify and predict regions where solely accretion will occur. © 2019 John Wiley & Sons, Ltd. 相似文献