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R. H. Webb

Publications and source records attributed to R. H. Webb.

At least 19 recordsLinked to original sources
USGS2017

Late Holocene vegetation changes in Greenwater Valley, Mojave Desert, California

Small-scale late Holocene vegetation changes were determined from a series of 13 modern and fossil packrat middens collected from a site in the Greenwater Valley, northern Mojave Desert, California. Although the site is above the modern lower limit of Coleogyne ramosissima (black-brush), macrofossils of this shrub are only present in samples younger than 270 yr B.P. In order to measure changes more subtle than presence vs absence, macrofossil concentrations were quantified, and principal components and factor analyses were used to distinguish midden plant assemblages. Both the presence/absence data and the statistical analyses suggest a downward shift of 50 to 100 m for Coleogyne (blackbrush) communities between 1435 and 1795 A.D. ?? 1985.

Quaternary Research↗
USGS2017

Peak discharge of a Pleistocene lava-dam outburst flood in Grand Canyon, Arizona, USA

The failure of a lava dam 165,000 yr ago produced the largest known flood on the Colorado River in Grand Canyon. The Hyaloclastite Dam was up to 366 m high, and geochemical evidence linked this structure to outburst-flood deposits that occurred for 32 km downstream. Using the Hyaloclastite outburst-flood deposits as paleostage indicators, we used dam-failure and unsteady flow modeling to estimate a peak discharge and flow hydrograph. Failure of the Hyaloclastite Dam released a maximum 11 ?? 109 m3 of water in 31 h. Peak discharges, estimated from uncertainty in channel geometry, dam height, and hydraulic characteristics, ranged from 2.3 to 5.3 ?? 105 m3 s-1 for the Hyaloclastite outburst flood. This discharge is an order of magnitude greater than the largest known discharge on the Colorado River (1.4 ?? 104 m3 s-1) and the largest peak discharge resulting from failure of a constructed dam in the USA (6.5 ?? 104 m3 s-1). Moreover, the Hyaloclastite outburst flood is the oldest documented Quaternary flood and one of the largest to have occurred in the continental USA. The peak discharge for this flood ranks in the top 30 floods (>105 m3 s-1) known worldwide and in the top ten largest floods in North America. ?? 2005 University of Washington. All rights reserved.

Quaternary Research↗
USGS2012

Long-term change along the Colorado River in Grand Canyon National Park (1889-2011)

The Colorado River and its riverine resources have undergone profound changes since completion of Glen Canyon Dam in 1963, as every river runner with any history in Grand Canyon will attest. Long-term monitoring data are difficult to obtain for high-value resource areas (Webb et al. 2009), particularly in remote parts of national parks, yet these data are important to determining appropriate actions for restoration of resources and (or) potential modifications of flow releases on regulated rivers. The river corridor through the bottom of Grand Canyon creates a challenging environment for change-detection monitoring techniques (Belnap et al. 2008).

Arizona↗
USGS2011

A comparison of methods to assess long-term changes in Sonoran Desert vegetation

Knowledge about the condition of vegetation cover and composition is critical for assessing the structure and function of ecosystems. To effectively quantify the impacts of a rapidly changing environment, methods to track long-term trends of vegetation must be precise, repeatable, and time- and cost-efficient. Measuring vegetation cover and composition in arid and semiarid regions is especially challenging because vegetation is typically sparse, discontinuous, and individual plants are widely spaced. To meet the goal of long-term vegetation monitoring in the Sonoran Desert and other arid and semiarid regions, we determined how estimates of plant species, total vegetation, and soil cover obtained using a widely-implemented monitoring protocol compared to a more time- and resource-intensive plant census. We also assessed how well this protocol tracked changes in cover through 82 years compared to the plant census. Results from the monitoring protocol were comparable to those from the plant census, despite low and variable plant species cover. Importantly, this monitoring protocol could be used as a rapid, "off-the shelf" tool for assessing land degradation (or desertification) in arid and semiarid ecosystems.

Arizona↗
USGS2009

Plants and ventifacts delineate late Holocene wind vectors in the Coachella Valley, USA

Strong westerly winds that emanate from San Gorgonio Pass, the lowest point between Palm Springs and Los Angeles, California, dominate aeolian transport in the Coachella Valley of the western Sonoran Desert. These winds deposit sand in coppice dunes that are critical habitat for several species, including the state and federally listed threatened species Uma inornata , a lizard. Although wind directions are generally defined in this valley, the wind field has complex interactions with local topography and becomes more variable with distance from the pass. Local, dominant wind directions are preserved by growth patterns of Larrea tridentata (creosote bush), a shrub characteristic of the hot North American deserts, and ventifacts. Exceptionally long-lived, Larrea has the potential to preserve wind direction over centuries to millennia, shaped by the abrasive pruning of windward branches and the persistent training of leeward branches. Wind direction preserved in Larrea individuals and clones was mapped at 192 locations. Compared with wind data from three weather stations, Larrea vectors effectively reflect annual prevailing winds. Ventifacts measured at 24 locations record winds 10° more westerly than Larrea and appear to reflect the direction of the most erosive winds. Based on detailed mapping of local wind directions as preserved in Larrea , only the northern half of the Mission-Morongo Creek floodplain is likely to supply sand to protected U. inornata habitat in the Willow Hole ecological reserve.

California↗
USGS2008

Holocene debris flows on the Colorado Plateau: The influence of clay mineralogy and chemistry

Holocene debris flows do not occur uniformly on the Colorado Plateau province of North America. Debris flows occur in specific areas of the plateau, resulting in general from the combination of steep topography, intense convective precipitation, abundant poorly sorted material not stabilized by vegetation, and the exposure of certain fine-grained bedrock units in cliffs or in colluvium beneath those cliffs. In Grand and Cataract Canyons, fine-grained bedrock that produces debris flows contains primarily single-layer clays—notably illite and kaolinite—and has low multilayer clay content. This clay-mineral suite also occurs in the colluvium that produces debris flows as well as in debris-flow deposits, although unconsolidated deposits have less illite than the source bedrock. We investigate the relation between the clay mineralogy and major-cation chemistry of fine-grained bedrock units and the occurrence of debris flows on the entire Colorado Plateau. We determined that 85 mapped fine-grained bedrock units potentially could produce debris flows, and we analyzed clay mineralogy and major-cation concentration of 52 of the most widely distributed units, particularly those exposed in steep topography. Fine-grained bedrock units that produce debris flows contained an average of 71% kaolinite and illite and 5% montmorillonite and have a higher concentration of potassium and magnesium than nonproducing units, which have an average of 51% montmorillonite and a higher concentration of sodium. We used multivariate statistics to discriminate fine-grained bedrock units with the potential to produce debris flows, and we used digital-elevation models and mapped distribution of debris-flow producing units to derive a map that predicts potential occurrence of Holocene debris flows on the Colorado Plateau.

Arizona, Colorado, New Mexico, Utah↗
USGS2007

Impact of recent extreme Arizona storms

Heavy rainfall on 27–31 July 2006 led to record flooding and triggered an historically unprecedented number of debris flows in the Santa Catalina Mountains north of Tucson, Ariz. The U.S. Geological Survey (USGS) documented record floods along four watercourses in the Tucson basin, and at least 250 hillslope failures spawned damaging debris flows in an area where less than 10 small debris flows had been documented in the past 25 years. At least 18 debris flows destroyed infrastructure in the heavily used Sabino Canyon Recreation Area (http://wwwpaztcn.wr.usgs.gov/rsch_highlight/articles/20061 l.html). In four adjacent canyons, debris flows reached the heads of alluvial fans at the boundary of the Tucson metropolitan area. While landuse planners in southeastern Arizona evaluate the potential threat of this previously little recognized hazard to residents along the mountain front, an interdisciplinary group of scientists has collaborated to better understand this extreme event.

Arizona↗
USGS2006

Effects of tributary debris on the longitudinal profile of the Colorado River in Grand Canyon

The Colorado River in Grand Canyon has long been known as a "rapids-and-pools" river, with the rapids owing their existence primarily to tributary debris flows. The debris flows deposit subaerial debris fans that constrict the channel laterally and, when they enter the river, raise the bed elevation. The rapids are short-wavelength (???0.1 to ???1 km), small-amplitude (??????5 m) convexities in the river's longitudinal profile, arising from the shallow gradient in the upstream pool and the steep gradient through the rapid itself. Analysis of the entire longitudinal profile through Grand Canyon reveals two long-wavelength (???100 km), large-amplitude (15-30 m) river profile convexities: the eastern canyon convexity between river mile (RM) 30 and RM 80 and the western canyon convexity between RM 150 and RM 250. Convexities of intermediate scale are also identified in the longitudinal profile. These longer-wavelength, larger-amplitude convexities have strong spatial correlations with high rates of debris flow occurrence, high densities of Holocene debris fans, the largest debris fans along the river, and alluvial thicknesses of 10 m or more. River profile convexities are unstable and require an active and powerful geologic process to maintain them, in this case the abundant, frequent, and voluminous Holocene debris flow activity in Grand Canyon. At all wavelengths the most likely cause for these river profile convexities is Holocene aggradation of the riverbed beneath them, driven by the coarse particles of tributary debris flows. Large enough debris flows will slow river flow for kilometers upstream, causing it to drop much of its suspended load. Integrated over time and all of the tributary point source contributions, this process will build short-wavelength convexities into long-wavelength convexities. For most if not all of the Holocene the Colorado River has been dissipating most of its energy in the rapids and expending the remainder in transporting fine sediment through Grand Canyon, with little or no regional incision of bedrock.

Journal of Geophysical Research F: Earth Surface↗
USGS2006

Sediment yield and runoff frequency of small drainage basins in the Mojave Desert, U.S.A

Sediment yield from small arid basins, particularly in the Mojave Desert, is largely unknown owing to the ephemeral nature of these fluvial systems and long recurrence interval of flow events. We examined 27 reservoirs in the northern and eastern Mojave Desert that trapped sediment from small (< 1 km 2 ) drainage basins on alluvial fans over the past 100 yr, calculated annual sediment yield, and estimated the average recurrence interval (RI) of sediment-depositing flow events. These reservoirs formed where railbeds crossed and blocked channels, causing sediment to be trapped and stored upslope. Deposits are temporally constrained by the date of railway construction (1906–1910), the presence of 137 Cs in the reservoir profile (post-1952 sediment), and either 1993, when some basins breached during regional flooding, or 2000–2001, when stratigraphic analyses were performed. Reservoir deposits are well stratified at most sites and have distinct fining-upward couplets indicative of discrete episodes of sediment-bearing runoff. Average RI of runoff events for these basins ranges from 2.6 to 7.3 yr and reflects the incidence of either intense or prolonged rainfall; more than half the runoff events occurred before 1963. A period of above-normal precipitation, from 1905 to 1941, may have increased runoff frequency in these basins. Mean sediment yield (9 to 48 tons km − 2 yr − 1 ) is an order of magnitude smaller than sediment yields calculated elsewhere and may be limited by reduced storm intensity, the presence of desert pavement, and shallow gradient of fan surfaces. Sediment yield decreases as drainage area increases, a trend typical of much larger drainage basins where sediment-transport processes constrain sediment yield. Coarse substrate and low-angle slopes of these alluvial fan surfaces likely limit sediment transport capacity through transmission losses and channel storage.

California↗
USGS2006

Precipitation history and ecosystem response to multidecadal precipitation variability in the Mojave Desert region, 1893-2001

Precipitation varied substantially in the Mojave Desert through the 20th century in a manner broadly similar to the other warm North American deserts. Episodes of drought and prolonged dry conditions (1893-1904, ca. 1942-1975, and 1999-present) alternated with relatively wet periods (1905-ca. 1941 and ca. 1976-1998), probably because of global-scale climate fluctuations. These are the El Nin??o-Southern Oscillation that affects interannual climate and the Pacific Decadal Oscillation that evidently causes decadal-scale variability such as prolonged dry and wet episodes. Studies done in the late 20th century demonstrate that precipitation fluctuations affected populations of perennial vegetation, annuals, and small herbivores. Landscape rephotography reveals that several species, particularly creosote bush, increased in size and density during the ca. 1976-1998 wet period. A brief, intense drought from 1989 to 1991 and the ongoing drought caused widespread mortality of certain species; for example, chenopods and perennial grasses suffered up to 100% mortality. Drought pruning, the shedding of above-ground biomass to reduce carbon allocation, increased substantially during drought. Overall, drought had the greatest influence on the Mojave Desert ecosystem. ?? 2006.

Journal of Arid Environments↗
USGS2006

Ground-water surface-water interactions and long-term change in riverine riparian vegetation in the southwestern United States

Riverine riparian vegetation has changed throughout the southwestern United States, prompting concern about losses of habitat and biodiversity. Woody riparian vegetation grows in a variety of geomorphic settings ranging from bedrock-lined channels to perennial streams crossing deep alluvium and is dependent on interaction between ground-water and surface-water resources. Historically, few reaches in Arizona, southern Utah, or eastern California below 1530 m elevation had closed gallery forests of cottonwood and willow; instead, many alluvial reaches that now support riparian gallery forests once had marshy grasslands and most bedrock canyons were essentially barren. Repeat photography using more than 3000 historical images of rivers indicates that riparian vegetation has increased over much of the region. These increases appear to be related to several factors, notably the reduction in beaver populations by trappers in the 19th century, downcutting of arroyos that drained alluvial aquifers between 1880 and 1910, the frequent recurrence of winter floods during discrete periods of the 20th century, an increased growing season, and stable ground-water levels. Reductions in riparian vegetation result from agricultural clearing, excessive ground-water use, complete flow diversion, and impoundment of reservoirs. Elimination of riparian vegetation occurs either where high ground-water use lowers the water table below the rooting depth of riparian species, where base flow is completely diverted, or both. We illustrate regional changes using case histories of the San Pedro and Santa Cruz Rivers, which are adjacent watersheds in southern Arizona with long histories of water development and different trajectories of change in riparian vegetation.

Conference Paper↗
USGS2004

Geochemical discrimination of five pleistocene lava-dam outburst-flood deposits, western Grand Canyon, Arizona

Pleistocene basaltic lava dams and outburst‐flood deposits in the western Grand Canyon, Arizona, have been correlated by means of cosmogenic 3 He ( 3 He c ) ages and concentrations of SiO 2 , Na 2 O, K 2 O, and rare earth elements. These data indicate that basalt clasts and vitroclasts in a given outburst‐flood deposit came from a common source, a lava dam. With these data, it is possible to distinguish individual dam‐flood events and improve our understanding of the interrelations of volcanism and river processes. At least five lava dams on the Colorado River failed catastrophically between 100 and 525 ka; subsequent outburst floods emplaced basalt‐rich deposits preserved on benches as high as 200 m above the current river and up to 53 km downstream of dam sites. Chemical data also distinguishes individual lava flows that were collectively mapped in the past as large long‐lasting dam complexes. These chemical data, in combination with age constraints, increase our ability to correlate lava dams and outburst‐flood deposits and increase our understanding of the longevity of lava dams. Bases of correlated lava dams and flood deposits approximate the elevation of the ancestral river during each flood event. Water surface profiles are reconstructed and can be used in future hydraulic models to estimate the magnitude of these large‐scale floods.

Arizona↗
USGS2002

Recovery of severely compacted soils in the Mojave Desert, California, USA

Often as a result of large-scale military maneuvers in the past, many soils in the Mojave Desert are highly vulnerable to soil compaction, particularly when wet. Previous studies indicate that natural recovery of severely compacted desert soils is extremely slow, and some researchers have suggested that subsurface compaction may not recover. Poorly sorted soils, particularly those with a loamy sand texture, are most vulnerable to soil compaction, and these soils are the most common in alluvial fans of the Mojave Desert. Recovery of compacted soil is expected to vary as a function of precipitation amounts, wetting-and-drying cycles, freeze-thaw cycles, and bioturbation, particularly root growth. Compaction recovery, as estimated using penetration depth and bulk density, was measured at 19 sites with 32 site-time combinations, including the former World War II Army sites of Camps Ibis, Granite, Iron Mountain, Clipper, and Essex. Although compaction at these sites was caused by a wide variety of forces, ranging from human trampling to tank traffic, the data do not allow segregation of differences in recovery rates for different compaction forces. The recovery rate appears to be logarithmic, with the highest rate of change occurring in the first few decades following abandonment. Some higher-elevation sites have completely recovered from soil compaction after 70 years. Using a linear model of recovery, the full recovery time ranges from 92 to 100 years; using a logarithmic model, which asymptotically approaches full recovery, the time required for 85% recovery ranges from 105-124 years.

Arid Land Research and Management↗
USGS1999

Cosmogenic 3He ages and frequency of late Holocene debris flows from Prospect Canyon, Grand Canyon, USA

Lava Falls Rapid, which was created and is maintained by debris flows from Prospect Canyon, is the most formidable reach of whitewater on the Colorado River in Grand Canyon and is one of the most famous rapids in the world. Debris flows enter the Colorado River at tributary junctures, creating rapids. The frequency of debris flows is an important consideration when management of regulated rivers involves maintenance of channel morphology. We used cosmogenic 3He, 14C, and historical photographs to date 12 late Holocene and historic debris flows from Prospect Canyon. The highest and oldest deposits from debris flows on the debris fan yielded a 3He date of about 3 ka, which indicates predominately late Holocene aggradation of one of the largest debris fans in Grand Canyon. The deposit, which has a 25-m escarpment caused by river reworking, crossed the Colorado River and raised its base level by 30 m for an indeterminate although likely short period. We mapped depositional surfaces of 11 debris flows that occurred after 3 ka. Two deposits inset against the highest deposit yielded 3He ages of about 2.2 ka, and at least two others followed shortly afterwards. At least one of these debris flows also dammed the Colorado River. The most recent prehistoric debris flow occurred no more than 0.5 ka. The largest historic debris flow, which constricted the river by 80%, occurred in 1939. Five other debris flows occurred after 1939; these debris flows constricted the Colorado River by 35-80%. Assuming the depositional volumes of late Holocene debris flows can be modeled using a lognormal distribution, we calculated recurrence intervals of 15 to more than 2000 years for debris flows from Prospect Canyon.

Geomorphology↗