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Robert Webb

Publications and source records attributed to Robert Webb.

14 recordsLinked to original sources
USGS2016

Comparison of methods to monitor the distribution and impacts of unauthorized travel routes in a border park

The distribution and abundance of human-caused disturbances vary greatly through space and time and are cause for concern among land stewards in natural areas of the southwestern border-lands between the USA and Mexico. Human migration and border protection along the international boundary create Unauthorized Trail and Road (UTR) networks across National Park Service lands and other natural areas. UTRs may cause soil erosion and compaction, damage to vegetation and cultural resources, and may stress wildlife or impede their movements. We quantify the density and severity of UTR disturbances in relation to soils, and compare the use of previously established targeted trail assessments (hereafter — targeted assessments) against randomly placed transects to detect trail densities at Coronado National Memorial in Arizona in 2011. While trail distributions were similar between methods, targeted assessments estimated a large portion of the park to have the lowest density category (0–5 trail encounters per/km 2 ), whereas the random transects in 2011 estimated more of the park as having the higher density categories (e.g., 15–20 encounters per km 2 category). Soil vulnerability categories that were assigned, a priori, based on published soil texture and composition did not accurately predict the impact of UTRs on soil, indicating that empirical methods may be better suited for identifying severity of compaction. While the estimates of UTR encounter frequencies were greater using the random transects than the targeted assessments for a relatively short period of time, it is difficult to determine whether this difference is dependent on greater cross-border activity, differences in technique, or from confounding environmental factors. Future surveys using standardized sampling techniques would increase accuracy.

Arizona↗
USGS2012

ADV point measurements within rapids of the Colorado River in Grand Canyon

Rapids on the Colorado River in Grand Canyon attract over 20,000 white-water enthusiasts a year and are considered one of the premiere collections of rapids in North America. While this collection of rapids is an important recreational resource, relatively little is known of the specific hydraulics of individual rapids. Flow measurements are occasionally made in the low-velocity reaches between rapids, but the turbulent and dangerous nature of rapids makes in-situ data collection challenging. The present study measured hydraulics within a small rapid in Grand Canyon as well as an alluvial reach of the Colorado River in Glen Canyon using a Sontek Argonaut acoustic Doppler velocimeter (ADV) 2 . The ADV was mounted near the center-front of a motor-powered 19-foot J-snout boat; the instrument sample volume was located 80 cm below the surface. The quality of the measurements was best in the slower water above the rapid and in Glen Canyon. Waves, aeration, and high-velocity water rendered specific measurements in the core of the rapid difficult as the ADV instrument could only measure velocities less than about 3.0 m/s. Nonetheless, velocity, bathymetry, and water-surface maps were constructed for the rapid and the reach in Glen Canyon. The compiled data sets can be used for predicting the erosion potential of debris fans forming the rapid and the development of numerical models to better characterize rapids.

Colorado↗
USGS2011

Unravelling long-term vegetation change patterns in a binational watershed using multitemporal land cover data and historical photography

A significant amount of research conducted in the Sonoran Desert of North America has documented, both anecdotally and empirically, major vegetation changes over the past century due to human land use activities. However, many studies lack coincidental landscape-scale data characterizing the spatial and temporal manifestation of these changes. Vegetation changes in a binational (USA and Mexico) watershed were documented using a series of four land cover maps (1979-2009) derived from multispectral satellite imagery. Cover changes are compared to georeferenced, repeat oblique photographs dating from the late 19 th century to present. Results indicate the expansion of grassland over the past 20 years following nearly a century of decline. Historical repeat photography documents early-mid 20 th century mesquite invasions, but recent land cover data and rephotography demonstrate declines in xeroriparian/riparian mesquite communities in recent decades. These vegetation changes are variable over the landscape and influenced by topography and land management.

Arizona, California↗
USGS2006

Researchers consider U.S. Southwest's response to warmer, drier conditions

In 2000, the popular press frequently referred to reports that the southwestern United States might experience a shift from relatively wet to dry conditions during the next couple of decades (see http://topex‐www.jpl.nasa.gov/discover/PDO.html). These predictions stemmed from observations that the Pacific Decadal Oscillation (PDO) appeared to abruptly change from a “positive” to a “negative” phase in 1999 (Figure 1). During the mid‐twentieth century, a similar negative phase of the PDO was accompanied by prolonged dry conditions in the southwest. By extrapolation, some climatologists predicted future drought in the southwest. Such a change would heavily affect land use planning in the region, because national demographics have stressed the region's resources over the past century From 1990 to 2000, for instance, the population of Nevada and Arizona increased by almost 2.3 million people (http://www.census.gov/population/www/cen2000/respop.html). To discuss potential scenarios of landscape and ecosystem response to 25 years of hot and dry climate, scientists from diverse disciplines gathered at the University of Arizona in April 2001. The objectives of this workshop were to address evidence supporting predictions of warmer and drier climate and the possible landscape responses (http://geology.wr.usgs.gov/sw‐workshop/).

Eos, Transactions, American Geophysical Union↗
USGS2004

Frequency and initiation of debris flows in Grand Canyon, Arizona

Debris flows from 740 tributaries transport sediment into the Colorado River in Grand Canyon, Arizona, creating rapids that control its longitudinal profile. Debris flows mostly occur when runoff triggers failures in colluvium by a process termed “the fire hose effect.” Debris flows originate from a limited number of geologic strata, almost exclusively shales or other clay‐rich, fine‐grained formations. Observations from 1984 through 2003 provide a 20 year record of all debris flows that reached the Colorado River in Grand Canyon, and repeat photography provides a 100 year record of debris flows from 147 tributaries. Observed frequencies are 5.1 events/year from 1984 to 2003, and historic frequencies are 5.0 events/year from 1890 to 1983. Logistic regression is used to model historic frequencies based on drainage basin parameters observed to control debris flow initiation and transport. From 5 to 7 of the 16 parameters evaluated are statistically significant, including drainage area, basin relief, and the height of and gradient below debris flow source areas, variables which reflect transport distance and potential energy. The aspect of the river channel, which at least partially reflects storm movement within the canyon, is also significant. Model results are used to calculate the probability of debris flow occurrence at the river over a century for all 740 tributaries. Owing to the variability of underlying geomorphic controls, the distribution of this probability is not uniform among tributaries of the Colorado River in Grand Canyon.

Arizona↗
USGS2001

Techniques for estimating sediment yield of ungaged tributaries on the southern Colorado Plateau

Numerous regional sediment transport data are used to evaluate three techniques for estimating streamflow sediment yield from ungaged tributaries of the Colorado River in Grand Canyon. These techniques include: (1) a regression equation relating drainage area to sediment yield for all relevant sediment-yield data from northern Arizona, (2) an empirical relation developed by Renard (1972) selected from 8 potentially relevant methods, and (3) a new procedure that combines regional flood-frequency analysis with sediment-rating curves. Results based on techniques (1) and (2) are not significantly different. The third technique requires numerous assumptions, most notably that sediment yield on a decadal average can be described by several floods of recurrence intervals of 2 yr, 5 yr, and 10 yr described by regional flood-frequency relations. Using data collected at gaging stations, we develop a relation between peak discharge and total-event sediment yield derived from hydrographs and sediment-rating curves. This third technique produces sediment yield estimates comparable to those of the regional data regression and Renard (1972) relations and may be a more robust technique for estimating sediment yield when streamflow data are available.

Arizona↗
USGS1997

Debris flows in Grand Canyon National Park: Peak discharges, flow transformations, and hydrographs

Direct measurements of debris-flow hydrograph and flow behavior in remote drainage areas are rare. We infer hydrographs and flow behavior for recent debris flow in bedrock tributaries of the Colorado River from preserved stratigraphic relations, sedimentology and surface morphology of debris fans and evidence of flow-surface elevations. We propose that 3 types of debris-flow hydrographs occur in Grand Canyon: Type I flows have a single debris-flow peak followed by recessional 'hyperconcentrated flow' or streamflow; Type II flows have multiple debris-flow peaks with intervening 'hyperconcentrated flow' and (or) streamflow phases; and Type III flows begin as either Type I or Type II flows, but late-stage recessional streamflow is higher than the stage(s) of the debris-flow phase(s) and extensively reworks debris-flow deposits of buries them beneath streamflow sand and gravel. Field evidence shows that debris-flow peaks last for seconds to minutes, while recessional flows have durations of several hours to a day.

Arizona↗
USGS1997

Initiation of debris flows in tributaries of the Colorado River in Grand Canyon, Arizona

Debris flows are initiated in tributaries of the Colorado River in Grand Canyon when intense rainfall causes failures in colluvium and (or) bedrock. Most debris flows occur in the summer during localized convective thunderstorms with rainfall intensities as high as 40 mm/hr. Rarer and larger debris flows occur during unusually warm frontal storms in winter. Hourly precipitation data suggest that storms that cause debris flows terminate with a period of intense rainfall, a characteristic that complicates the use of daily rainfall records in assessing debris-flow hazard. Recurrence intervals for 1-day rainfall associated with 37 recent debris flows range from <1 to >50 years, with most <10 years. Recurrence intervals for the multi-day rainfall of storms associated with debris flows range from <1 to 158 years, but most were >10 years. The low recurrence intervals of debris-flow producing rainfalls, compared with the 10-50 yr recurrence intervals for most debris flows, underscores the co-dependence of debris-flow initiation on geologic factors, including bedrock type and antecedent soil-moisture conditions. The primary geologic factor influencing debris-flow initiation in Grand Canyon is the exposure of shale units at heights >100 m above the river. Exposed shale bedrock fails readily, either producing debris flows directly or contributing source material to wedges of colluvium that may fail later. Shales also provide silt- and clay-size particles that in part determine the rheological properties of debris flows.

Arizona↗
USGS1987

Dynamics of Mojave Desert shrub assemblages in the Panamint Mountains, California.

We studied shrub communities in the Panamint Mountains of the Mojave Desert to determine whether vegetational changes after disturbance can be characterized as succession according to modern successional theory. We found, on a variety of disturbed and undisturbed sites, that the rate of change was a function of the type and age of disturbance. Recent debris—flow deposits were colonized by shrub assemblages of different species composition than those on the surrounding, older debris—flow deposits and other geomorphically stable surfaces. Colonization of human—disturbed sites was highly variable, but species compositions were different from the predisturbance species composition. In Grayia—Lycium assemblages, Grayia spinosa reasserted its dominance over colonizers relatively quickly. In Coleogyne assemblages, typically found on older geomorphic surfaces, species composition differences persisted considerably longer, depending on the severity of the initial disturbance. Extremely stable assemblages, dominated by Coleogyne ramosissima, occurred on the oldest, least disturbed surfaces. The variability of species composition among disturbed sites was greater than the variability among undisturbed and geomorphically stable sites, in accord with convergent succession. Models of desert succession should consider several factors: (1) colonization is dependent largely on the severity of disturbances and residual biotic components; (2) the time span for recovery may be longer than past periods of climatic and geomorphic stability; and (3) colonizing species may have considerable range in their life—history strategies.

California↗