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R. H. Webb

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

25 records · Page 2Linked to original sources
USGS1997

Succession of desert plants on debris flow terraces, Grand Canyon, Arizona, U.S.A.

Vegetation sampling on 11 debris flow terraces in Grand Canyon National Park, Arizona, U.S.A., showed that plant assemblages changed as age of surface increased. The terraces ranged in age from about 5 to about 3100 years. There were distinct differences among sites in the life history characteristics of the dominant plants. Young terraces (5-55 years) were dominated by short-lived plants that had high reproductive potential. Older surfaces were dominated by species with longer life-spans and lower reproductive potential. Density and cover of long-lived species increased with age of surface; for short-lived plants, density was inversely related to surface age. Species composition was also correlated with site age; however, location, exposure, and other factors ensured that no two debris flows supported identical mixtures of species. Succession on recent Grand Canyon debris flows is driven in part by life-history strategies, particularly life-span and seed dispersal traits, and also by climatic factors, especially those that control germination and establishment of the long-lived dominants.

Journal of Arid Environments↗
USGS1992

Accuracy of post-bomb 137Cs and 14C in dating fluvial deposits

The accuracy and precision of 137Cs and 14C for dating post-1950 alluvial deposits were evaluated for deposits from known floods on two rivers in Arizona. The presence of 137Cs reliably indicates that deposition occurred after intensive above-ground nuclear testing was initiated around 1950. There was a positive correlation between the measured level of 137Cs activity and the clay content of the sediments, although 137Cs was detected even in sandy flood sediments with low clay content. 137Cs is a valuable dating tool in arid environments where organic materials for 14C or tree-ring dating are scarce and observational records are limited. The 14C activity measured in different types of fine organic detritus yielded dates within 1 to 8 yr of a 1980 flood deposit, and the accuracy was species-dependent. However, undifferentiated mixtures of fine organic materials from several post-bomb deposits of various ages repeatedly yielded dates between 1958 and 1962, and detrital charcoal yielded a date range of 1676-1939. In semiarid environments, the residence time of most types of organic debris precludes accurate annual resolution of post-bomb 14C dates. ?? 1992.

Quaternary Research↗
USGS1991

Relation of sediment load and flood-plain formation to climatic variability, Paria River drainage basin, Utah and Arizona

Suspended-sediment load, flow volume, and flood characteristics of the Paria River were analyzed to determine their relation to climate and flood-plain alluviation between 1923 and 1986. Flood-plain alluviation began about 1940 at a time of decreasing magnitude and frequency of floods in winter, summer, and fall. No floods with stages high enough to inundate the flood plain have occurred since 1980, and thus no flood-plain alluviation has occurred since then. The decrease in magnitude and frequency of floods appears to have resulted from a decrease in frequency of large storms, particularly dissipating tropical cyclones, and not from a decrease in annual or seasonal precipitation. Suspended-sediment load is highest in summer and fall, whereas flow volume is highest in winter. Fall shows the greatest interannual variability in suspended-sediment load, flow volume, and flood size because climatic conditions are most variable in fall. The relation between sediment load and discharge apparently did not change within the period of sediment sampling (1949-1976), even though the channel elevation and width changed significantly. Annual suspended-sediment loads estimated for periods before and after 1949-1976 show that decrease in suspended-sediment load caused by floodplain alluviation in the Paria River and other tributaries could have been a significant part of the decrease of suspended-sediment load in the Colorado River in the early 1940s.

Arizona, Utah↗
USGS1987

Debris flows from tributaries of the Colorado River, Grand Canyon National Park, Arizona; executive summary

Debris flows are a major process of sediment transport to the Colorado River from ungaged tributaries in Grand Canyon National Park, Arizona. Debris flows are slurries of clay to boulder-sized particles of large magnitude and short duration that occur infrequently. They are the source for potential large volumes of sand for beaches on the Colorado River. Debris flows create and maintain hydraulic controls (rapids) on the Colorado River at tributary mouths. (See also W89-09240) (Author 's abstract)

Open-File Report↗
USGS1987

Debris flows from tributaries of the Colorado River, Grand Canyon National Park, Arizona

A reconnaissance of 36 tributaries of the Colorado River indicates that debris flows are a major process by which sediment is transported to the Colorado River in Grand Canyon National Park. Debris flows are slurries of sediment and water that have a water content < 40% by volume. Debris flows occur frequently in arid and semiarid regions. Slope failures commonly trigger debris flows, which can originate from any rock formation in the Grand Canyon. The largest and most frequent flows originate from the Permian Hermit Shale, the underlying Esplanade Sandstone of the Supai Group, and other formations of the Permian and Pennsylvanian Supai Group. Debris flows have reached the Colorado River on an average of once every 20 to 30 yr in the Lava-Chuar Creek drainage since about 1916. Two debris flows have reached the Colorado River in the last 25 yr in Monument Creek. The Crystal Creek drainage has had an average of one debris flow reaching the Colorado River every 50 yr, although the debris flow of 1966 has been the only flow that reached the Colorado River since 1900. Debris flows may actually reach the Colorado River more frequently in these drainages because evidence for all debris flows may not have been preserved in the channel-margin stratigraphy. Discharges were estimated for the peak flow of three debris flows that reached the Colorado River. The debris flow of 1966 in the Lava-Chuar Creek drainage had an estimated discharge of 4,000 cu ft/sec. The debris flow of 1984 in the Monument Creek drainage had a discharge estimated between 3,600 and 4,200 cu ft/sec. The debris flow of 1966 in the Crystal Creek drainage had a discharge estimated between 9,200 and 14,000 cu ft/sec. Debris flows in the Grand Canyon generally are composed of 10 to 40% sand by weight and may represent a significant source of beach-building sand along the Colorado River. The particle size distributions are very poorly sorted and the largest transported boulders were in the Crystal Creek drainage. Reworking of debris fans by the Colorado River creates debris bars that constrain the size of eddy systems and forms secondary rapids and riffles below tributary mouths. (See also W89-09239) (Lantz-PTT)

Arizona↗
USGS1987

Distribution of radionuclide and trace-elements in ground water, grasses, and surficial sediments associated with the alluvial aquifer along the Puerco River, northeastern Arizona; a reconnaissance sampling program

The concentrations of gross alpha radioactivity minus uranium equaled or exceeded 15 picoCuries/L (pCi/L) in five of 14 wells sampled. The concentration of radium-226 plus radium-228 exceeded the primary water quality standard of 5 pCi/L in one well. The concentration of uranium exceeded a recommended limit of 0.035 mg/L in two wells. Perennial grass and sediment samples had low concentrations of radionuclides. The concentration of trace elements in the sediment samples was not unusual. Water quality of surface water in the Puerco River at Chambers varied as a function of the suspended sediment concentration. Concentrations of total gross alpha radiation fluctuated from 12 to 11,200 pCi/L. Concentrations of total gross beta radiation fluctuated from 45 to 4,500 pCi/L. (Author 's abstract)

Open-File Report↗