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K. S. Thompson

Publications and source records attributed to K. S. Thompson.

8 recordsLinked to original sources
USGS2017

Numerical ages of Holocene tributary debris fans inferred from dissolution pitting on carbonate boulders in the Grand Canyon of Arizona

Carbonate boulders transported down steep tributary channels by debris flow came to rest on Holocene debris fans beside the Colorado River in Grand Canyon National Park. Weakly acidic rainfall and the metabolic activity of blue-green algae have produced roughly hemispheric dissolution pits as much as 2-cm deep on the initially smooth surfaces of the boulders. The average depth of dissolution pits increases with relative age of fan surfaces. The deepening rate averages 2.4 mm/1000 yr (standard error = 0.2 mm/1000 yr), as calculated from several radiometrically dated surfaces and an archeological structure. This linear rate, which appears constant over at least the past 3000 yr, is consistent with field relations limiting the maximum age of the fans and with the physical chemistry of limestone dissolution. Dissolution-pit measurements (n = 6973) were made on 617 boulders on 71 fan surfaces at the 26 largest debris fans in Grand Canyon. Among these fan surfaces, the average pit depth ranges from 1.2 to 17.4 mm, and the resulting pit dissolution ages range from 500 to 7300 cal yr B.P. Most (75%) surfaces are younger than 3000 yr, probably because of removal of older debris fans by the Colorado River. Many of the ages are close to 800, 1600, 2300, 3100, or 4300 cal yr B.P. If not the result of differential preservation of fan surfaces, this clustering implies periods of heightened debris-flow activity and increased precipitation.

Quaternary Research↗
USGS1996

Tributary debris fans and the late Holocene alluvial chronology of the Colorado River, eastern Grand Canyon, Arizona

Bouldery debris fans and sandy alluvial terraces of the Colorado River developed contemporaneously during the late Holocene at the mouths of nine major tributaries in eastern Grand Canyon. The age of the debris fans and alluvial terraces contributes to understanding river hydraulics and to the history of human activity along the river, which has been concentrated on these surfaces for at least two to three millennia. Poorly sorted, coarse-grained debris-flow deposits of several ages are interbedded with, overlie, or are overlapped by three terrace-forming alluviums. The alluvial deposits are of three age groups: the striped alluvium, deposited from before 770 b.c. to about a.d. 300; the alluvium of Pueblo II age deposited from about a.d. 700 to December 1900; and the alluvium of the upper mesquite terrace, deposited from about a.d. 1400 to 1880. Two elements define the geomorphology of a typical debris fan: the large, inactive surface of the fan and a smaller, entrenched, active debris-flow channel and fan that is about one-sixth the area of the inactive fan. The inactive fan is segmented into at least three surfaces with distinctive weathering characteristics. These surfaces are conformable with underlying debris-flow deposits that date from before 770 b.c. to around a.d. 660, a.d. 660 to before a.d. 1200, and from a.d. 1200 to slightly before 1890, respectively, based on late-19th-century photographs, radiocarbon and archaeologic dating of the three stratigraphically related alluviums, and radiocarbon dating of fine-grained debris-flow deposits. These debris flows aggraded the fans in at least three stages beginning about 2.8 ka, if not earlier in the late Holocene. Several main-stem floods eroded the margin of the segmented fans, reducing fan symmetry. The entrenched, active debris-flow channels contain deposits <100 yr old, which form debris fans at the mouth of the channel adjacent to the river. Early and middle Holocene debris-flow and alluvial deposits have not been recognized, as they were evidently not preserved adjacent to the river or are buried by younger deposits.

Arizona↗
USGS1993

Surficial geology, geomorphology, and erosion of archeologic sites along the Colorado River, eastern Grand Canyon, Grand Canyon National Park, Arizona

The average number of archeologic sites along the Colorado River in eastern Grand Canyon between River Miles 65-72 exceeds 12 km -1 ; the largest concentration from Glen Canyon Dam to the mouth of Grand Canyon. The sites are mostly of Anasazi affiliation, dating from the Pueblo Ito Pueblo II periods (A.D. 800-1200), although older sites of Basketmaker II affinity (about 200 B.C. to A.D. 400) and younger sites of Native American and Anglo affiliation are also present. All of the sites are closely associated with late-Holocene alluvial, debris-flow, and eolian deposits that accumulated in the river corridor during the past 2,500 years. The majority of sites occur on or beneath the surface of ancient alluvial deposits of the Colorado River, which form distinctive high terraces. Lateral shifts of the river extensively eroded these deposits and associated archeologic sites twice in prehistoric times, between A.D.300700 and between A.D. 1200-1400. In late historic times, numerous sites have been damaged or destroyed by erosion, which has accelerated since 1965-73. The daily operation of Glen Canyon Dam probably did not cause accelerated erosion in eastern Grand Canyon, although the presence of the dam indirectly effects erosion. Generally, sites are eroded by arroyo cutting in the short, ephemeral streams that drain the terraces of the river corridor. These streams are small; 90 percent have catchment area less than 20,000-30,000 m 2 and channel length of less than 300-400 m. Driven by excessive rainfall, arroyo cutting deepens, widens, and expands the channel system. The extent of arroyo cutting is related to past and present depositional levels of the river, which are local baselevels of erosion. The post-dam level is 3-4 m below the lowest pre-dam level; this decrease resulted from elimination of the annual flood and a six-fold reduction of sediment load. Eighty percent of tributary streams end above or on the post-dam depositional level, but during large runoff the channels are free to extend upslope as well as downslope toward the river. These channels will eventually extend downslope to the river, where the channel gradient will be lowered 3-4 m. Arroyo cutting will be intensified until channel gradients adjust to the post-dam baselevel.

Arizona↗