Lower Colorado River Valley Research Articles

Early Pliocene marine transgression into the lower Colorado River valley, southwestern USA, by re-flooding of a former tidal strait

Abstract Marine straits and seaways are known to host a wide range of sedimentary processes and products, but the role of marine connections in the development of large river systems remains little studied. This study explores a hypothesis that shallow-marine waters flooded the lower Colorado River valley at c. 5 Ma along a fault-controlled former tidal strait, soon after the river was first integrated into the northern Gulf of California. The upper bioclastic member of the southern Bouse Formation provides a critical test of this hypothesis. The upper bioclastic member contains wave ripple-laminated bioclastic grainstone with minor red mudstone, pebbly grainstone with hummocky cross-stratification (HCS)-like stratification and symmetrical gravelly ripples, and calcareous-matrix conglomerate. Fossils include upward-branching segmented coralline-like red algae with no known modern relatives but confirmed as marine calcareous algae, echinoid spines, barnacles, shallow-marine foraminifers, clams, and serpulid worm tubes. These results provide evidence for deposition in a shallow-marine bay or estuary seaward of the transgressive backstepping Colorado River delta. Tsunamis generated by seismic and meteorological sources likely produced the HCS-like and wave-ripple cross-bedding in poorly-sorted gravelly grainstone. Marine waters inundated a former tidal strait within a fault-bounded tectonic lowland that connected the lower Colorado River to the Gulf of California. Delta backstepping and transgression resulted from a decrease in sediment output due to sediment trapping in upstream basins and relative sea-level rise produced by regional tectonic subsidence.

Tectonostratigraphic record of late Miocene–early Pliocene transtensional faulting in the Eastern California shear zone, southwestern USA

AbstractThe Eastern California shear zone (ECSZ; southwestern USA) accommodates ~20%–25% of Pacific–North America relative plate motion east of the San Andreas fault, yet little is known about its early tectonic evolution. This paper presents a detailed stratigraphic and structural analysis of the uppermost Miocene to lower Pliocene Bouse Formation in the southern Blythe Basin, lower Colorado River valley, where gently dipping and faulted strata provide a record of deformation in the paleo-ECSZ. In the western Trigo Mountains, splaying strands of the Lost Trigo fault zone include a west-dipping normal fault that cuts the Bouse Formation and a steeply NE-dipping oblique dextral-normal fault where an anomalously thick (~140 m) section of Bouse Formation siliciclastic deposits filled a local fault-controlled depocenter. Systematic basinward thickening and stratal wedge geometries in the western Trigo and southeastern Palo Verde Mountains, on opposite sides of the Colorado River valley, record basinward tilting during deposition of the Bouse Formation. We conclude that the southern Blythe Basin formed as a broad transtensional sag basin in a diffuse releasing stepover between the dextral Laguna fault system in the south and the Cibola and Big Maria fault zones in the north. A palinspastic reconstruction at 5 Ma shows that the southern Blythe Basin was part of a diffuse regional network of linked right-stepping dextral, normal, and oblique-slip faults related to Pacific–North America plate boundary dextral shear. Diffuse transtensional strain linked northward to the Stateline fault system, eastern Garlock fault, and Walker Lane, and southward to the Gulf of California shear zone, which initiated ca. 7–9 Ma, implying a similar age of inception for the paleo-ECSZ.

Bedrock And Boulder Mortars, Basins, Slicks, And Cupules In The Southern Southwest

This article describes mortars, basins, slicks, and cupules created in bedrock and boulders in the Southern Southwest, and discusses the distribution and possible functions of these features. It defines the Southern Southwest as the region of the U.S. south of 34 degrees north latitude that includes the Califor­nia portion of the Lower Colorado River valley and southern portions of Arizona and New Mexico, and the portion of western Texas that includes El Paso, Hudspeth, Culber­son, Loving, Winkler, Ward, Reeves, and Jeff Davis counties (roughly the part of Texas from El Paso eastward just past the south­eastern corner of New Mexico, then south and south­westward to a point south of Fort Davis, then westward to the Río Grande).

A. G. Tassin’s 1877 Manuscript Account of Natural Conditions and Historical Settlements on the Lower Colorado River

A. G. Tassin’s 1877 Manuscript Account of Natural Conditions and Historical Settlements on the Lower Colorado River Jerry Schaefer (bio) Introduction On October 31, 1877, Second Lieutenant Captain Augustus G. Tassin (1842–1893) of the 12th U.S. Infantry at Camp (or Fort) Mohave submitted to his commanding officer a handwritten and self-illustrated report on the natural history, historical geography, and ethnography of the lower Colorado River valley. His report was prepared in response to a Circular of October 5, 1877, from the Engineer Office, Military Division of the Pacific, for use by the U.S. Department of Agriculture. It might be expected that a 47-page manuscript of such meticulous detail and obvious personal knowledge of the region would have been published like so many of the early explorers’ and military accounts of the lower Colorado River area (Emory 1848; Farquhar 1991; Ives 1861; Möllhausen 1858, 1861; Whipple et al. 1856; Williamson 1853). It was obviously read by various officials and even a small section on the local flora was published, but for the most part it remained in its original form even after it was deposited in the archives of the Bureau of American Ethnology at the Smithsonian Institution in April 1926. Today the manuscript can be found at the National Anthropological Archives (NAA), Department of Anthropology, National Museum of Natural History. Known only to a few scholars, and occasionally referenced, the substance of the manuscript and remarkable illustrations remained largely unknown to the wider academic community, until now. There are several possible reasons that Tassin’s report was never published by the U.S. government, unlike many other field reports on the region. Although the penmanship is neat and regular, Tassin’s handwriting can still be difficult to read. Secondly, much of the information in this portion of the report was known [End Page 177] and more professionally presented from other sources by 1877. For example, take Wheeler’s (1876) nearly contemporary report with objective descriptions of the terrain, scientific data, maps drawn by professional cartographers, and, for the first time in Colorado River explorations, photographs. Some of Tassin’s plans may actually be based on the Wheeler report. Thirdly, French-born Tassin’s prose may have been considered too idiosyncratic even by contemporary literary conventions, peppered as it is with personal opinions and allusions to European history. Nevertheless, Tassin does provide details, data sets, and plans that are not readily available from other sources. We are fortunate that the manuscript was preserved and a high-quality digital scan was produced and posted by the NAA in association with this transcription project (Tassin 1877). I am grateful to them for permission to publish.1 Augustus G. Tassin (October 12, 1842–October 19, 1893) had a colorful personal history and unusual abilities. Much of his life was shaped by his role in the American military’s pacification of Native Americans in the West (Altshuler 1991; Murray 2013; Patterson 2005; Schaefer and Laylander 2014; Thrapp 1988–1994:1403). His biography is well documented although sources differ in minor details. Born in Paris, France, Tassin was only ten years old when he immigrated through the New Orleans port of entry. He was living in Leopold, Indiana, when the Civil War broke out. Five months into the war, he enlisted in “D” Company of the 35th Indiana Infantry, just a month short of his twentieth birthday. He was assigned the rank of first lieutenant shortly after his enlistment on September 15, 1861. By the end of the war, he had risen to the rank of full colonel, and later was awarded several honorary breveted ranks for his meritorious service in the Battle of Missionary Ridge and the Atlanta and Nashville campaigns. After the war he married Mary Tilley and had three children. Tassin left the army on December 1, 1870, to fight in the Franco-Prussian War on the French side. Upon his return he reenlisted in 1872 as a private in the Signal Corps, but was commissioned a second lieutenant in the 12th Infantry the following year. He continued his military career for most of his life, attaining the rank of first lieutenant in 1878 and...

The Bouse Formation, a controversial Neogene archive of the evolving Colorado River: a scientific drilling workshop report (28 February–3 March 2019 – BlueWater Resort & Casino, Parker, AZ, USA)

Abstract. Neogene deposits of the lower Colorado River valley, especially the Miocene(?) and early Pliocene Bouse Formation, have been the focus of intense debate regarding the early paleoenvironmental history of this important continental-scale river system in southwestern North America and its integration with the proto-Gulf of California. Fine-grained units within these Neogene deposits also hold a promising archive of Pliocene paleoclimate history for this part of the world. Because the depocenter deposits of the Bouse Formation and the deposits that overlie and underlie it are poorly exposed and highly weathered, the formation is ripe for study through collection of drill cores. A workshop was held 28 February–3 March 2019 in Parker, AZ, USA, to discuss how scientific drilling might be employed to help resolve the Bouse controversies and improve our understanding of paleoclimate history in the region.

Comparisons of Reptile Assemblages in Two Subdivisions of the Sonoran Desertscrub Biotic Community

Anthropogenic disturbances can have negative effects on species assemblages. This study was established to form baseline data on the environmental structure and reptile assemblages within a planned energy corridor in Pinal County, Arizona, prior to construction. We emphasized evaluating the differences in reptile assemblages in two subdivisions of the Sonoran Desertscrub, the Lower Colorado River Valley (LCV) and Arizona Uplands (AZU). Surveys were conducted on 50 sites (LCV = 15; AZU = 35) along the proposed 67.1-km long energy corridor through environmental surveys and 50 drift-fence trapping arrays with 400 box funnel traps. Vegetation height, number of burrows, and percent rock, ground cover, and coarse woody debris were significantly higher in the AZU than in LCV. Eighteen reptile species (n = 995 captures) were detected on the energy corridor including 8 lizard species (n = 952 captures) and 10 snake species (n = 43 captures). Species richness, evenness, and capture rates were not significantly different between the LCV and AZU; however, species diversity was significantly higher in the LCV. Reptile abundance (LCV = 281; AZU = 714) differed in the two subdivisions, yet rank-abundance curves revealed no difference in dominance of species. Post hoc examination revealed that the geographic separation of sites within the LCV and the location of the study area (along the ecotone) may have contributed to our results. We conclude that both subdivisions are equally important to the maintenance of local biotic diversity and recommend that any future land set-asides consider both subdivisions.

Punctuated Sediment Discharge during Early Pliocene Birth of the Colorado River: Evidence from Regional Stratigraphy, Sedimentology, and Paleontology

The Colorado River in the southwestern U.S. provides an excellent natural laboratory for studying the origins of a continent-scale river system, because deposits that formed prior to and during river initiation are well exposed in the lower river valley and nearby basinal sink. This paper presents a synthesis of regional stratigraphy, sedimentology, and micropaleontology from the southern Bouse Formation and similar-age deposits in the western Salton Trough, which we use to interpret processes that controlled the birth and early evolution of the Colorado River. The southern Bouse Formation is divided into three laterally persistent members: basal carbonate, siliciclastic, and upper bioclastic members. Basal carbonate accumulated in a tide-dominated marine embayment during a rise of relative sea level between ~6.3 and 5.4Ma, prior to arrival of the Colorado River. The transition to green claystone records initial rapid influx of river water and its distal clay wash load into the subtidal marine embayment at ~5.4–5.3Ma. This was followed by rapid southward progradation of the Colorado River delta, establishment of the earliest through-flowing river, and deposition of river-derived turbidites in the western Salton Trough (Wind Caves paleocanyon) between ~5.3 and 5.1Ma. Early delta progradation was followed by regional shut-down of river sand output between ~5.1 and 4.8Ma that resulted in deposition of marine clay in the Salton Trough, retreat of the delta, and re-flooding of the lower river valley by shallow marine water that deposited the Bouse upper bioclastic member. Resumption of sediment discharge at ~4.8Ma drove massive progradation of fluvial-deltaic deposits back down the river valley into the northern Gulf and Salton Trough.These results provide evidence for a discontinuous, start-stop-start history of sand output during initiation of the Colorado River that is not predicted by existing models for this system. The underlying controls on punctuated sediment discharge are assessed by comparing the depositional chronology to the record of global sea-level change. The lower Colorado River Valley and Salton Trough experienced marine transgression during a gradual fall in global sea level between ~6.3 and 5.5Ma, implicating tectonic subsidence as the main driver of latest Miocene relative sea-level rise. A major fall of global sea level at 5.3Ma outpaced subsidence and drove regional delta progradation, earliest flushing of Colorado River sand into the northern Gulf of California, and erosion of Bouse basal carbonate and siliciclastic members. The lower Colorado River valley was re-flooded by shallow marine waters during smaller changes in global sea level~5.1–4.8Ma, after the river first ran through it, which requires a mechanism to stop delivery of sand to the lower river valley. We propose that tectonically controlled subsidence along the lower Colorado River, upstream of the southern Bouse study area, temporarily trapped sediment and stopped delivery of sand to the lower river valley and northern Gulf of California for ~200–300 kyr. Massive progradation of the fluvial-deltaic system back down the river valley into the Salton Trough starting ~4.8–4.5Ma apparently was driven by a huge increase in sediment discharge that overwhelmed the sediment-storage capacity of sub-basins along the lower river corridor and established the fully integrated river channel network.

Cosmogenic 3He age estimates of Plio-Pleistocene alluvial-fan surfaces in the Lower Colorado River Corridor, Arizona, USA

Plio-Pleistocene deposits of the Lower Colorado River (LCR) and tributary alluvial fans emanating from the Black Mountains near Golden Shores, Arizona record six cycles of Late Cenozoic aggradation and incision of the LCR and its adjacent alluvial fans. Cosmogenic 3He (3Hec) ages of basalt boulders on fan terraces yield age ranges of: 3.3–2.2Ma, 2.2–1.1Ma, 1.1Ma to 110ka, <350ka, <150ka, and <63ka. T1 and Q1 fans are especially significant, because they overlie Bullhead Alluvium, i.e. the first alluvial deposit of the LCR since its inception ca. 4.2Ma. 3Hec data suggest that the LCR began downcutting into the Bullhead Alluvium as early as 3.3Ma and as late as 2.2Ma. Younger Q2a to Q4 fans very broadly correlate in number and age with alluvial terraces elsewhere in the southwestern USA. Large uncertainties in 3Hec ages preclude a temporal link between the genesis of the Black Mountain fans and specific climate transitions. Fan-terrace morphology and the absence of significant Plio-Quaternary faulting in the area, however, indicate regional, episodic increases in sediment supply, and that climate change has possibly played a role in Late Cenozoic piedmont and valley-floor aggradation in the LCR valley.

Pleistocene Impacts on the Phylogeography of the Desert Pocket Mouse(Chaetodipus penicillatus)

The desert pocket mouse (Chaetodipus penicillatus) comprises 6 nominate subspecies that occupy warm, sandy desert-scrub habitats across the Mojave and Sonoran deserts. The most thorough morphological assessment within the species noted variable levels of distinctiveness, leading to uncertainty regarding the geographic distributions of subspecies. Subsequent genetic assessments using chromosomal, allozymic, and mitochondrial DNA (mtDNA) sequence data detected a general east–west divergence centered on the Colorado River, but few locations were included in these assessments. We investigated phylogeographic structure in C. penicillatus by sequencing regions of mtDNA for 220 individuals from 51 locations representing all continental subspecies. We identify 2 major monophyletic mtDNA lineages (clades) roughly centered in the Mojave and Sonoran deserts. These clades broadly overlap along the Lower Colorado River valley and adjacent desert regions across most of the range of C. p. penicillatus. Outside this zone of mtDNA clade overlap, Sonoran clade haplotypes occur in populations from across the range of C. p. pricei and extend to the northwestern edge of the Sonoran Desert within the southern range of C. p. angustirostris. Northern clade haplotypes occur in populations within the ranges of C. p. sohrinus and C. p. stephensi and in populations from the western Mojave Desert in the northern range of C. p. angustirostris. Based on rough estimates for rates of sequence evolution, divergence among the major clades appears to have occurred during the Pleistocene, but well before the latest glacial maximum. The secondary contact among the major clades appears to have some longevity, with little evidence of recent, postglacial range expansion. We develop ecological niche models (EMNs) for the major lineages of C. penicillatus, and project these models onto reconstructions of climatic conditions during the latest glacial maximum (LGM; 18,000–21,000 years ago). The ENMs for each clade indicate differences in predicted current geographic distributions as well as distributions during the LGM. Models for the LGM indicate broad retention of potential habitat within the area of contact among the major clades. Furthermore, the ENM for the Mojave clade in particular indicates retention of suitable habitat during the LGM in small isolated patches within northern areas, consistent with the haplotype network that supports the perspective that some populations from the Mojave clade were isolated within northern refugia during the last glacial period.

Phylogenetic relationships of assimineid gastropods of the Death Valley–lower Colorado River region: relicts of a late Neogene marine incursion?

AbstractAim A small fauna of amphibious snails (genus Assiminea Fleming, 1828) living in association with highly mineralized springs in the Death Valley–lower Colorado River region (DVLCR) is thought to be a relict of the Bouse Embayment, a putative late Miocene–early Pliocene transgression of the ancestral Gulf of California along the lower Colorado River valley. We analysed the phylogenetic relationships of this fauna using mtDNA sequence data (1171 bp) to determine whether, as would be consistent with this hypothesis, it forms a substantially divergent unit sister to marine coastal congeners.Location South‐western Great Basin and lower Colorado River region, USA.Methods Two genes [mitochondrial cytochrome c oxidase subunit I (COI) and the mitochondrial 16S ribosomal RNA gene] were sequenced for 10 populations of DVLCR assimineas (Assiminea infimaBerry, 1947; Assiminea sp.). We also sequenced an undescribed population from a spring in the Colorado River delta; western North American Pacific Coastal Assiminea californica (Tryon, 1865); the three other congeners that live on the continent; and three Old World assimineids (outgroups). Phylogenies based on the combined data set were obtained using Bayesian methods, and divergence times were estimated using a COI molecular clock for related gastropods.Results Composite haplotypes of the DVLCR assimineas, together with that observed in the Colorado River delta population, formed a weakly supported clade that was sister to a clade composed of populations of North American Pacific and Atlantic coastal species. The genetic distance between members of these two clades was 3.46 ± 0.47% for COI and 1.69 ± 0.38% for 16S. The former clade was composed of five subunits that differed from each other by 1.29–2.84% (COI) and 0.52–1.98% (16S) sequence divergence.Main conclusions Application of the COI clock suggests that progenitors of the DVLCR fauna diverged from coastal ancestors 2.13–1.89 Ma (late Pliocene), several million years after the Bouse Embayment would have been terminated by the establishment of the lower (freshwater) Colorado River. This finding, together with shallow genetic structuring of several DVLCR lineages that are widely distributed across the topographically complex regional landscape, suggests that the Assiminea fauna of this inland area was more likely to have been founded by coastal colonists transported on water birds than through a direct connection with the sea.

Development of spatially diverse and complex dune‐field patterns: Gran Desierto Dune Field, Sonora, Mexico

AbstractThe pattern of dunes within the Gran Desierto of Sonora, Mexico, is both spatially diverse and complex. Identification of the pattern components from remote‐sensing images, combined with statistical analysis of their measured parameters demonstrate that the composite pattern consists of separate populations of simple dune patterns. Age‐bracketing by optically stimulated luminescence (OSL) indicates that the simple patterns represent relatively short‐lived aeolian constructional events since ∼25 ka. The simple dune patterns consist of: (i) late Pleistocene relict linear dunes; (ii) degraded crescentic dunes formed at ∼12 ka; (iii) early Holocene western crescentic dunes; (iv) eastern crescentic dunes emplaced at ∼7 ka; and (v) star dunes formed during the last 3 ka. Recognition of the simple patterns and their ages allows for the geomorphic backstripping of the composite pattern. Palaeowind reconstructions, based upon the rule of gross bedform‐normal transport, are largely in agreement with regional proxy data. The sediment state over time for the Gran Desierto is one in which the sediment supply for aeolian constructional events is derived from previously stored sediment (Ancestral Colorado River sediment), and contemporaneous influx from the lower Colorado River valley and coastal influx from the Bahia del Adair inlet. Aeolian constructional events are triggered by climatic shifts to greater aridity, changes in the wind regime, and the development of a sediment supply. The rate of geomorphic change within the Gran Desierto is significantly greater than the rate of subsidence and burial of the accumulation surface upon which it rests.

Identifying sources of groundwater in the lower Colorado River valley, USA, with δ18O, δD, and 3H: implications for river water accounting

Isotope measurements (δ18O, δD, 3H) indicate groundwater origin in the Lower Colorado River Valley (LCRV) and provide an alternative, or supplement, to the US Bureau of Reclamation’s proposed “accounting surface” method. The accounting surface method uses a hydraulic criterion to identify certain wells away from the flood plain that will eventually yield mainstream Colorado River water. New isotope data for 5 surface-water and 18 groundwater sites around Topock Marsh, Arizona, are compared with river-water data (1974–2002) from 11 sites between Utah and Mexico and with groundwater data from previous LCRV studies. Three groundwater sources are repeatedly identified in the LCRV: (1) local recharge derived from precipitation, usually winter rain, plots slightly below the global meteoric water line (GMWL) and has δD values that are 20‰ greater than those of recent river water; (2) “older” (pre-1950) upper basin river-water plots on or near the GMWL, distinct from local rainfall and recent river water; and (3) recent (post-1950) Colorado River water, including Topock Marsh samples, plots below the GMWL along an evaporation trend. Large floods, as in 1983, complicate interpretation by routing less evaporated upper basin water into the LCRV; however, tritium content can indicate the age of a water. River-water tritium has declined steadily from its peak of 716 TU in 1967 to about 11 TU in 2002. Mixtures of all three groundwater sources are common.

Late Quaternary sedimentation, lower Colorado River, Gulf Coastal Plain of Texas

Research Article| August 01, 1994 Late Quaternary sedimentation, lower Colorado River, Gulf Coastal Plain of Texas MICHAEL D. BLUM; MICHAEL D. BLUM 1Department of Geology, Southern Illinois University, Carbondale, Illinois 62901 Search for other works by this author on: GSW Google Scholar SALVATORE VALASTRO, JR. SALVATORE VALASTRO, JR. 2Radiocarbon Laboratory, Balcones Research Center, University of Texas, Austin, Texas 78758 Search for other works by this author on: GSW Google Scholar Author and Article Information MICHAEL D. BLUM 1Department of Geology, Southern Illinois University, Carbondale, Illinois 62901 SALVATORE VALASTRO, JR. 2Radiocarbon Laboratory, Balcones Research Center, University of Texas, Austin, Texas 78758 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1994) 106 (8): 1002–1016. https://doi.org/10.1130/0016-7606(1994)106<1002:LQSLCR>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation MICHAEL D. BLUM, SALVATORE VALASTRO; Late Quaternary sedimentation, lower Colorado River, Gulf Coastal Plain of Texas. GSA Bulletin 1994;; 106 (8): 1002–1016. doi: https://doi.org/10.1130/0016-7606(1994)106<1002:LQSLCR>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Investigations in the lower Colorado River Valley, Gulf Coastal Plain of Texas, have resulted in the development of a spatially and temporally controlled history of changes in channel and flood-plain erosional and depositional processes. When combined with paleoclimatic and stratigraphic data from the upper Colorado River drainage and the record of glacio-eustasy in the Gulf of Mexico, this study permits evaluation of the relative influence of different external controls on channel and flood-plain behavior, the development of alluvial landforms, and the development of alluvial stratigraphic sequences.Late Pleistocene and Holocene alluvial deposits of the lower Colorado River have been subdivided into allostratigraphic units, with chronological control afforded by radiocarbon ages. In the bedrock-confined valley, up to 10 m of late Pleistocene (∼20,000-14,000 yr B.P.) sediments referred to as the Eagle Lake Alloformation (ELA) underlie a terrace at 17-20 m above the present-day channel. Deposition of the ELA was followed by bedrock valley incision, then deposition of a complex Holocene valley fill referred to as the Columbus Bend Alloformation (CBA). Columbus Bend Allomembers 1 and 2 (CBA-1 and CBA-2) underlie a terrace at 12-14 m above the present-day channel. CBA-1 was deposited ∼12,000-5,000 yr B.P., whereas CBA-2 was deposited ∼5,000-1,000 yr B.P. Columbus Bend Allomember 3 (CBA-3) consists of channel and flood-plain deposits that represent the past 600 yr of activity.Allostratigraphic units within the lower Colorado valley correlate with allostratigraphic units in major valley axes of the upper Colorado drainage and with records of climatic and environmental change, suggesting that alluvial deposits record basinwide responses to climatically controlled changes in discharge regimes and sediment supply. Basal unconformities for Holocene valley fills, however, appear to be 1,000-2,000 yr younger in the upper Colorado drainage than they are in the lower Colorado valley. This time-transgressioe episode of bedrock valley cutting was initiated by climatically controlled reductions in sediment supply, but conditioned by limits on rates of up-stream propagation of incision through a large drainage basin. By contrast, unconformities within Holocene valley fills document time-parallel episodes of flood-plain abandonment and soil formation, but little additional bedrock valley cutting, and indicate decreased flood magnitudes following shifts to drier climatic conditions.Flood-plain morphology and sedimentary facies changed through time in response to changes in climate coupled with a protracted degradation of upland soil mantles, which altered the rate at which precipitation was transferred to stream channels as runoff. During the late Pleistocene through middle Holocene, runoff was filtered through deep upland soils, floods were for the most part less flashy and contained within channel perimeters, and flood plains were constructed by lateral migration without significant vertical accretion; hence, the ELA and CBA-1 contain few vertical accretion facies. Exposure of bedrock surfaces during the late Holocene resulted in increased flood stages, deep overbank flooding, and construction of flood plains by vertical accretion; hence, CBA-2 and CBA-3 contain thick vertical accretion facies.Allostratigraphic units and bounding unconformities persist through the bedrock-confined valley to the Quaternary alluvial plain, but stratigraphic architecture changes substantially in the downstream direction as a result of the last glacio-eustatic cycle. On the alluvial plain, late Holocene CBA-2 and modern CBA-3, deposited contemporaneously with the present interglacial highstand, onlap and bury the ELA and CBA-1, which were emplaced during the last full glacial lowstand and the transgression that followed. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Late pleistocene and holocene loess deposits of the Southeastern Buenos Aires province, Argentina

The Late Pleistocene and Holocene loess deposits of the SE Buenos Aires province are composed of four allostratigraphic units that represent four episodes of loess deposition. The first and the second episodes occurred in Late Pleistocene times. The second episode was followed by a soil forming interval (Early Holocene to Mid-Holocene times). The third episode took place at about 5000 yr BP, after the Holocene sea-level maximum when marine regression began. The fourth episode constitutes a historical event of only local significance. Loess shows a fairly constant granulometric and mineralogical composition. The modal fraction consists of very fine sand and coarse silt (3 to 5 phi). They are classified as sandy silts or silty sands. Three grain-size subpopulations are differentiated: coarse, medium and fine. The medium-size subpopulation, which is the most important, consists of most of the very fine sand and coarse silt. It is thought to be transported by modified saltation and short-term suspension during local dust-storms. The mineralogical composition of loess consists of a volcaniclastic assemblage derived mainly from reworked pyroclastic deposits, primary tephras and volcaniclastic sediments. The source area of these materials was located in the lower Colorado river valley about 400 km SW of the studied area. There was also a direct supply by volcanic ash falls.

Avian Community Structure Changes in a Mature Floodplain Forest after Extensive Flooding

Effects of flooding on the vegetation and avian communities of the Bill Williams River, Arizona, were evaluated from 1976 to 1983. High water flows in 1978 through 1980 caused the death of 99% of all Fremont cottonwoods (Populus fremontii) and 64% of all Goodding willows (Salix gooddingii) on a 120-ha area near the confluence with the Colorado River. By 1982, cattails (Typha spp.) had become the dominant vegetation. Groundand canopy-dwelling avian insectivores and cavity nesters decreased, whereas passerine marsh insectivores and rail-like species increased. Water released from Alamo Dam resulted in loss of the last remaining large stand of mature cottonwood-willow habitat in the lower Colorado River valley, causing declines in numbers of some bird species possibly threatened with local extirpation. Need for flood management procedures to conserve vegetation is paramount when beleaguered habitats are involved. J. WILDL. MANAGE. 51(2):495-502 Creation of reservoirs and channelization of rivers have extensively altered plant and animal communities along many major water courses. In some areas controlled water releases in spring and early summer have resulted in a spread of native riparian tree species, as along the Platte River in Colorado and Nebraska (Williams 1978), thus increasing riparian wildlife in that area. However, in other areas, water flow management has been detrimental to both native vegetation and wildlife, as along the Missouri River (Anderson 1971, Johnson et al. 1976). Such problems are encountered also in the Southwest, where a prolific exotic tree, Chinese tamarisk (Tamarix chinensis), largely has replaced native tree species in many areas (Horton 1977). The lower Colorado River and its tributaries are such areas and serve as dramatic testimonials to the ecological consequences of heavily managed riverine systems. Examination of historical records shows that until the mid-1800's, the lower Colorado River was lined with gallery forests consisting of cottonwoods and willows. Channelization of the river, cutting of trees for firewood, fires, and land clearing for agriculture removed most of these forests (Ohmart et al. 1977). Cessation of natural floods and high soil salinities have prevented the return of these communities. Today, most Fremont cottonwood and Goodding willow areas that remain in the lower Colorado River valley are decadent and form mixed stands with exotic tamarisk, screwbean mesquite (Prosopis pubescens), honey mesquite (P. juliflora glandulosa), and arrowweed (Tessaria sericea). Only along the Bill Williams River does the cottonwood-willow forest remain relatively intact in the lower Colorado River mainstem and adjacent tributary deltas. A water release from Alamo Dam (near the confluence of the Bill Williams and Big Sandy rivers, Mohave County, Arizona) of unusually long duration flooded a 120-ha stand of cottonwood and willow on the Bill Williams River at its confluence with the Colorado River during October 1978-April 1981. Management of native vegetation is presently given very low priority in flood control procedures. This study was conducted to determine the effects of longterm flooding in a riparian forest and to denote subsequent changes in the avifauna. Results from this study will be useful in planning management options during future flooding events. Special thanks are due R. W. Engel-Wilson, M. J. Kasprzyk, J. M. Jackson, and D. Krueper for organizing and assisting with the fieldwork. D. E. Busch provided water flow data and insightful views into the workings of water management agencies. K. V. Rosenberg, A. W. Laurenzi, and V. C. Hink critically reviewed the manuscript and helped with interpolation of preand during-flood bird and vegetation data. S. M. Cook and J. R. Durham provided editorial assistance, and C. D. Zisner typed the manuscript. The research was funded partly by the U.S. Bur. Reclamation, Contract 7-07-30-V0009.

Nonlinear Relationships between Birds and Vegetation

We tested for the presence and importance of nonlinear relationships among bird species abundances and vegetation characteristics of the lower Colorado River valley. We transformed vegetation variables into polynomials and then ran multiple regressions, with bird densities as the dependent variables. Polynomial variables were selected in 36% of first steps and 45% of second steps in the regressions. In a significant number of cases there was a response to a polynomial variable but not to the original linear variable. Importance of curvilinear responses varied among bird species. This method offers a more comprehensive examination of responses to envirnomental gradients because unimodal and some skewed relationships can be detected.

Habitat Selection Attributes of an Avian Community: A Discriminant Analysis Investigation

Habitat use characteristics of all bird species occurring in the lower Colorado River valley were studied. Seventy—two transects, each through homogeneous stands of vegetation and together representing all riparian habitat types occurring in the area, were censused three times monthly for 2 yr (1977, 1978) to provide distributional records for all species. From the census data we divided the transects into three groups for each bird species each season: transects where the species was recorded in both 1977 and 1978, transects where the species was absent both years, and transects where the species occurred in only one of the years. We used discriminant analyses to classify the habitat on each transect as suitable or unsuitable for each species, using a set of eight species composition measures, four foliage density measures, and foliage height diversity (FHD). Mean habitat discriminability of all species varied from a low of 86% in summer to a high of 91% in late summer. Most species showed extensive variability among seasons in both the degree of habitat selectivity and the criteria used in habitat selection. This variability reflected both changes in population structure of the species and responses to changing resource and climatic pressures. Ecological attributes of the species were examined to determine which factors shaped the habitat use patterns of each. Feeding guild played only a small role in both the degree and criteria of habitat selectivity, except for highly specialized feeders such as nectivores and frugivores. Species with restricted distributions were usually habitat specialists, but species with widespread distributions included both habitat generalists and specialists. During spring and summer nonresident, short—stay species showed greater habitat selectivity than did extended—stay species, but during nonbreeding seasons both short—stay and extended—stay species showed equivalent habitat selectivity. That both the mix of habitat generalists and specialists and the habitat selectivity of individual species varied seasonally implies that the organization of habitat use of the community is the dynamic product of many interacting factors. The importance of individual habitat attributes was also examined. Species showed preferences for particular habitat attributes significantly more frequently than they avoided habitat attributes. FHD was the most frequently important variable in differentiating areas used from areas not used. Proportions of honey mesquite, cottonwood, and willow in the vegetation were also frequently important variables in habitat selection by avian species, but usually in conjunction with FHD. Foliage density characteristics were less frequently significant, but for species where these measures were important, they tended to be the primary habitat selection criterion. In summer and late summer, habitat selection criteria of individual species seemed independent of the selection criteria of the other species in the community. In other seasons there was evidence of species interactions in these habitat selection patterns. These empirical findings for an entire avian community over all seasons are an advance in our understanding of how habitat use patterns of communities are organized. The extensive seasonal variability in habitat use by avian species and variability of distributions of individual species between years imply that studies of community organization can best be done with large—scale, long—term studies.

Brood Parasitism of the Abert's Towhee: Timing, Frequency, and Effects

The effects of brood parasitism by the dwarf race of the Brown-headed Cowbird (Molothrus ater obscurus) on the nesting success of the Abert's Towhee (Pipilo aberti) in the lower Colorado River valley were studied. The frequency of cowbird parasitism varied significantly with time of season. The laying season of cowbirds paralleled that of migratory songbirds, but began a month later than that of the sedentary Abert's Towhee. After an influx of breeding cowbirds, 44% of towhee nests were parasitized. Egg ejection by adult cowbirds caused towhee clutches to decline significantly from a laying average of 2.9 eggs to 1.7 eggs at hatching. The probability of nesting success declined from 0.278, before cowbirds arrived on the breeding grounds, to 0.046 for all parasitized nests. The lack of a significant difference between daily mortality rates of nonparasitized nests before and after cowbirds started breeding indicated that the reduced success of late towhee nests was primarily due to cowbird parasitism. The success of cowbird eggs in towhee nests was also low, suggesting that towhees may not be favorable hosts.

Wintering Biology of Ruby-Crowned Kinglets in the Lower Colorado River Valley

The Ruby-crowned Kinglet (Regulus calendula) was studied from October-April during four years, 1975-1979, among six major plant communities in the riparian corridor of the lower Colorado River valley. Kinglets were most abundant in cottonwood-willow and least abundant in arrowweed. Within each community, the greatest density of kinglets generally occurred in areas of tall, dense vegetation. Density differed significantly among the four years in November, the month of peak abundance, and in February, winter's end. Populations decreased significantly through the winter in each year and decreases were not uniform among plant communities. Differences in relative abundances and winter population decreases were related to winter weather conditions. During the study period, kinglets were subject to marked seasonality as determined by decreases in average monthly minimum temperatures, changes in tree phenology, and changes in insect abundance and biomass. Kinglet responses to seasonality were most apparent in population dynamics, distribution among plant communities, foraging substrate utilization and choice of prey. Analysis of these data emphasizes the selective importance of the nonbreeding season ecology to kinglets and implies that these birds may be winter-limited.

Evidence for Social Regulation in Some Riparian Bird Populations

Patterns in the regulation of seven permanent resident riparian bird species were studied in the lower Colorado River valley. Populations may be predator-food regulated or selection might favor dispersal tendencies that keep the population at a given location low enough to avoid resource depletion as well as the attraction of predators. Theory states that socially regulated species should have relatively short breeding seasons and less postbreeding mortality, and should begin population declines later than nonsocially regulated species. Among the seven species studied we found a continuum, with ladder-backed and Gila wood-peckers in agreement with the theoretical criteria of socially regulated species and the cactus wren, verdin, and Gambel quail at the opposite end of the spectrum. The Abert towhee and crissal thrasher showed criteria of both groups, but had more in common with socially regulated species.