Western Grand Canyon Research Articles

Temporal Patterns of Fish Occurrence in the Colorado River, Grand Canyon in Response to Temperature, Largescale Drought, and Newly Exposed Habitat

ABSTRACTTwo large dams on the Colorado River, Glen Canyon and Hoover Dam, have profoundly altered the river within Grand Canyon. Owing to widespread drought and increased water usage throughout the Southwest, water levels in Lake Mead declined by over 40 m since 2000, exposing over 80 km of newly emerged riverine habitat in the western Grand Canyon that experiences higher water temperatures and increased turbidity relative to upstream portions of the Colorado River within the Grand Canyon. Our objective was to investigate how the probability of fish occurrence has changed within the Colorado River in Grand Canyon as abiotic conditions have shifted in relation to these altered environmental conditions. We used 22 years of long‐term fish monitoring data, from 2000 to 2022, on the mainstem of the Colorado River in the Grand Canyon to map the probability of occurrence based upon modeled water temperature for native and common nonnative salmonid species through space (river kilometer) and time in relation to declining water levels in Lake Mead. Our results suggest that native fish occurrence is strongly correlated with longitudinally increasing water temperature, while nonnative trout species were negatively correlated with increased water temperature downstream from the dam. Further, we found that nonnative trout species have experienced decreases in their probability of occurrence through time, while natives have experienced an increase in probability of occurrence. Information on distribution of fish prior to reservoir declines, and how the fish assemblage has shifted coincident with environmental changes, can help inform future management of native and nonnative fish in highly regulated river systems.

Vital rates of a burgeoning population of Humpback Chub in western Grand Canyon

AbstractObjectiveThe Colorado River ecosystem has experienced habitat alterations and nonnative species invasions; as a result, many of its native species have had extirpations, abundance declines, and range constrictions. Despite these pitfalls, the Humpback Chub Gila cypha has persisted and in the last 10–15 years has expanded its range to become abundant in western Grand Canyon, a river segment in which it had been rare for the prior three decades. The goal of this study was to evaluate the population dynamics (i.e., growth, survival, movement, abundance) for this “new” group of Humpback Chub.MethodsWe analyzed a 6‐year mark–recapture study from a fixed monitoring reach in western Grand Canyon, and we provide the first estimates of survival and growth (vital rates) for these fish. We compared vital rates in western Grand Canyon to those of two life history forms (residents and migrants, representing fast and slow life history trajectories, respectively) from the more established Little Colorado River (LCR) aggregation in eastern Grand Canyon.ResultCompared to LCR migrants and LCR residents, Humpback Chub in western Grand Canyon had intermediate values for apparent survival, growth, and asymptotic length. Relatively high survival of subadults coupled with fast growth allowed for rapid population growth in western Grand Canyon. However, a large cohort in 2017 failed to lead to noticeable increases in adults. Seasonal survival patterns were distinct in all three groups, and apparent survival was lowest in western Grand Canyon during spring months. Adult Humpback Chub in western Grand Canyon were mobile and had a high probability of transience (i.e., just passing through the reach) and temporary emigration, demonstrating the need for future movement studies in western Grand Canyon to better distinguish emigration from survival.ConclusionThere is considerable spatial variability in viral rates for Humpback Chub in Grand Canyon. We discuss how observations are related to disparate temperature regimes experienced by the three groups and whether (how) the relationship between metabolism and temperature influences vital rates within the river network.

Assessment of Potential Recovery Viability for Colorado Pikeminnow Ptychocheilus lucius in the Colorado River in Grand Canyon

Abstract Colorado Pikeminnow Ptychocheilus lucius, the Colorado River's top native predatory fish, was historically distributed from the Gulf of California delta to the upper reaches of the Green, Colorado, and San Juan rivers in the Colorado River basin in the Southwestern United States. In recent decades Colorado Pikeminnow population abundance has declined, primarily as a result of predation by warmwater nonnative fish and habitat modification following dam construction. Small, reproducing populations remain in the Green and upper Colorado rivers, but their current population trajectory is declining and the San Juan River population is maintained primarily through stocking. As such, establishment of an additional population could aid recovery efforts and increase the species' resilience and population redundancy. The Colorado River in Grand Canyon once supported Colorado Pikeminnow, but until recently habitat suitability in this altered reach was considered low as a result of depressed thermal regime and abundant nonnative predators. Climate change and ongoing drought has presented an opportunity to evaluate the feasibility of native fish restoration in a system where declining reservoir storage has led to warmer releases and re-emergence of riverine habitat. These changes in the physical attributes of the river have occurred in concert with a system-wide decline in nonnative predators. Conditions 10 y ago were not compatible with reintroduction feasibility in Grand Canyon; however, as a result of rapidly changing conditions an expert Science Panel was convened to evaluate whether the physical and biological attributes of this reach could now support various life stages of Colorado Pikeminnow. Here, we report on the evaluation process and outcome from the Science Panel, which developed a science-based recommendation to the U.S. Fish and Wildlife Service on reintroduction feasibility. The Science Panel concluded that current habitat attributes in Grand Canyon could satisfy some, but perhaps not all, Colorado Pikeminnow life history requirements. This reach has the potential to support adult and subadult growth, foraging, migrations, and spawning, but low juvenile survival may limit recruitment. However, populations of other native species are successfully reproducing and increasing in western Grand Canyon, even in areas once considered suboptimal habitat. Should managers decide to move to the next phase of this process, actions such as experimental stocking and monitoring, telemetry studies, bioenergetics modeling, and laboratory-based research may provide additional information to further evaluate a potential reintroduction effort in this rapidly changing but highly altered system.

SYSTEMATIC LARVAL FISH SURVEYS AND ABIOTIC CORRELATES CHARACTERIZE EXTANT NATIVE FISH ASSEMBLAGE REPRODUCTIVE SUCCESS IN THE COLORADO RIVER, WESTERN GRAND CANYON, ARIZONA

Systematic larval fish surveys increase the probability of detecting rare species and provide ecological insights for more common species that can be difficult to infer from surveys of older life-stages. To characterize the reproductive success of the extant fish assemblage in the western Grand Canyon portion of the Colorado River, we conducted systematic larval fish surveys in 2014 and 2015. The effort identified endangered razorback sucker (Xyrauchen texanus) as initiating spawning in February with a continuation through July. We collected recently hatched protolarval fish throughout the study area including the most upstream sample sites, suggesting fish spawned throughout and upstream of the study area. Our collection of a protolarval endangered humpback chub (Gila cypha) supports the hypothesis that the population increased in range within the Grand Canyon from its dramatic 2001 contraction into the Little Colorado River and suggests localized spawning. Back-calculating hatching dates indicated humpback chub began hatching in late April and continued through mid-May. Native fishes numerically dominated larval fish collections (97.6 and 99.3% for 2014 and 2015, respectively) and recruitment to the early juvenile life-stage was documented for all fish species captured except razorback sucker. Abundance, measured as catch per unit effort and frequency of occurrence, was different between years for each native species captured (n = 5). Interannual differences in abiotic factors (discharge, diel fluctuations in discharge, and water temperature) were significantly different for most months between years. A redundancy analysis, evaluating the relationship between abiotic environmental factors and catch rates, suggested both discharge and water temperature were associated with speckled dace (Rhinichthys osculus), bluehead sucker (Catostomus discobolus), and flannelmouth sucker (Catostomus latipinnis), with a positive correlation to water temperature and a negative correlation to discharge. We were unable to identify associations between abiotic factors and the endangered fishes, which may have been due to the lack of variability in those species' catch rates. Improving our understanding of the relationship between reproductive success and abiotic factors would enhance management of the system to benefit native fishes.

A constraint on post–6 Ma timing of western Grand Canyon (Arizona, USA) incision removed: Local derivation indicated by ca. 5.4 Ma fluvial deposits below Shivwits Plateau basalts north of Grand Canyon

Abstract The timing of integration of the Colorado River system is central to understanding the landscape evolution of much of the southwestern United States. However, the time at which the Colorado River started incising the westernmost Grand Canyon (Arizona) is still an unsettled question, with conflicting interpretations of both geologic and thermochronologic data from western Grand Canyon. Fluvial gravels on the Shivwits Plateau, north of the canyon, have been reported to contain clasts derived from south of the modern canyon, suggesting the absence of western Grand Canyon at the time of their deposition. In this study, we reassess these deposits using modern geochronologic measurements to determine the age of the deposits and the presence or absence of clasts from south of the Grand Canyon. We could not identify southerly derived clasts, so cannot rule out the existence of a major topographic barrier such as Grand Canyon prior to the age of deposition of the gravels. 40Ar/39Ar analysis of a basalt clast entrained in the upper deposit (in combination with prior data) supports a maximum age of deposition of ca. 5.4 Ma, limiting deposition to post-Miocene, a period from which very few diagnostic and dated fluvial deposits remain in the western Colorado Plateau. Analysis of detrital zircon composition of the sand matrix supports interpretation of the deposit as being locally derived and not part of a major throughgoing river. We suggest that the published constraint of <6 Ma timing of Grand Canyon incision may be removed, given that no clasts that must be sourced from south of Grand Canyon were found in the only known outcrop of gravels under the Shivwits Plateau basalts at Grassy Mountain north of Grand Canyon.

Evaluating the Shinumo-Sespe drainage connection: Arguments against the “old” (70–17 Ma) Grand Canyon models for Colorado Plateau drainage evolution

AbstractThe provocative hypothesis that the Shinumo Sandstone in the depths of Grand Canyon was the source for clasts of orthoquartzite in conglomerate of the Sespe Formation of coastal California, if verified, would indicate that a major river system flowed southwest from the Colorado Plateau to the Pacific Ocean prior to opening of the Gulf of California, and would imply that Grand Canyon had been carved to within a few hundred meters of its modern depth at the time of this drainage connection. The proposed Eocene Shinumo-Sespe connection, however, is not supported by detrital zircon nor paleomagnetic-inclination data and is refuted by thermochronology that shows that the Shinumo Sandstone of eastern Grand Canyon was >60 °C (∼1.8 km deep) and hence not incised at this time. A proposed 20 Ma (Miocene) Shinumo-Sespe drainage connection based on clasts in the Sespe Formation is also refuted. We point out numerous caveats and non-unique interpretations of paleomagnetic data from clasts. Further, our detrital zircon analysis requires diverse sources for Sespe clasts, with better statistical matches for the four “most-Shinumo-like” Sespe clasts with quartzites of the Big Bear Group and Ontario Ridge metasedimentary succession of the Transverse Ranges, Horse Thief Springs Formation from Death Valley, and Troy Quartzite of central Arizona. Diverse thermochronologic and geologic data also refute a Miocene river pathway through western Grand Canyon and Grand Wash trough. Thus, Sespe clasts do not require a drainage connection from Grand Canyon or the Colorado Plateau and provide no constraints for the history of carving of Grand Canyon. Instead, abundant evidence refutes the “old” (70–17 Ma) Grand Canyon models and supports a <6 Ma Grand Canyon.

Insights into post-Miocene uplift of the western margin of the Colorado Plateau from the stratigraphic record of the lower Colorado River

AbstractThe spatial and temporal distribution of Pliocene to Holocene Colorado River deposits (southwestern USA and northwestern Mexico) form a primary data set that records the evolution of a continental-scale river system and helps to delineate and quantify the magnitude of regional deformation. We focus in particular on the age and distribution of ancestral Colorado River deposits from field observations, geologic mapping, and subsurface studies in the area downstream from Grand Canyon (Arizona, USA). A new 4.73 ± 0.17 Ma age is reported for a basalt that flowed down Grand Wash to near its confluence with the Colorado River at the eastern end of what is now Lake Mead (Arizona and Nevada). That basalt flow, which caps tributary gravels, another previously dated 4.49 ± 0.46 Ma basalt flow that caps Colorado River gravel nearby, and previously dated speleothems (2.17 ± 0.34 and 3.87 ± 0.1 Ma) in western Grand Canyon allow for the calculation of long-term incision rates. Those rates are ∼90 m/Ma in western Grand Canyon and ∼18–64 m/Ma in the eastern Lake Mead area. In western Lake Mead and downstream, the base of 4.5–3.5 Ma ancestral Colorado River deposits, called the Bullhead Alluvium, is generally preserved below river level, suggesting little if any bedrock incision since deposition. Paleoprofiles reconstructed using ancestral river deposits indicate that the lower Colorado River established a smooth profile that has been graded to near sea level since ca. 4.5 Ma. Steady incision rates in western Grand Canyon over the past 0.6–4 Ma also suggest that the lower Colorado River has remained in a quasi–steady state for millions of years with respect to bedrock incision. Differential incision between the lower Colorado River corridor and western Grand Canyon is best explained by differential uplift across the Lake Mead region, as the overall 4.5 Ma profile of the Colorado River remains graded to Pliocene sea level, suggesting little regional subsidence or uplift. Cumulative estimates of ca. 4 Ma offsets across faults in the Lake Mead region are similar in magnitude to the differential incision across the area during the same approximate time frame. This suggests that in the past ∼4 Ma, vertical deformation in the Lake Mead area has been localized along faults, which may be a surficial response to more deep-seated processes. Together these data sets suggest ∼140–370 m of uplift in the past 2–4 Ma across the Lake Mead region.

Ceramic Production and Exchange among the Virgin Anasazi, 30 Years Later

At the 1988 SAA annual meeting in Phoenix, Margaret Lyneis presented a paper with the title Ceramic Production and Exchange among the Virgin Anasazi. In that paper, she presented evidence that much of the pottery found on archaeological sites in the Moapa Valley of southeastern Nevada was in fact produced 70–100 km to the east. This pottery was made from distinctive raw materials found near the north rim of the western Grand Canyon. That 1988 SAA paper inspired much subsequent research, including my doctoral dissertation, which examined ceramic distributions across the western part of the Virgin region. In this paper, I update and expand on my earlier study. This analysis adds detail to Lyneis's original arguments, but demonstrates that she was largely correct. From about AD. 1050–1125, small-scale Virgin region settlements were linked by intensive ceramic exchange networks that crossed long distances and rugged terrain.

Carboniferous chondrichthyan assemblages from the Surprise Canyon and Watahomigi formations (latest Mississippian–Early Pennsylvanian) of the western Grand Canyon, Northern Arizona

AbstractTwo chondrichthyan assemblages of Late Mississippian/Early Pennsylvanian age are now recognized from the western Grand Canyon of northern Arizona. The latest Serpukhovian Surprise Canyon Formation has yielded thirty-one taxa from teeth and dermal elements, which include members of the Phoebodontiformes, Symmoriiformes, Bransonelliformes, Ctenacanthiformes, Protacrodontoidea, Hybodontiformes, Neoselachii (Anachronistidae), Paraselachii (Gregoriidae, Deeberiidae, Orodontiformes, and Eugeneodontiformes), Petalodontiformes, and Holocephali. The euselachian grade taxa are remarkably diverse with four new taxa recognized here; the Protacrodontidae:Microklomax carrieaenew genus new species andNovaculodus billingsleyinew genus new species, and the Anchronistidae:Cooleyella plateranew species andAmaradontus santuciinew genus new species The Surprise Canyon assemblage also has the youngest occurrence of the elasmobranchClairina, previously only known from the Upper Devonian. The Surprise Canyon Formation represents a nearshore fluvial infilling of karstic channels, followed by a shallow marine bioherm reef, and finally deeper open water deposition. The early Bashkirian Watahomigi Formation represents open marine deposition and contains only two taxa: a new xenacanthiform,Hokomata parvanew genus new species, and the holocephalanDeltodus. The relationship between the Surprise Canyon and Watahomigi chondrichthyan assemblages and other significant coeval chondrichthyan assemblages suggests that there may have been eastern and western distinctions among the Euamerican assemblages during the Serpukhovian due to geographic separation by the formation of Pangea.UUID:http://zoobank.org/54a906b6-4873-4f84-92b5-ca0752de01aa

Humpback Chub (Gila cypha) Range Expansion in the Western Grand Canyon

The Colorado River in the Grand Canyon holds the largest remaining population of Humpback Chub Gila cypha, an endangered fish endemic to the Colorado River basin. Early surveys in the 1990s found that most Humpback Chub occupied the Little Colorado River and nearby areas in the mainstem Colorado River and were uncommon in the western Grand Canyon (below Kanab Creek, at river kilometer [rkm] 257.2). From 1939 to 2002, the Colorado River was typically inundated by Lake Mead to rkm 407 (at full pool). Since 2000, Lake Mead water levels have declined, and the current inflow is located almost 100 km downstream at rkm 503.1. Thus, the Arizona Game and Fish Department (AGFD) extended its system-wide monitoring downstream to Pearce Ferry rapid (rkm 478.6) in 2011. Electrofishing is relatively inefficient at capturing Humpback Chub, but a small number of captures from 2011 to 2015 in the western Grand Canyon suggested that the Humpback Chub may have expanded its range downstream. Subsequently, hoop nets were added to AGFD system-wide monitoring in 2016 to better describe Humpback Chub distribution. The distribution of Humpback Chub observed in 2016 and 2017 differed from previous descriptions; we documented relatively large numbers of Humpback Chub, including age-0 fish, in the western Grand Canyon and observed the highest catches downstream of Diamond Creek (rkm 389.1). This suggests that a recent range expansion has occurred and that Humpback Chub in the western Grand Canyon do not utilize the Little Colorado River for reproduction and might therefore constitute a new subpopulation.

Population Expansion of Humpback Chub In Western Grand Canyon and Hypothesized Mechanisms

Humpback chub, Gila cypha, is an endangered warm water fish endemic to the Colorado River basin of southwestern North America. In Grand Canyon National Park, cold hypolimnetic water-release temperatures from Glen Canyon Dam have largely precluded successful spawning and recruitment of humpback chub in the mainstem Colorado River. Therefore, the species has utilized the warmer, more saline, and free-flowing Little Colorado River for its primary spawning habitat and continued existence. Based on long-term fish sampling efforts, we document local recruitment and population expansion of humpback chub in the western Grand Canyon and hypothesize that this is a result of recent warmer mainstem water temperatures. Continued recruitment and population expansion of humpback chub in the western Grand Canyon could potentially reduce extinction risk by providing population redundancy and less reliance upon the Little Colorado River for the species survival in the Grand Canyon.

Paleoproterozoic orogenesis and quartz-arenite deposition in the Little Chino Valley area, Yavapai tectonic province, central Arizona, USA

New field mapping and laboratory studies of Paleoproterozoic rock units around Little Chino Valley in central Arizona clarify the timing of magmatism, deformation, and sedimentation in part of the Yavapai tectonic province and yield new insights into sources of sands and weathering environments. Mafic lavas, calc-silicate rocks, and pelitic and psammitic strata in the Jerome Canyon area west of Little Chino Valley were deposited, deformed, and intruded by the 1736 ± 21 (2σ) Ma Williamson Valley Granodiorite. U-Pb geochronologic analysis of detrital zircons from a sample of psammitic strata yielded a maximum depositional age of ca. 1738 Ma. Approximately 25% of the detrital-zircon grains were derived from a ca. 2480 Ma source, as previously identified in Grand Canyon schist units. Kolmogorov-Smirnov statistical comparison of the Jerome Canyon detrital-zircon analyses with Grand Canyon schist analyses indicates that three of the 12 samples analyzed by Shufeldt et al. (2010) are statistically indistinguishable from the Jerome Canyon sample at the 95% confidence level and supports the concept that the Jerome Canyon sequence and Paleoproterozoic schists in the eastern and western Grand Canyon are part of the same tectonostratigraphic terrane. The Del Rio Quartzite on the northeast side of Little Chino Valley, previously considered an outlier of Mazatzal Quartzite, consists of poorly sorted quartz arenite, pebbly quartz arenite, and conglomerate deposited in a braided-stream environment. Microscope examination of 32 thin sections stained for potassium and calcium failed to identify any feldspar, mica, or mafic silicate grains. Similarly, conglomerate clasts consist entirely of vein quartz and less abundant argillite and jasper. A rock unit interpreted as a paleosol beneath the Del Rio Quartzite contains no surviving minerals except quartz, some of which is embayed and rounded as in corrosive saprolitic soils. U-Pb geochronologic analyses of detrital zircons from the 1400-m-thick Quartzite indicate maximum depositional ages of ca. 1745 Ma for the base and ca. 1737 Ma for the top. The unit is folded but is unaffected by the penetrative deformation and metamorphism that affected other Paleoproterozoic volcanic and sedimentary strata in the area, and it is probably significantly younger. We infer that the physically immature but chemically super-mature Del Rio Quartzite was deposited during a time of extreme weathering during a hot, humid climate with exceptionally high atmospheric CO 2 concentrations and associated corrosive rainwater rich in carbonic acid.

Rates of river incision and scarp retreat in eastern and central Grand Canyon over the past half million years: Evidence for passage of a transient knickzone: COMMENT

Abbott et al. (2015) and Crow et al. (2014), two Grand Canyon incision studies, come to very different conclusions despite apparent methodological similarities. Crow et al. (2014) used U-Th, Ar-Ar, and cosmogenic burial dating of material associated with Colorado River (CR) strath terrace sequences at 6 sites throughout Grand Canyon and concluded that average bedrock incision rates have been temporally steady at each site over at least the last 650 ka, but vary spatially from 100 to 160 m/Ma due to differential mantle-driven uplift. Abbott et al. (2015) used our unpublished Ar-Ar dating on a dike in western Grand Canyon, near river mile 159, plus U-Th dating of travertine-cement in sidestream alluvium, near Hermit Rapid, to suggest that incision rates were 1–4 km/Ma from 500 to 400 ka and <200 m/Ma after 400 ka, a difference they ascribe to a migrating knickpoint. The conclusions of these studies are contradictory.

A new model for Quaternary lava dams in Grand Canyon based on40Ar/39Ar dating, basalt geochemistry, and field mapping

The geomorphic response to volcanic incursions is spectacularly documented in western Grand Canyon, where numerous Quaternary lava flows dammed the Colorado River. This paper uses new 40 Ar/ 39 Ar ages, geochemistry, paleomagnetism, and field relationships to suggest 17 damming events, requiring major revision to previously published intracanyon flow sequences. From ca. 850 to 400 ka and at ca. 320 ka, numerous lava dams formed near the modern-day Lava Falls area. Starting around 250 ka, major volcanism shifted to the Whitmore Wash area, where additional dams formed. From ca. 200 to 100 ka, cascades flowed over the north rim in areas between Lava Falls and Whitmore Wash to form the youngest set of lava dams. Field observations and new dam reconstructions require a new model for how the Colorado River interacted with ephemeral lava dams in Grand Canyon. Specifically, the structure of lava dams, the position, character, and provenance of basaltic gravels within and above dams, and cooling structures in intracanyon flows suggest that unstable upstream dam portions failed quickly, while stable downstream dam segments were dismantled by the Colorado River more slowly. Time scales of dam removal are hard to assess, but we infer that lava dams that are overlain by monomictic basalt gravels were removed by the river in tens of years to centuries. In contrast, dams overlain by far-traveled gravel may have persisted for millennia.

Why the 16th Century Spanish Conquistadors likely did not descend into Grand Canyon near Desert View: A new perspective

There is probably no more historically significant or culturally pivotal event in the history of the south-western USA than the ambitious expedition of Francisco Vazquez de Coronado. Although the exploits of Coronado’s army have been examined for hundreds of years, new information has emerged from on-going studies of surviving documents and field work. This paper presents a new hypothesis and supportive evidence which suggest that a dispatch of Coronado’s men (led by Don Garcia Lopez de Cardenas) with the goal of discovering the Colorado River, likely did not descend off the south rim of Grand Canyon near the present day Desert View area in eastern Grand Canyon, as proposed by Bartlett in 1940. Surviving documents indicate that the Cardenas Expedition traveled 20 days and 130 miles before attempting a descent into the canyon; only three or four days of travel are needed to arrive at the Desert View area from the Hopi Village of Awat’ovi (the starting location of the Cardenas expedition). In this paper, topographic, anthropologic and historic evidence along with new distance calculations are presented which suggest that the 16th Century Spanish descent into Grand Canyon did not occur near the present day Desert View area, but likely occurred at least 35 to 40 (linear) miles west of this area near the present day South Bass Trail. Ramifications of the proposed hypothesis include: (1) focusing the search for Spanish artefacts and surviving evidence at a new location in western Grand Canyon; and, (2) rectifying the historical record.

Geomorphic constraints on the age of the western Grand Canyon

Hypotheses for the age of the western Grand Canyon (WGC) range from younger than 6 Ma to older than 70 Ma. We study the relationships among topography, geology, and available erosion rates in space and time to place constraints on plausible canyon incision histories. Evidence suggests that lateral retreat of the Shivwits Plateau escarpment left a lithologically controlled bench on the Sanup Plateau, but the Hualapai Plateau is beveled indiscriminately across rock types of the Paleozoic stratigraphic section. A period of accelerated base-level fall in the Tertiary is implicated by the canyon incised into the beveled Hualapai Plateau surface, consistent with higher erosion rates observed in canyons than on the surrounding plateau. Streams draining the Hualapai Plateau preserve relict headwater segments that were equilibrated with a slower base-level-fall rate before canyon incision. These relict segments are now separated from the Grand Canyon by knickpoints indicative of a transient landscape. Relief production since the beveling of the Hualapai Plateau is ∼1000 m in the WGC. Comparison of hillslope and channel morphologies between the Grand Wash Cliffs and the WGC provides a test to distinguish hypothesized ages of canyon incision. The data strongly suggest that carving of the WGC is younger than relief production due to slip on the Grand Wash fault ca. 18–12 Ma. Thus the geomorphic data are only fully consistent with the late Tertiary, transient incision model of canyon incision beginning at integration after 6 Ma.

Constraints on the evolution of vertical deformation and Colorado River incision near eastern Lake Mead, Arizona, provided by quantitative structural mapping of the Hualapai Limestone

The 12–6 Ma Hualapai Limestone was deposited in a series of basins that lie in the path of the Colorado River directly west of the Colorado Plateau and has been deformed by an en-echelon normal fault pair (Wheeler and Lost Basin Range faults). Therefore, this rock unit represents an opportunity to study the sedimentological and structural setting over which the Colorado River first flowed after integration through western Grand Canyon and Lake Mead. In this study, we quantify the structural geometry of the Hualapai Limestone and separate the deformation into syn- and postdepositional episodes. Both the Wheeler and Lost Basin Range faults were active during Hualapai Limestone deposition, as shown by thickening of strata and fanning of time lines toward half-graben faults that bound the Hualapai subbasins. The structure is characterized by a prominent reverse-drag fold and broad, shallow syncline adjacent to the Lost Basin Range fault, and a small-magnitude reverse-drag fold and short-wavelength normal-drag fold adjacent to the Wheeler fault. We find ∼450 m of throw between the footwall and hanging-wall Hualapai Limestone sections, suggesting faulting was ongoing after Hualapai Limestone deposition ceased and during Colorado River incision. To investigate a range of possible fault geometries that may have been responsible for Hualapai Limestone deformation, we compared our structural results against surface deflections calculated by a two-dimensional (2-D) geomechanical model. While nonunique, our results are consistent with a scenario in which the Wheeler fault was surface rupturing, or nearly surface rupturing throughout deposition of the Hualapai Limestone, but was inundated at ca. 6 Ma by coalescing paleolakes in Gregg and Grand Wash Basins as sedimentation kept pace with deformation. In contrast, we find evidence suggesting the Lost Basin Range fault was deeply buried by the Hualapai Limestone and likely propagated upward and laterally to break the surface sometime after 6 Ma. Therefore, we interpret the landscape over which the Colorado River first flowed to be of low relief within the terrain bounded by the Grand Wash Cliffs, the Hiller Mountains, and subtle topographic highs to the north and south of our field area. This original low-relief depositional surface was deflected into the structure exposed today by continuing deformation by the Wheeler and Lost Basin Range faults, allowing for calculation of apparent incision rates of the modern Colorado River drainage system that spatially vary between 33 and 42 m/m.y. in the hanging wall and between 108 and 115 m/m.y. in the footwall. Hanging-wall incision rate values are similar to, but faster than, a previously published point measurement, and footwall values are similar to measured incision rates in the western Grand Canyon, suggesting the Wheeler fault system may resolve as much as ∼410 m of Colorado Plateau uplift in the last 6 m.y.

Detrital-zircon U-Pb evidence precludes paleo–Colorado River sediment in the exposed Muddy Creek Formation of the Virgin River depression

Only since 5–6 Ma has the Colorado River flowed through the western Grand Canyon into the Grand Wash Trough at the eastern end of Lake Mead. Before then, the river may have flowed through a paleocanyon transiting the Kaibab uplift at a stratigraphic level above the present eastern Grand Canyon to cross the Shivwits Plateau north of the western Grand Canyon and enter the Virgin River drainage, which exits the Colorado Plateau into the Virgin River depression north of Lake Mead. U-Pb age spectra of detrital zircons from Miocene–Pliocene basin fill of the Muddy Creek Formation in the Virgin River depression preclude any paleo–Colorado River sand in Muddy Creek exposures but fail to show that a Miocene paleo–Colorado River never flowed into the Virgin River depression, because all exposed Muddy Creek horizons sampled for detrital zircons postdate ca. 6 Ma. Detrital-zircon populations from available surface collections cannot test for the presence of Colorado River sediment within unexposed lower Muddy Creek or sub–Muddy Creek strata in the subsurface of the Virgin River depression. Alternate scenarios include rejection of paleo–Colorado flow into the Virgin River drainage at any time, or exit of the pre–6 Ma paleo–Colorado River from the upper Virgin paleodrainage toward the northwest into the eastern Great Basin without flowing southwest into the Virgin River depression through the Virgin River gorge, which may be younger than ca. 6 Ma.

Patterns in the Landscape and Erosion of Cultural Sites Along the Colorado River Corridor in Grand Canyon, USA

The geologic and geomorphic template of Grand Canyon influences patterns in the archaeological record, including sites where apparent increases in erosion may be related to Glen Canyon Dam. To provide geoarchaeological context for the Colorado River corridor and such issues, we explore first‐order trends in a database of field observations and topographic metrics from 227 cultural sites. The patterns revealed may be expected in other river‐canyon settings of management concern. The spatial clustering of sites along the river follows variations in width of the valley bottom and the occurrence of alluvial terraces and debris fans, linking to bedrock controls. In contrast, the pattern of more Formative (Ancestral Puebloan, 800–1250 A.D.) sites in eastern Grand Canyon and Protohistoric (1250–1776 A.D.) sites in western Grand Canyon does not follow any evident geomorphic trends. In terms of site stability, wider reaches with more terrace and debris fan landforms host a disproportionate number of sites with acute erosion. This links most directly to weak alluvial substrates, and the primary erosion process is gullying with diffusive‐creep processes also pervasive. Although Glen Canyon Dam does not directly influence these erosion processes, overall sediment depletion and the loss of major flooding leaves erosion unhampered along the river corridor.

Paleogene Grand Canyon incompatible with Tertiary paleogeography and stratigraphy

The Hualapai Plateau in northwest Arizona, the location of the western Grand Canyon, contains an unusually lengthy Tertiary stratigraphic record dominated by fluvial deposition and extending from at least late Paleocene through late Miocene time. The thickest and oldest Tertiary sections are best exposed in a system of partially re-exhumed Laramide paleocanyons. The Paleogene drainage system was locally disrupted and ponded by Laramide monoclines. In pre-Oligocene time, extensive alluvial fans spread southward from the Shivwits Plateau scarp across the current location of the modern Colorado River gorge to the northern margin of the Laramide drainage system at Hindu Canyon. Locally derived, fluvial Buck and Doe Conglomerate subsequently filled the disrupted Paleogene channels, spilled out over the local interfluves, and formed an extensive aggradational surface of low relief by late Oligocene time. Early Miocene volcanism filled in much of the relict Laramide relief. Erosional recession of the adjacent Shivwits Plateau escarpment shifted the northern Hualapai Plateau margin 8 km northeastward after the Laramide drainage episode and before the incision by the modern Colorado River. Partially exhumed tributaries to the Hindu Canyon paleochannel and associated sedimentary deposits bordering the southern edge of the Grand Canyon gorge demonstrate that local surface runoff flowed south, away from the modern Grand Canyon location, during early Paleogene time. Headwardly eroding Colorado River tributaries exhumed, captured, and reversed the flow of these tributaries to the Laramide canyon, beginning in late Miocene or Pliocene time. The geomorphic and stratigraphic records show no evidence of, and provide no space for, incision of a Late Cretaceous–Paleogene ancestral precursor to the modern Colorado River gorge. Instead, all the field evidence clearly supports a late Miocene–Pliocene origin for integration of the western Grand Canyon on the central Hualapai Plateau with the upper Colorado River.