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The Southwestern community of Gilbert, Ariz., chose a proactive approach to addressing dramatic changes in its watershed and resulting treatment challenges. Comprehensive analysis showed that building a new treatment plant was the best way to go. [ABSTRACT FROM AUTHOR]

Laramide northeast-flowing streams from the ancestral Mogollon highland beveled gently northeast-dipping Late Proterozoic to Cretaceous strata across the southern Colorado Plateau Transition Zone. Late Eocene renewed uplift rejuvenated northeast-flowing streams incising paleocanyons. Apache paleocanyon was incised into the Mogollon highland, including the north-trending Laramide Apache uplift bounded by major reverse faults/monoclines. The Mogollon Rim sequence aggraded in Apache paleocanyon and on a broad alluvial plain to the east. Middle Cenozoic tectonic subsidence of the Transition Zone, aridification, and volcanism combined to aggrade Apache paleocanyon with sedimentary and volcanic rocks from 37.6 to 18.63 Ma. Emplacement of the Mogollon-Datil caldera complex and Chuska erg on the southeastern Colorado Plateau forced streamflow to northwest-dispersal of fluvio-eolian sediment from 34 to 26 Ma. Following erosion by northwest-flowing streams on the southern Colorado Plateau from ~26 to 16 Ma, lake sediments of the lower Bidahochi Fm were deposited. Southwestward reactivation of Laramide faults was underway by ~25 Ma coeval with extensive 25 to 20 Ma Natanes Plateau basalt flows and extreme crustal thinning southwest of the Transition Zone. As northeastward streamflow gradually diminished, a Superstition field ash flow tuff ended northeastward flow at 18.63 Ma and was followed by a period of sluggish southwest stream flow and ponding until after ~14.84 Ma. Southwestward structural subsidence and possible spillover from ancestral Lake Hopi on the Colorado Plateau southern margin caused incision of the southwest-directed Dagger Canyon paleovalley after 14.84, which followed the path of the earlier Apache paleocanyon and possibly to the Sespe delta on the California coast before opening of Gulf of California. Structural collapse of the Tonto Basin to the west induced deposition of the Dagger Canyon Conglomerate in the Dagger Canyon paleovalley before the modern Salt River incised all previous deposits and became integrated with the Gila River during the Plio-Pleistocene. [Display omitted] • East-northeast Laramide streams incised paleocanyons and beveled landforms. • Mogollon highlands structurally subsided during Oligocene crustal extension. • Ignimbrite volcanism deflected streams northwest on Colorado Plateau southern margin. • Northwest streams carved Little Colorado River valley and eastern Grand Canyon. • Spillover of Bidahochi lake initiated southwest river flow from Colorado Plateau. [ABSTRACT FROM AUTHOR]

Abstract: Sulfides from the upper sulfide zone of the Salt River VMS deposit (South Africa) were analyzed for PGE (platinum group elements), Re, Au, Sb, As, Se, Te, Cr, Co, Ni, Mn, Mo, Ag, Hg and Pb using in-situ LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) techniques. The analysis of a plethora of trace elements during the ablation of one single spot was possible by using a multi-standard approach, involving the analysis of several external standards before, after and between the analysis of unknowns, including a new in-house standard for As, Sb, Se, and Te. Chondrite normalized PGE patterns best fit data from Besshi-type deposits, and show that metamorphism was isochemical with respect to the PGE and Au. Although most sulfide minerals experienced melting no migration into discrete pockets could be observed. [Copyright &y& Elsevier]

Abstract: Recent archaeological excavations along the lower Salt River, Arizona resulted in the unexpected discovery of buried late Pleistocene soils and cultural features dating 5800–7100cal YBP (Early Archaic), the latter representing the earliest evidence of human activity in the lower Salt River floodplain thus far identified. Because the lower Salt River floodplain has been heavily impacted by recent agriculture and urbanization and contains few stratigraphic exposures, our understanding of the river''s geological history is limited. Here we present a late Quaternary alluvial chronology for a segment of the lower Salt River based on 19 accelerator mass spectrometry 14C and four optically stimulated luminescence ages obtained during two previous geoarchaeological investigations. Deposits are organized into allostratigraphic units and reveal a buried late Pleistocene terrace inset into middle-to-late Pleistocene terrace deposits. Holocene terrace fill deposits unconformably cap the late Pleistocene terrace tread in the site area, and the lower portion of this fill contains the Early Archaic archaeological features. Channel entrenchment and widening ~900cal YBP eroded much of the older terrace deposits, leaving only a remnant of fill containing the buried latest Pleistocene and middle-to-late Holocene deposits preserved in the site area. Subsequent overbank deposition and channel filling associated with a braided channel system resulted in the burial of the site by a thin layer of flood sediments. Our study confirms that the lower Salt River is a complex mosaic of late Quaternary alluvium formed through vertical and lateral accretion, with isolated patches of buried soils preserved through channel avulsion. Although channel avulsion is linked to changes in sediment load and discharge and may have climatic linkages, intrinsic geomorphic and local base level controls limit direct correlations of lower Salt River stratigraphy to other large rivers in the North American Southwest. [Copyright &y& Elsevier]

ABSTRACT / Data from historical aerial photographs analyzed with a GIS show that river channel change on the Salt River in the Phoenix metropolitan area of central Arizona has been driven by large-scale regional flood events and local human activities. Mapping of functional surfaces such as low-flow channels, high-flow channels, islands, bars attached to channel banks, and engineered surfaces shows that during the period from 1935 to 1997, the relative areal coverage of these surfaces has changed. Flood events have caused general changes in sinuosity of the low-flow channel, but islands have remained remarkably consistent in location and size, while channel-side bars have waxed and waned. The most important determinant of local channel form and process is sand and gravel mining, which in some reaches occupies more than 70% of the active channel area. The general location of mining is closely related to the location of the moving urban fringe, which serves as a market for sand and gravel during construction. Quantitative spatial analysis of imagery supplemented by field mapping shows that for each location within the general channel area, it is possible to specify a probability of encountering a low-flow channel or other fluvial features. Maps showing the distribution of these probabilities of occurrence reveal the most probable location and configuration of the channel as it occurred in the past. Some reaches have the low-flow channel located persistently within a limited area as a result of bedrock or sinuosity controls, but other reaches dominated by flow separation or shallow gradient have almost no persistence in channel location from one flood to another. [ABSTRACT FROM AUTHOR]

U Pb detrital zircon and 40Ar 39Ar detrital sanidine dating of paleoriver deposits refines the timing of the mid-Cenozoic drainage reversal from NE- to SW-flowing rivers across the southern edge of the Colorado Plateau. NE-flowing paleorivers of the Mogollon Rim Formation were of multiple ages: ≤59.38 Ma for the Flying V outcrop and ≤37–33 Ma for other outcrops. These NE-flowing paleorivers were defeated by construction of the 38–23 Ma Mogollon–Datil volcanic field and extensional collapse of the Mogollon Highlands. The 37.6–21.8 Ma Whitetail Conglomerate of the Salt River paleocanyon records the transition from NE-flowing rivers to internal drainage that persisted from 30 to 14.67 Ma during continued extensional collapse. The SW-flowing proto-Salt River was established by about 12 Ma and flowed into Tonto Basin ~7 Ma. The Salt River extended its length to near Phoenix via basin spillover from the Tonto and Verde basins about 2.8 Ma and became established in its present path only in the past ~500 ka. Between 3.0 and 0.52 Ma, incision rates of the Salt River's headwaters have been steady at 95 m/Ma as calculated using the age and height of four far-traveled basalt runouts from the Springerville volcanic field. These incision rates contrast with 10 m/Ma rates for downstream areas near the Sentinel-Arlington volcanic field on the Gila River over the past 2.37 Ma. This 85 m/Ma of differential incision was a geomorphic response of headwater drainages to changes in base level (base level fall plus headwater uplift) that, at least in part, was a consequence of top-down integration of the Salt and Verde river systems via spillover. Neogene headwater uplift is proposed to have set the stage for and perhaps driven this downward integration by increasing river gradients, and progressive uplift in the past 3 Ma may help explain post-integration steady headwater incision. Surface uplift components include construction of the Springerville and San Francisco volcanic fields and related mantle-driven epeirogenic uplift of the southern rim of the Colorado Plateau. Long-term differential incision rates for the Salt River (85 m/Ma) are less than for the Colorado River system (140 m/Ma across Grand Canyon) suggesting west-up neotectonic tilting of the Colorado Plateau. [ABSTRACT FROM AUTHOR]

Queen Creek exemplifies a moderately-sized tributary drainage of the Salt River, central Arizona, USA. In contrast to the exoreic Salt River and its immediate neighbor to the south, the exoreic Gila River, Queen Creek has remained an ephemeral stream, losing its discharge as it debouched from its mountain drainage basin throughout the late Pliocene and Quaternary. As a result, only the largest discharge events extended out into the center of the endorheic Higley Basin and then, later, to the Salt River. Data presented here demonstrate that the Queen Creek drainage existed long before the arrival of the Salt River in the Higley Basin, and continued to exist after aggradational piracy diverted the early Salt River around the north side of South Mountain. Thus, Queen Creek not only provides information about the history of internal drainage within the Higley Basin, but it also provides valuable insight about how a local endoreic stream reacts to drainage integration. [ABSTRACT FROM AUTHOR]

• Freshwater flux (E-P) and entrainment modulate the maximum salinity region. • Ekman transport of near-equatorial low salinity waters controls maximum E-P region. • Fate of maximum salinity waters is modulated by the seasonal migration of the SEC. • A subducted salt river (2.6 Sv) flows northward along the North Brazil Current. Like most other ocean basins, the maximum sea surface salinity region (MSR) in the South Atlantic shows a large displacement from the region of maximum difference between evaporation and precipitation (E-P), suggesting that ocean processes play a key role in determining the location of the MSR. We use outputs from a general circulation model (ECCO v4r3) to analyze the mixed layer salinity balance and disentangle the interaction of atmospheric forcing and oceanic processes in both regions. The MSR balance is dominated by evaporative surface fluxes and entrainment, while advection and diffusion play a secondary role. On the other hand, in the region of maximum E-P, the high surface freshwater loss is partially compensated by horizontal advection of low salinity waters, which is responsible for decreasing the salinity below that observed in the MSR. Using a particle tracking model, we find that MSR waters originate mostly from re-circulation in the Tropical South Atlantic and from the Tropical North Atlantic and Indian Oceans. After reaching the MSR, most of those waters flow southward in austral summer along the Brazil Current (1.6 Sv, 1 Sv = 106 m3 s−1), and northward in winter along the North Brazil Current (3.5 Sv). This seasonal variability in the fate of the salty water is modulated by the seasonal migration of the South Equatorial Current bifurcation region. Tracking of particles released at the base of the MSR mixed layer shows a subducted salt river with an estimated transport of 2.6 Sv on the 25.2 kg m−3 neutral density surface that flows northward along the North Brazil Current and retroflects just north of the equator as part of the Equatorial Undercurrent. These high-salinity waters are a significant contributor to the upper limb of the Atlantic meridional overturning circulation and the eastern Tropical Atlantic and their variability. [ABSTRACT FROM AUTHOR]

Studies of the evolution of drainage systems in extensional settings like the Basin and Range Province of western North America benefit from well drilling as a means of acquiring valuable insight. Cuttings from two wells drilled into sediments of the Phoenix metropolitan region, Arizona, USA, offer new insights into the drainage history of the Salt and Verde river drainage basins. Analyses of detrital zircons with U Pb dating reveal a different signature for Ancestral Salt River Deposits (ASRD) as compared to the underlying basin fill. Trace element, 87Sr/86Sr ratios, and electron microprobe analyses of basalt fragments in the basal deposits of the ASRD in two different wells from Mesa, Arizona, show matches for outcrop sources near Bartlett Dam in the Verde drainage and near Stewart Mountain Dam in the Salt drainage. This indicates that the Salt and Verde rivers were transporting these basalts when the Salt River first flowed into the metropolitan Phoenix area. We employed tephrochronology to determine that the 3.3-Ma Nomlaki tuff accumulated in closed-basin playa deposits located near the present-day junction of the Salt and Verde rivers, providing a maximum-limiting age for the integration of both rivers. Because the age of mountains crossed by the Salt and Verde rivers pre-date <3.3 Ma by tens of millions of years, we rule out antecedence and superimposition as possible mechanisms to explain these transverse drainages. Multiple lines of evidence presented here are inconsistent with drainage piracy from headward erosion for both drainages: (i) headward erosion would have eroded analyzed basalt clasts from outcrop positions prior to drainage integration, and yet these clasts only occur in the lowest deposits of the ASRD; (ii) headward erosion would not be expected to create transverse streams in two distinct river drainages at the same time, and yet basalt clasts eroded from outcrops in the Salt and Verde rivers arrived together in the basal layer of the ASRD deposits, within the temporal resolution of 3 m sampling interval for well cuttings; (iii) headward erosion of the Verde River from the Nomlaki Tuff (providing maximum age of river integration at 3.3 Ma) to the breached Verde Formation-depositing lake at 2.5 Ma would require an extraordinarily fast rate of >12 cm/yr across multiple bedrock uplands; and (iv) Mescal Limestone clasts on the highest Salt River strath terrace cannot be explained by headward erosion. However, all analyzed evidence are consistent with the process of lake overflow. Based on our findings and the use of geomorphic criteria (Douglass et al., 2009), we conclude that lake overflow is the most likely cause of drainage integration of both the Salt River and the Verde River. Unlabelled Image • Verde and Salt rivers integrated drainages after Nomlaki 3.3 Ma eruption • Detrital zircon analyses reveal substantially different source areas of sediment before and after river integration • Mineralogic and geochemical analyses indicate penecontemporaneous arrival of Salt and Verde rivers into the Phoenix area • Antecedence, superimposition, and headward erosion via piracy processes in conflict with new evidence • Lake overflow best explains Verde and Salt rivers integration across the Basin and Range Province of Arizona, USA [ABSTRACT FROM AUTHOR]

The dynamics of river systems are most easily studied by observing their present geomorphology exposed at Earth's surface. However, the bulk of these often long-lived transportation and depositional systems is mostly buried at depth and hidden from view. Accessing these subsurface archives is often not possible. The Phoenix 'basin' of Arizona, USA, with its rich collection of water well drilling information, offers a unique opportunity. Clast assemblages of sand and gravel deposits contained in subsurface drill cuttings enable estimates of sediment provenance and, importantly, reveal changes of provenance over time. This study represents more than a decade of drill cuttings analysis and presents a method for evaluating the depositional history of how the Phoenix basin developed a large external river system that crossed a long-lived internally drained basin. This configuration developed after Basin and Range faulting and extension had ceased, allowing the basin to fill to capacity with sediment. When placed in the context of a series of new isopach maps of the subsurface ancestral Salt River deposits, our clast assemblage analysis supports the conclusion that the Salt and Verde rivers appeared suddenly in the geologic record. The ancestral Salt River deposits and other nearby streams crossing the basin each possess a unique clast assemblage that can be used to 'fingerprint' sediments with similar sources. We use these observations to draw conclusions about the age and origin of these streams. Used in conjunction with other types of analyses such as cosmogenic burial dating and detrital zircon analyses, determining clast assemblages of drill cuttings in the context of mapping these assemblages is a powerful new method for estimating the source of sediment and for understanding how the Phoenix basin, and basins in general, fill with sediment. This information is of value in modeling both basin development and groundwater resources in similar contexts in Arizona and elsewhere. [ABSTRACT FROM AUTHOR]