Hydraulic gold-mining tailings produced in the late nineteenth century in the Sierra Nevada foothills of California caused severe channel aggradation in the lower Feather and Yuba Rivers. Topographic and planimetric data from historical accounts, maps, topographic surveys, vertical sections, aerial photographs, and LiDAR (light detection and ranging) data reveal contrasting styles of channel change and floodplain evolution between these two rivers. For example, levee cross-channel spacings up to 4 km along the lower Yuba River contrast with spacings <2 km on the larger Feather River. More than a quarter billion cubic meters of hydraulic-mining sediment were stored along the lower Yuba River, and the wide levee spacing was intentionally maintained during design of the flood-control system to minimize delivery of sediment to navigable waters downstream. Consequently, the lower Yuba floodplain has a multithread high-water channel system with braiding indices >12 in some reaches. Some of the larger of these channels remain clearly visible on aerial photographs and LiDAR imagery in spite of intensive agricultural leveling. Narrow levee spacings on the Feather River were designed to encourage transport of mining sediment downstream and keep the channel clear for navigation. Levee spacings on the lower Feather River reached a minimum near the turn of the twentieth century, when floodplain widths were reduced at several constricted reaches to <250 m. Historical data indicate that the general channel location of the lower Yuba River had stabilized by the end of the nineteenth century, whereas substantial channel avulsions began later and continued into the twentieth century on the lower Feather River. The striking contrasts in channel change between the Yuba and Feather Rivers are due, at least in part, to different river-management strategies, although the Yuba River received much more sediment. Early river engineering of these channels represented the first efforts at integrated river-basin management west of the Mississippi, so the observed long-term effects are instructive. Modern river management should consider how the disturbance factors in these channels and the imprint of early river management affect the modern morphologic stability and sediment-production potential of the channel and floodplain.

The Hunter Creek Sandstone of the Verdi Basin, Nevada, yielded a succession of superposed continental faunal assemblages ranging in age from the late Clarendonian (late Miocene) through the late Blancan (late Pliocene) in the North American land mammal age framework, or ca. 10.5–2.5 Ma. We describe two new local faunas from the Hunter Creek Sandstone: the East Verdi local fauna, of late-medial to late Clarendonian age, which includes Dinohippus cf. D. leardi , Camelidae, ?Antilocapridae, and Mammutidae or Gomphotheriidae; and the Mogul local fauna, of Hemphillian age, which includes Dinohippus sp., Rhinocerotidae, Camelidae (at least two species), Mammut sp., and possibly Gomphotheriidae. A third unnamed assemblage, of latest Hemphillian or earliest Blancan age, is represented by a small sample of fossils from W.M. Keck Museum locality P-105. The only taxa recovered from this locality are cf. Megatylopus and Gomphotheriidae or Mammutidae. A single late Blancan locality, the Byland locality, yielded Equus idahoensis . The recognition of this faunal succession provides a biostratigraphic framework for the Hunter Creek Sandstone that corroborates and is consistent with the previous chronostratigraphy based on radioisotopic and tephrochronologic dating methods.

The Neogene Boca basin, located 15 km northeast of Truckee, California, records the depositional and deformational history for the late Miocene–Pliocene period in this part of the northern Sierra Nevada. This study consists of fine-scale analysis of the well-exposed Neogene sedimentary rocks in an otherwise poorly exposed area of the northern Sierra Nevada. The Neogene Boca basin sedimentary section is >500 m thick and dips generally west to southwest. Four distinct lithologic intervals are deposited unconformably over lahars and intermediate lavas of the Miocene Kate Peak Formation. An ~180-m-thick section of conglomerate and conglomeratic litharenite represents a generally southwest directed fluvial system that existed from at least 4.4 Ma (interval I). This is overlain by and locally interfingered with a ca. 4.38 Ma basalt flow of Boca Hill. Above this basalt, an ~107-m-thick section of quartz wacke and siltstone deposits represents a deltaic system controlled by local volcanic topography from ca. 4.4 to 4.1 Ma (interval II). Conformably above interval II, an ~122-m-thick section of silty diatomite deposits with interbedded tephra and litharenite represents a lacustrine environment from ca. 4.1 to 2.7 Ma (interval III). Overlying the diatomite along a disrupted surface, a >91-m-thick section of medium- to coarse-grained litharenite and cobble conglomerate represents an abrupt change in depositional environment, to a west directed fluvial system (interval IV). Pliocene westward tilting and change in base level began during deposition of interval IV (ca. 2.7 Ma) and prior to eruption of the Boca Ridge Formation (ca. 2.61 Ma). Four orientations of large faults (>0.1 m displacement) are distributed evenly across the basin: (1) northeast to north-northeast striking sinistral faults; (2) northwest to north-northwest striking dextral faults; (3) west to west-northwest striking oblique-reverse faults; and (4) other fault orientations that have apparent motions not included in these categories. Strike-slip faulting is thought to have occurred during tilting of the Neogene section. The distributed conjugate strike-slip faults in the rocks of Boca basin accommodated east-southeast directed extension and south-southwest directed contraction. These new stratigraphic and structural data provide information on late Miocene–Pliocene deformation at the eastern edge of the Sierra Nevada. The Boca basin appears to have been an isolated basin controlled by volcanic topography. A late Miocene deformation event is not recorded in Boca basin; however, a Pliocene event is recorded in the termination of deposition and deformation of the section through tilting, incision, and distributed faulting. Pliocene deformational style is consistent with generally east-west extension associated with westward encroachment of the Basin and Range or northward migration of normal faults at Lake Tahoe. The structural data cannot disprove migration of Walker Lane deformation into the Sierra Nevada but merely show that this did not occur in the area occupied by the Neogene Boca basin. The Pliocene deformation event coincided with local eruption of high-potassium lavas and a regional base-level change, and it may represent rollback of the Juan de Fuca plate after ca. 3 Ma.

Biostratigraphic data, based mostly on radiolarian assemblages, establish synchronous deposition in the northern Sierra terrane and the Havallah basin beginning in the Late Devonian and extending into the early Late Permian. Lower Mississippian and mid-Permian arc-derived volcaniclastic debris was deposited in parts of the Havallah basin during episodes of arc volcanism in the northern Sierra terrane. Between these episodes of arc volcanism, from late Early Mississippian to at least Middle Pennsylvanian, the northern Sierra terrane collected siliceous pelagic deposits that correlate with dominantly chert-argillite sections in the Havallah sequence. These intermixed lithic assemblages suggest shared stratigraphic evolution and geographic proximity between the Sierran arc terrane and the Havallah basin during the late Paleozoic. During Late Devonian and Early Mississippian arc volcanism in the northern Sierra terrane, lower Paleozoic rocks of the Roberts Mountains allochthon were thrust over coeval deposits on the North American shelf. Chert-quartz-rich siliciclastic debris, derived from the Antler orogenic belt, is interbedded with Upper Devonian and Lower Mississippian distal volcanic rocks in the northern Sierra terrane and with Kinderhookian volcaniclastic rocks and chert in the Schoonover sequence. These quartzose-clastic deposits not only provide an independent lithologic link between the Sierran arc terrane and the Havallah basin, they also tie the arc terrane and basin to North America at the time of the Antler orogeny. Late Devonian and Early Mississippian arc volcanism in the northern Sierra terrane occurred in an extensional regime. Extensional tectonism began locally in the Havallah basin during the Famennian and continued into the early Meramecian. Contemporaneous extension in the arc and basin during emplacement of the Roberts Mountains allochthon is difficult to reconcile with existing arc-continent collision models for the Antler orogeny.