The Columbia River system is one of the great river systems of North America, draining much of the Pacific Northwest, as well as parts of the western United States and British Columbia. The river system has had a long and complex history, slowly evolving over the past 17 m.y. The Columbia River and its tributaries have been shaped by flood basalt volcanism, Cascade volcanism, regional tectonism, and finally outburst floods from Glacial Lake Missoula. The most complex part of river development has been in the northern part, the Columbia Basin, where the Columbia River and its tributaries were controlled by a subsiding Columbia Basin with subtle anticlinal ridges and synclinal valleys superimposed on a flood basalt landscape. After negotiating this landscape, the course to the Pacific Ocean led through the Cascade Range via the Columbia Trans-Arc Lowland, an ancient crustal weakness zone that separates Washington and Oregon. The peak of flood basalt volcanism obliterated the river paths, but as flood basalt volcanism waned, the rivers were able to establish courses within the growing fold belt. As the folds grew larger, the major pathways of the rivers moved toward the center of the Columbia Basin where subsidence was greatest. The finishing touches to the river system, however, were added during the Pleistocene by the Missoula floods, which caused local repositioning of river channels.

ABSTRACT This field trip will highlight landform sediment assemblages and the geomorphic consequences and timing of multiple significant deglacial flood events at and near the confluences of the Minnesota and St. Croix rivers with the Upper Mississippi Valley. This geographic position is also near the former margins of two different Late Wisconsin glacial ice fronts. New radiocarbon and optical spectral luminescence (OSL) ages collected from these landform sediment assemblages are presented to help date the geomorphically transformative Late Wisconsin and earliest Holocene flood events of this complicated river confluence setting. Trip discussions will include pre–Late Wisconsin bedrock valley fill; geomorphology and gradients of genetically related terraces in the Upper Mississippi Valley and major west-side tributaries; comparisons between radiocarbon and OSL dating results; GIS mapping technologies and data sets; and contexts and predictions for buried archaeological resources.

Stratigraphic and geomorphic analyses reveal that the regional drainage basin of the modern Amargosa River formed via multistage linkage of formerly isolated basins in a diachronous series of integration events between late Miocene and latest Pleistocene–Holocene time. The 275-km-long Amargosa River system drains generally southward across a large (15,540 km 2 ) watershed in southwestern Nevada and eastern California to its terminus in central Death Valley. This drainage basin is divided into four major subbasins along the main channel and several minor subbasins on tributaries; these subbasins contain features, including central valley lowlands surrounded by highlands that form external divides or internal paleodivides, which suggest relict individual physiographic-hydrologic basins. From north to south, the main subbasins along the main channel are: (1) an upper headwaters subbasin, which is deeply incised into mostly Tertiary sediments and volcanic rocks; (2) an unincised low-gradient section within the Amargosa Desert; (3) a mostly incised section centered on Tecopa Valley and tributary drainages; and (4) a west- to northwest-oriented mostly aggrading lower section along the axis of southern Death Valley. Adjoining subbasins are hydrologically linked by interconnecting narrows or canyon reaches that are variably incised into formerly continuous paleodivides. The most important linkages along the main channel include: (1) the Beatty narrows, which developed across a Tertiary bedrock paleodivide between the upper and Amargosa Desert subbasins during a latest Miocene–early Pliocene to middle Pleistocene interval (ca. 4–0.5 Ma); (2) the Eagle Mountain narrows, which cut into a mostly alluvial paleodivide between the Amargosa Desert and Tecopa subbasins in middle to late Pleistocene (ca. 150–100 ka) time; and (3) the Amargosa Canyon, which formed in late middle Pleistocene (ca. 200–140 ka) time through a breached, actively uplifting paleodivide between the Tecopa and southern Death Valley subbasins. Collectively, the interconnecting reaches represent discrete integration events that incrementally produced the modern drainage basin starting near Beatty sometime after 4 Ma and ending in the Salt Creek tributary in the latest Pleistocene to Holocene (post–30 ka). Potential mechanisms for drainage integration across paleodivides include basin overtopping from sedimentary infilling above paleodivide elevations, paleolake spillover, groundwater sapping, and (or) headward erosion of dissecting channels in lower-altitude subbasins. These processes are complexly influenced by fluvial responses to factors such as climatic change, local base-level differences across divides, and (or) tectonic activity (the latter only recognized in Amargosa Canyon).

Where the lower Colorado River traverses the Basin and Range Province below the Grand Canyon, significant late Pleistocene aggradation and subsequent degradation of the river are indicated by luminescence, paleomagnetic, and U-series data and stratigraphy. Aggradational, finely bedded reddish mud, clay, and silt are underlain and overlain by cross-bedded to plane-bedded fine sand and silt. That sequence is commonly disconformably overlain by up to 15 m of coarse sand, rounded exotic gravel, and angular, locally derived gravel. Luminescence dates on the fine sediments range from ca. 40 ka to 70 ka, considering collective uncertainties. A section of fine-grained sediments over a vertical range of 15 m shows normal polarity magnetization and little apparent secular variation beyond dispersion that can be explained by compaction. Aggradation on large local tributaries such as Las Vegas Wash appears to have been coeval with that of the Colorado River. The upper limits of erosional remnants of the sequence define a steeper grade above the historical river, and these late Pleistocene deposits are greater than 100 m above the modern river north of 35°N. Terrace gravels inset below the upper limit of the aggradational sequence yield 230 Th dates that range from ca. 32 ka to 60 ka and indicate that degradation of the river system in this area closely followed aggradation. The thick sequence of rhythmically bedded mud and silt possibly indicates settings that were ponded laterally between valley slopes and levees of the aggrading river. Potential driving mechanisms for such aggradation and degradation include sediment-yield response to climate change, drought, fire, vegetation-ecosystem dynamics, glaciation, paleofloods, groundwater discharge, and building and destruction of natural dams produced by volcanism and landslides.

The Dead Sea (drainage area of ∼42,200 km 2 ) is a terminal lake fed from the north by the Jordan River. The main water source into the Dead Sea is runoff, and its level is highly sensitive to the annual rainfall in the upper Jordan River drainage basin. Here, we summarize relevant data about present and past surface-water hydrology in the drainage basin and water input into the Dead Sea. The lower Jordan River, with a natural mean annual discharge of ∼1100 10 6 m 3 yr −1 , drains Mediterranean to semiarid areas in northern Israel, Jordan, Syria, and Lebanon. Additional water is contributed from tributaries such as the Zarqa River (64 10 6 m 3 yr −1 ). The diversion of water from these and other sources since the 1960s to an inflow of only ∼210 10 6 m 3 yr −1 caused a 20 m decline of the Dead Sea level. The ephemeral Nahal Arava drains hyperarid areas south of the Dead Sea and contributes small volumes of water (∼5 10 6 m 3 yr −1 ) but experiences occasional large floods (up to 1000 m 3 s −1 ); the smaller, steep ephemeral tributaries west of the Dead Sea produce relatively large floods (up to 775 m 3 s −1 ) due to their wetter headwaters, but their volumes are relatively small (<3 10 6 m 3 ). The annual inflow (∼200 10 6 m 3 yr −1 ) to the Dead Sea from the eastern, semiarid tributaries is divided between base flows (∼65% of annual discharge) and winter floods. In the arid part of the basin, transmission losses and recharge into the shallow alluvial aquifers and probably also to the deeper aquifers during floods are directly related to flow volume. Transmission losses in Nahal Zin decrease with flood magnitude from ∼86% to 100% in small floods of <10 m 3 s −1 to ∼10% for large floods of up to 1500 m 3 s −1 . During the largest floods, the volume of these losses can exceed 4 10 6 m 3 . Rare paleofloods during the past 2000 yr in the Negev were 2–3 times larger than present-day measured floods. The temporal distribution of paleofloods shows that periods with a high frequency of large floods alternate with periods of few large floods. Periods with a high frequency of large floods correlate with high Dead Sea levels, and a low frequency of floods correlates with low lake levels.