ABSTRACT Uplift of the central Andes during the Miocene was followed by large-scale reorganization of Atlantic-draining rivers in Argentine Patagonia. Here, we document the abandonment of one large river in the late Pliocene and the establishment of the modern drainage in the Early Pleistocene. A chronology for these events is provided by 40 Ar/ 39 Ar ages on basalt flows. Remnants of the Pliocene paleovalley system are well preserved in the Lago Cardiel–Gobernador Gregores area, where they are eroded into flat-lying basalt flows dated from ca. 13.9 Ma to 8.6 Ma. Younger basalts that erupted onto the abandoned floor of the paleovalley are as young as 3.7 Ma. Abandonment of the Pliocene paleovalley and establishment of the modern Río Chico and Río Shehuen catchments happened near the close of the Pliocene when Andean glaciers incised the east-sloping pediment on which the late Miocene drainage was established. Lago Cardiel sits within a large endorheic basin that is inset into the late Pliocene paleovalley. The basin began to develop just before 4 Ma, after the paleovalley was abandoned. It became larger and deeper during the Pleistocene due to mass movements along its margins, deflation of the basin floor during times when Lago Cardiel was dry or nearly dry, and possibly lowering along bounding faults. The Pliocene–Pleistocene landscape and drainage changes that we have documented are not unique to the Lago Cardiel–Gobernador Gregores area; similar changes are apparent elsewhere in Patagonia east of the crest of the Andes.

ABSTRACT High-resolution light detection and ranging (lidar) data and new stratigraphic, lake sediment, and radiocarbon constraints help to resolve a long-standing dispute regarding the timing and nature of the Everson interstade and the Sumas stade, the last major events of the Cordilleran ice sheet in the Fraser Lowland. The new data indicate that: (1) an early, maximum Sumas advance occurred roughly 14,500 cal yr B.P. (calibrated 14 C years before 1950), extending into the Salish Sea near Bellingham, Washington; (2) ice retreated north of the International Boundary long enough for forests to establish in deglaciated lowland sites; (3) a rapid, short-lived rise in local relative sea level (RSL) of ~20–30 m, possibly related to meltwater pulse 1A or the collapse of a glacio-isostatic forebulge, inundated the U.S. portion of the lowlands up to ~130 m above modern sea level; and (4) directly following this transgression at ca. 14,000 cal yr B.P., ice readvanced across the border to nearly the same limit as reached during the early Sumas period. Distinct crosscutting marine strandlines (erosional and depositional remains of emerged marine shorelines), subaerial moraines, and till plains imaged in lidar data indicate that following the maximum extent of the second Sumas advance, local RSL progressively lowered as the glacier fluctuated and gradually thinned. By ca. 13,000 cal yr B.P., ice had retreated north of the border, and local RSL had fallen to within ~4 m of modern. A layer of possible loess in sediments in Squalicum Lake suggests a possible third and final Sumas readvance between 13,000 and 11,150 cal yr B.P., at which time a moraine was constructed ~8 km south of the border near the town of Sumas, Washington. Together, our results suggest that the concept of a distinct Everson interstade and Sumas stade should be abandoned in favor of a more nuanced “Sumas episode” that encompasses the sequence of events recorded in the Fraser Lowland.

Abstract Mountain rivers, like alpine glaciers, are sensitive indicators of climate change. Some rivers may provide a more complete record of Holocene climate change than the glaciers in their headwaters. We illustrate these points by examining the record preserved in the upper part of the alluvial fill in the Nostetuko River valley in the southern Coast Mountains, British Columbia (Canada). Glacier advances in the upper part of the watershed triggered valley-wide aggradation and complex changes in river planform. Periods when glaciers were restricted in extent coincide with periods of incision of the valley fill and floodplain stability. As many as 10 overbank aggradation units are separated by peat layers containing tree roots and stems in growth position. Twenty-five radiocarbon ages on roots, tree stems and woody plant detritus in several of the peat layers closely delimit periods of aggradation. The oldest phase of aggradation occurred about 6500 years bp and coincides with the Garibaldi Advance documented elsewhere in the southern Coast Mountains. A second phase of aggradation, recorded by several units of clastic sediment, dates to about 2500 years bp , near the peak of the middle Neoglacial Tiedemann Advance. The third phase occurred shortly after 1400 years bp during or shortly after the First Millennium Advance, which has been recently documented in coastal British Columbia and Alaska. The most recent phase of aggradation began about 800 years bp and continued until recently. It coincides with the Little Ice Age, when glaciers in the Nostetuko River basin and elsewhere in the southern Coast Mountains attained their greatest Holocene size. Several periods of peat deposition during the Little Ice Age indicate periods of floodplain stability separated by brief intervals of floodplain aggradation that coincide with Little Ice Age glacier advances in western Canada. The results imply that the west fork of Nostetuko River is sensitive to upvalley glacier fluctuations and, indirectly, to relatively minor changes in climate.

Abstract An important paradigm in geomorphology is paraglacial sedimentation , a phrase first used almost 40 years ago to describe reworking of glacial sediment by mass wasting and streams during and after continental-scale deglaciation. The concept has been extended to include non-glacial landforms and landscapes conditioned by glaciation. In this paper we apply the paraglacial concept to volcanoes in southern British Columbia, Canada, that formed, in part, in contact with glacier ice. The Cheekye River basin, a small watershed on the flank of a volcano that erupted against the decaying Cordilleran ice sheet, has a Holocene history marked by an exponential decay in debris-flow activity and sediment yield. Its history is consistent with the primary exhaustion model of the paraglacial cycle. At larger spatial scales, this primary sediment is reworked by rivers and transported downstream and augmented by stochastic geomorphic events. Repeated large landslides from Mount Meager volcano in southern British Columbia have delivered a disproportionate volume of sediment to the fluvial system: although occupying only 2% of the watershed area, 25–75% of the 10 km 3 of sediment deposited in Lillooet River valley during the Holocene originated from the volcano. In these cases a significant overall reduction in sediment yield must await the removal, by erosion, of volcanic edifices, a process that could take up to millions of years. These examples of paraglacial activity on Quaternary volcanoes are end members in the spectrum of landscape response to Pleistocene deglaciation.