Evolution of cutoffs across meander necks in Powder River,Montana, USA

Over a period of several decades, gullies have been observed in various stages of forming, growing and completing the cutoff of meander necks in Powder River. During one episode of overbank flow, water flowing over the down-stream bank of the neck forms a headcut. The headcut migrates up-valley, forming a gully in its wake, until it has traversed the entire neck, cutting off the meander. The river then follows the course of the gully, which is subsequently enlarged as the river develops its new channel. The complete process usually requires several episodes of high water: in only one of the five cases described herein was a meander cutoff initiated and completed during a single large flood. © 1998 John Wiley & Sons, Ltd.     

Nannofossils: the smoking gun for the Canarian hotspot

The origin of volcanism in the Canary Islands has been a matter of controversy for several decades. Discussions have hinged on whether the Canaries owe their origin to seafloor fractures associated with the Atlas Mountain range or to an underlying plume or hotspot of superheated mantle material. However, the debate has recently come to a conclusion following the discovery of nannofossils preserved in the products of the 2011–2012 submarine eruption at El Hierro, which tell us about the age and growth history of the western‐most island of the archipelago. Light coloured, pumice‐like ‘floating rocks’ were found on the sea surface during the first days of the eruption and have been shown to contain fragments of pre‐island sedimentary strata. These sedimentary rock fragments were picked up by ascending magma and transported to the surface during the eruption, and remarkably retained specimens of pre‐island Upper Cretaceous to Pliocene calcareous nannofossils (e.g. coccolithophores). These marine microorganisms are well known biostratigraphical markers and now provide crucial evidence that the westernmost and youngest island in the Canaries is underlain by the youngest sediment relative to the other islands in the archipelago. This finding supports an age progression for the onset of volcanism at the individual islands of the archipeligo. Importantly, as fracture‐related volcanism is known to produce non‐systematic age‐distributions within volcanic alignments, the now‐confirmed age progression corroberates to the relative motion of the African plate over an underlying mantle plume or hotspot as the cause for the present‐day Canary volcanism.     

Causes for the decline of suspended‐sediment discharge in the Mississippi River system, 1940–2007

Before 1900, the Missouri–Mississippi River system transported an estimated 400 million metric tons per year of sediment from the interior of the United States to coastal Louisiana. During the last two decades (1987–2006), this transport has averaged 145 million metric tons per year. The cause for this substantial decrease in sediment has been attributed to the trapping characteristics of dams constructed on the muddy part of the Missouri River during the 1950s. However, reexamination of more than 60 years of water‐ and sediment‐discharge data indicates that the dams alone are not the sole cause. These dams trap about 100–150 million metric tons per year, which represent about half the decrease in sediment discharge near the mouth of the Mississippi. Changes in relations between water discharge and suspended‐sediment concentration suggest that the Missouri–Mississippi has been transformed from a transport‐limited to a supply‐limited system. Thus, other engineering activities such as meander cutoffs, river‐training structures, and bank revetments as well as soil erosion controls have trapped sediment, eliminated sediment sources, or protected sediment that was once available for transport episodically throughout the year. Removing major engineering structures such as dams probably would not restore sediment discharges to pre‐1900 state, mainly because of the numerous smaller engineering structures and other soil‐retention works throughout the Missouri–Mississippi system. Published in 2009 by John Wiley & Sons, Ltd.     

Backwater effects in the Amazon River basin of Brazil

The Amazon River mainstem of Brazil is so regulated by differences in the timing of tributary inputs and by seasonal storage of water on floodplains that maximum discharges exceed minimum discharges by a factor of only 3. Large tributaries that drain the southern Amazon River basin reach their peak discharges two months earlier than does the mainstem. The resulting backwater in the lowermost 800 km of two large southern tributaries, the Madeira and Purús rivers, causes falling river stages to be as much as 2–3 m higher than rising stages at any given discharge. Large tributaries that drain the northernmost Amazon River basin reach their annual minimum discharges three to four months later than does the mainstem. In the lowermost 300–400 km of the Negro River, the largest northern tributary and the fifth largest river in the world, the lowest stages of the year correspond to those of the Amazon River mainstem rather than to those in the upstream reaches of the Negro River.     

Sacred ground; the Maipés necropolis of north‐west Gran Canaria

Gran Canaria, like most of the Canary Islands, shows evidence for young basaltic volcanism in the form of cinder cones and valley‐hugging lava flows. These landforms were of no particular use to the aboriginal population, nor to the subsequent Spanish settlers, and young lava flows and lava fields are still referred to as ‘malpaís’ (badlands) in the Canary Islands. In north‐west Gran Canaria, one such lava flow fills the bottom of a steep‐sided valley, which reaches the sea at the present day village of Agaete. The lava flow erupted c. 3030 ± 90 yr bp and displays a total length of ～ 11 km. At its distal end, just outside Agaete, it hosts one of Europe’s largest and most important pre‐historic burial sites constructed of volcanic rock: the Maipés necropolis. Over 700 pre‐historic tombs (or tumuli) constructed from the aa‐type clinker materials have been identified on top of the valley‐filling lava flow. The up to soccer‐ball sized vesicular clinker fragments are sufficiently low in density to provide abundant, workable basalt blocks for the construction of the tumuli, allowing the pre‐hispanic aboriginal population to create a large and magnificent ‘sacred ground’ in an otherwise barren landscape.     

Regional controls on geomorphology, hydrology, and ecosystem integrity in the Orinoco Delta, Venezuela

Interacting river discharge, tidal oscillation, and tropical rainfall across the 22,000 km² Orinoco delta plain support diverse fresh and brackish water ecosystems. To develop environmental baseline information for this largely unpopulated region, we evaluate major coastal plain, shallow marine, and river systems of northeastern South America, which serves to identify principal sources and controls of water and sediment flow into, through, and out of the Orinoco Delta.The regional analysis includes a summary of the geology, hydrodynamics, sediment dynamics, and geomorphic characteristics of the Orinoco drainage basin, river, and delta system. Because the Amazon River is a major source of sediment deposited along the Orinoco coast, we summarize Amazon water and sediment input to the northeastern South American littoral zone. We investigate sediment dynamics and geomorphology of the Guiana coast, where marine processes and Holocene history are similar to the Orinoco coast.Major factors controlling Orinoco Delta water and sediment dynamics include the pronounced annual flood discharge; the uneven distribution of water and sediment discharge across the delta plain; discharge of large volumes of water with low sediment concentrations through the Río Grande and Araguao distributaries; water and sediment dynamics associated with the Guayana littoral current along the northeastern South American coast; inflow of large volumes of Amazon sediment to the Orinoco coast; development of a fresh water plume seaward of Boca Grande; disruption of the Guayana Current by Trinidad, Boca de Serpientes, and Gulf of Paria; and the constriction at Boca de Serpientes.