Search

Abstract: Particles of fine sediment can clog interstitial pore spaces of coarser grained sediment in river beds and thereby impede the exchange of water, dissolved constituents, and particulate matter with consequent ecological impacts. The extent to which fine sediment reduces connectivity between the stream bed and overlying channel is a function, in part, of sediment residence time. Short residence times imply frequent exchange of matter and reduced impact of fine sediment on stream bed habitat, while long residence times indicate that with respect to fine sediment delivery, the stream bed is more isolated from the overlying channel. Here we present a novel technique to quantify the residence time of river bed sediment at various depths over annual and decadal timescales using the fallout radionuclides (FRNs) 7Be (t 1/2 =53days) and 210Pbex (t 1/2 =22.3years). We placed mesh cylinders filled with 7Be-free sediment into a stream bed to quantify the capture of 7Be-tagged particles in the absence of scour and fill. We also took cores to the depth of refusal in alluvial sediment in unregulated and regulated rivers in Vermont and New Hampshire. Sampled watershed areas ranged from 29 to 410km2, and core depths ranged from 19 to 77cm. The 210Pbex activity profiles of cores show that bed sediment is exchanged to the depth of refusal at decadal timescales. In contrast, 7Be activity profiles indicate that fine sediment infiltrating into the bed had residence times ranging from 4 to >300days in unregulated rivers. Cores from a regulated river are notably different—subsurface sediment residence times were always longer than in unregulated rivers at comparable depths, likely owing to restriction of bed mobilization and clogging of bed material by fine sediment. These results suggest that filtration can be an important component of bed material delivery to stream beds, but filtration does not deliver material as deeply into the bed as scour and fill. We find that fallout radionuclides provide direct measurements of time and depth scales of active exchange of particulate matter, which are key controls on hyporheic function. [Copyright &y& Elsevier]

Avulsions are rare events that cause major realignments to fluvial systems and hold potential for significant societal consequences. We present a case study of an avulsion in a minimally-aggrading landscape that was influenced by long-term human modifications to a fluvial environment that was initially setup for an avulsion during regional Pleistocene deglaciation. The 2006 Suncook River avulsion in New Hampshire, USA, is a valuable example because minimal environmental management took place for a decade following the event, allowing natural processes to dominate initial fluvial responses. Geospatial data collected during this period allow for the evaluation of the short- and long-term morphological evolution of a river that experienced an avulsion. We review data from technical reports on the event and present a new synthesis of aerial imagery and topographic data highlighting key geomorphic features associated with the avulsion. These features include headcutting and widening associated with a 0.6-km long channel created by the avulsion. Downstream of the avulsion site, extensive channel filling, deposition of meter-scale sand sheets on floodplain surfaces, and development of a cutoff channel were major manifestations of the event. The initial setup for the avulsion can be traced to Pleistocene deglaciation, when glaciofluvial ridges deposited in the valley blocked a more direct route for the river and shifted the channel towards an area of shallow bedrock. Human modifications to the channel—including mill dam construction, floodplain narrowing, road development, and gravel mining—contributed to the avulsion setup by increasing superelevation and undermining the glaciofluvial deposits that had directed the river during the Holocene. The avulsion was triggered by a ~50-yr flood event following a 70-yr interval with no major floods but marked by continued human modification to the landscape. The unusual nature of the event combined with the wealth of available geospatial and hydrologic data make the Suncook River system an attractive target for future studies that can examine avulsion processes in greater detail. • Case study of 2006 Suncook River avulsion in New Hampshire, USA • Avulsion setup through combination of regional deglaciation and human impacts • Triggered by ~50-year flood event that followed several decades of low-intensity floods • Avulsion incised floodplain and glaciogenic sediments to create new channel. • Sediment wave caused downstream channel filling and significant overbank deposition. [ABSTRACT FROM AUTHOR]