Geology topics

P.R. Wilcock

Publications and source records attributed to P.R. Wilcock.

6 recordsLinked to original sources

USGS2011

Large shift in source of fine sediment in the upper Mississippi River

Although sediment is a natural constituent of rivers, excess loading to rivers and streams is a leading cause of impairment and biodiversity loss. Remedial actions require identification of the sources and mechanisms of sediment supply. This task is complicated by the scale and complexity of large watersheds as well as changes in climate and land use that alter the drivers of sediment supply. Previous studies in Lake Pepin, a natural lake on the Mississippi River, indicate that sediment supply to the lake has increased 10-fold over the past 150 years. Herein we combine geochemical fingerprinting and a suite of geomorphic change detection techniques with a sediment mass balance for a tributary watershed to demonstrate that, although the sediment loading remains very large, the dominant source of sediment has shifted from agricultural soil erosion to accelerated erosion of stream banks and bluffs, driven by increased river discharge. Such hydrologic amplification of natural erosion processes calls for a new approach to watershed sediment modeling that explicitly accounts for channel and floodplain dynamics that amplify or dampen landscape processes. Further, this finding illustrates a new challenge in remediating nonpoint sediment pollution and indicates that management efforts must expand from soil erosion to factors contributing to increased water runoff.

Environmental Science & Technology↗

USGS2011

Landscape evolution in south-central Minnesota and the role of geomorphic history on modern erosional processes

The Minnesota River Valley was carved during catastrophic drainage of glacial Lake Agassiz at the end of the late Pleistocene. The ensuing base-level drop on tributaries created knickpoints that excavated deep valleys as they migrated upstream. A sediment budget compiled in one of these tributaries, the Le Sueur River, shows that these deep valleys are now the primary source of sediment to the Minnesota River. To compare modern sediment loads with pre-European settlement erosion rates, we analyzed incision history using fluvial terrace ages to constrain a valley incision model. Results indicate that even thoughthe dominant sediment sources are derived from natural sources (bluffs, ravines, and streambanks), erosion rates have increased substantially, due in part to pervasive changes in watershed hydrology.

Conference Paper↗

USGS2009

Geomorphic evolution of the Le Sueur River, Minnesota, USA, and implications for current sediment loading

There is clear evidence that the Minnesota River is the major sediment source for Lake Pepin and that the Le Sueur River is a major source to the Minnesota River. Turbidity levels are high enough to require management actions. We take advantage of the well-constrained Holocene history of the Le Sueur basin and use a combination of remote sensing, fi eld, and stream gauge observations to constrain the contributions of different sediment sources to the Le Sueur River. Understanding the type, location, and magnitude of sediment sources is essential for unraveling the Holocene development of the basin as well as for guiding management decisions about investments to reduce sediment loads. Rapid base-level fall at the outlet of the Le Sueur River 11,500 yr B.P. triggered up to 70 m of channel incision at the mouth. Slope-area analyses of river longitudinal profi les show that knickpoints have migrated 30-35 km upstream on all three major branches of the river, eroding 1.2-2.6 ?? 109 Mg of sediment from the lower valleys in the process. The knick zones separate the basin into an upper watershed, receiving sediment primarily from uplands and streambanks, and a lower, incised zone, which receives additional sediment from high bluffs and ravines. Stream gauges installed above and below knick zones show dramatic increases in sediment loading above that expected from increases in drainage area, indicating substantial inputs from bluffs and ravines.

Special Paper of the Geological Society of America↗

USGS2007

Reach‐averaged sediment routing model of a canyon river

Spatial complexity in channel geometry indicates that accurate prediction of sediment transport requires modeling in at least two dimensions. However, a one‐dimensional model may be the only practical or possible alternative, especially for longer river reaches of practical concern in river management or landscape modeling. We have developed a one‐dimensional model of the Colorado River through upper Grand Canyon that addresses this problem by reach averaging the channel properties and predicting changes in sand storage using separate source and sink functions coupled to the sand routing model. The model incorporates results from the application of a two‐dimensional model of flow, sand transport, and bed evolution, and a new algorithm for setting the near‐bed sand boundary condition for sand transported over an exposed bouldery bed. Model predictions were compared to measurements of sand discharge during intermittent tributary inputs and varying discharges controlled by dam releases. The model predictions generally agree well with the timing and magnitude of measured sand discharges but tend to overpredict sand discharge during the early stages of a high release designed to redistribute sand to higher‐elevation deposits.

Water Resources Research↗

USGS2005

Complex channel responses to changes in stream flow and sediment supply on the lower Duchesne River, Utah

Channel responses to flow depletions in the lower Duchesne River over the past 100 years have been highly complex and variable in space and time. In general, sand-bed reaches adjusted to all perturbations with bed-level changes, whereas the gravel-bed reaches adjusted primarily through width changes. Gravel-bed reaches aggraded only when gravel was supplied to the channel through local bank erosion and degraded only during extreme flood events. A 50% reduction in stream flow and an increase in fine sediment supply to the study area occurred in the first third of the 20th century. The gravel-bed reach responded primarily with channel narrowing, whereas bed aggradation and four large-scale avulsions occurred in the sand-bed reaches. These avulsions almost completely replaced a section of sinuous channel about 14 km long with a straighter section about 7 km long. The most upstream avulsion, located near a break in valley slope and the transition from a gravel bed upstream and a sand bed downstream, transformed a sinuous sand-bed reach into a braided gravel-bed reach and eventually into a meandering gravel-bed reach over a 30-year period. Later, an increase in flood magnitudes and durations caused widening and secondary bed aggradation in the gravel-bed reaches, whereas the sand-bed reaches incised and narrowed. Water diversions since the 1950s have progressively eliminated moderate flood events, whereas larger floods have been less affected. The loss of frequent flooding has increased the duration and severity of drought periods during which riparian vegetation can establish along the channel margins. As a result, the channel has gradually narrowed throughout the study area since the late 1960s, despite the occasional occurrence of large floods. No tributaries enter the Duchesne River within the study area, so all reaches have experienced identical changes in stream flow and upstream sediment supply. ?? 2004 Elsevier B.V. All rights reserved.

Geomorphology↗

USGS1998

Frequency of effective wave activity and the recession of coastal bluffs: Calvert Cliffs, Maryland

The Calvert Cliffs, Chesapeake Bay, Maryland, USA, erode by direct wave undercutting or by freeze/thaw erosion accompanied by wave removal of slope debris. Directly undercut slopes recede more rapidly, with long-term rates exceeding 1.0 m/yr; freeze/thaw slopes recede at rates approaching 0.5 m/yr. The frequency of wave height and water level at the shoreline is estimated for eleven sites based on a 37-year wind record, estimates of storm surge, offshore wave geometry, nearshore wave transformation, and breaking wave type. Locations experiencing the largest slope recession are not uniformly those with the largest cumulative wave energy; the resistance to erosion of the slope toe must also be accounted for. An index of relative wave strength is defined as the ratio of wave pressure T and the cohesive strength S of the slope material. For the Calvert Cliffs, a minimum relative wave strength for initiating erosion of intact material is 0.05 < T/S < 0.1. A cumulative duration of ???50 hours per year for T/S ??? 0.1 distinguishes undercut and nonundercut slopes and recession rates greater or lesser than 0.5 m/yr. The relative wave strength index may be used to identify sites at risk of increased erosion. At one site with a small historical erosion rate, the loss of a protective beach and associated decrease in toe elevation caused a positive shift in the frequency of large T/S. Direct wave undercutting and increased slope recession may be anticipated at this site, as indicated by the development of an undercut notch during the course of the study.

Journal of Coastal Research↗