Geology topics

E.B. Evenson

Publications and source records attributed to E.B. Evenson.

4 recordsLinked to original sources

USGS2017

Seasonal variability in hydrologic-system response to intense rain events, Matanuska Glacier, Alaska, U.S.A.

Two rain events at Matanuska Glacier illustrate how subglacial drainage system development and snowpack conditions affect hydrologic response at the terminus. On 21 and 22 September 1995, over 56 mm of rain fell in the basin during a period usually characterized by much drier conditions. This event caused an 8-fold increase in discharge and a 47-fold increase in suspended-sediment concentration. Peak suspended-sediment concentration exceeded 20 kg m-3, suggesting rapid evacuation of stored sediment. While water discharge returned to its pre-storm level nine days after the rain ceased, suspended-sediment concentrations took about 20 days to return to pre-storm levels. These observations suggest that the storm influx late in the melt season probably forced subglacial water into a more distributed system. In addition, subglacially transported sediments were supplemented to an unknown degree by the influx of storm-eroded sediments off hillslopes and from tributary drainage basins. A storm on 6 and 7 June 1997, dropped 28 mm of rain on the basin demonstrating the effects of meltwater retention in the snowpack and englacial and subglacial storage early in the melt season. Streamflow before the storm event was increasing gradually owing to warming temperatures; however, discharge during the storm and the following week increased only slightly. Suspended-sediment concentrations increased only a small amount, suggesting the drainage system was not yet well developed, and much of the runoff occurred across the relatively clean surface of the glacier or through englacial channels.

Annals of Glaciology↗

USGS2009

Quantifying periglacial erosion: Insights on a glacial sediment budget, Matanuska Glacier, Alaska

Glacial erosion rates are estimated to be among the highest in the world. Few studies have attempted, however, to quantify the flux of sediment from the periglacial landscape to a glacier. Here, erosion rates from the nonglacial landscape above the Matanuska Glacier, Alaska are presented and compare with an 8-yr record of proglacial suspended sediment yield. Non-glacial lowering rates range from 1??8 ?? 0??5 mm yr -1 to 8??5 ?? 3??4 mm yr -1 from estimates of rock fall and debris-flow fan volumes. An average erosion rate of 0??08 ?? 0??04 mm yr -1 from eight convex-up ridge crests was determined using in situ produced cosmogenic 10 Be. Extrapolating these rates, based on landscape morphometry, to the Matanuska basin (58% ice-cover), it was found that nonglacial processes account for an annual sediment flux of 2??3 ?? 1??0 ?? 10 6 t. Suspended sediment data for 8 years and an assumed bedload to estimate the annual sediment yield at the Matanuska terminus to be 2??9 ?? 1??0 ?? 10 6 t, corresponding to an erosion rate of 1??8 ?? 0??6 mm yr -1 : nonglacial sources therefore account for 80 ?? 45% of the proglacial yield. A similar set of analyses were used for a small tributary sub-basin (32% ice-cover) to determine an erosion rate of 12??1 ?? 6??9 mm yr -1 , based on proglacial sediment yield, with the nonglacial sediment flux equal to 10 ?? 7% of the proglacial yield. It is suggested that erosion rates by nonglacial processes are similar to inferred subglacial rates, such that the ice-free regions of a glaciated landscape contribute significantly to the glacial sediment budget. The similar magnitude of nonglacial and glacial rates implies that partially glaciated landscapes will respond rapidly to changes in climate and base level through a rapid nonglacial response to glacially driven incision. ?? 2009 John Wiley & Sons, Ltd.

Earth Surface Processes and Landforms↗

USGS2003

Bedload component of glacially discharged sediment: Insights from the Matanuska Glacier, Alaska

The flux of glacially derived bedload and the proportions of the suspended and bedload components carried by proglacial streams are highly debated. Published data indicate a large range-from <30% to >75%-in the bedload percentage of the total load. Two "vents," where supercooled subglacial meltwater and sediment are discharged, were sampled over the course of an entire melt season in order to quantify the flux of glacially delivered bedload at the Matanuska Glacier, Alaska. The bedload component contributed by these vents, for the one melt season monitored, is negligible. Furthermore, the bedload fluxes appear to be strongly supply limited, as shown by the poorly correlated discharge, bedload-flux magnitude, and grain-size caliber. Thus, in this case, any attempt to employ a predictive quantitative expression for coarse-sediment production based on discharge alone would be inaccurate. A nonglaciated basin proximal to the Matanuska Glacier terminus yielded higher bedload sediment fluxes and larger clast sizes than delivered by the two monitored vents. Such nonglaciated basins should not be overlooked as potentially major sources of coarse bedload that is reworked and incorporated into valley outwash.

Geology↗

USGS1990

A geochemical sampling technique for use in areas of active alpine glaciation: an application from the central Alaska Range

In mountainous regions containing extensive glacier systems there is a lack of suitable material for conventional geochemical sampling. As a result, in most geochemical sampling programs a few stream-sediment samples collected at, or near, the terminus of valley glaciers are used to evaluate the mineral potential of the glaciated area. We have developed and tested a technique which utilizes the medial moraines of valley glaciers for systematic geochemical exploration of the glacial catchment area. Moraine sampling provides geochemical information that is site-specific in that geochemical anomalies can be traced directly up-ice to bedrock sources. Traverses were made across the Trident and Susitna glaciers in the central Alaska Range where fine-grained (clay to sand size) samples were collected from each medial moraine. These samples were prepared and chemically analyzed to determine the concentration of specific elements. Fifty pebbles were collected at each moraine for archival purposes and for subsequent lithologic identification. Additionally, fifty cobbles and fifty boulders were examined and described at each sample site to determine the nature and abundance of lithologies present in the catchment area, the extent and nature of visible mineralization, the presence and intensity of hydrothermal alteration and the existence of veins, dikes and other minor structural features. Results from the central Alaska Range have delineated four distinct multi-element anomalies which are a response to potential mineralization up-ice from the medial moraine traverse. By integrating the lithologic, mineralogical and geochemical data the probable geological setting of the geochemical anomalies is determined.

Journal of Geochemical Exploration↗