Boulder, Colo. - The August-September issue of GEOSPHERE, published in electronic format only by the Geological Society of America, is now available online. Geology topics of interest include a continent-scale tectonic model for Precambrian growth and evolution of North America with time-slice maps and animations.
Natural and anthropogenic influences on the scaling of discharge with drainage area for multiple watersheds
Joshua C. Galster, Lehigh University, 31 Williams Rd., Bethlehem, Pennsylvania 18015, USA.
Keywords: watersheds, discharge, drainage area, Yellowstone River.
This paper investigates the relationship between discharge and drainage area for five rivers: the John Day, Salmon, Wabash, Greenbrier, and Yellowstone. The rivers were selected to minimize, but not eliminate, the impacts of dams and diversions such as for irrigation on river discharges. A minimum of 60 years of continuous discharge records from the U.S. Geological Survey national surface water database were analyzed for changes in discharge characteristics between watersheds as well as over time. The results show that the studied watersheds can be grouped into two broad categories. The John Day, Salmon, Wabash, and Greenbrier rivers have discharges that increase linearly as their drainage area increases. The Yellowstone watershed is unique because of its changing discharge values over the twentieth century, as well as its overall lower scaled discharges with drainage areas. Climatic trends that control the timing of winter snowpack melting, increased frequency and intensity of forest fires, and increased human consumptive water use in downstream areas may all contribute to the observed behavior in discharges for the Yellowstone watershed. The results from the study of this set of rivers have broad implications for studies ranging from the modeling of fluvial erosion to allocations of water resources for human and environmental purposes.
Position of the Snake River watershed divide as an indicator of geodynamic processes in the greater Yellowstone region, western North America
Karl W. Wegmann et al., corresponding author: Frank J. Pazzaglia: Lehigh University, 31 Williams, Bethlehem, PA 18015, USA.
Keywords: filtered topography, drainage divides, synthetic drainage divides, Snake River Plain, Yellowstone hotspot, dynamic topography.
This paper explores the mobility of regional watershed divides as a key geomorphic metric that can distinguish between various tectonic and dynamic mantle flow processes driving crustal deformation in the greater Yellowstone region of the northwestern United States. This new analysis quantifies the differences between the locations of the present-day Snake River drainage divide from divides synthetically generated from digital topography filtered at specific wavelengths. Results suggest that the topography of the Yellowstone area is supported by both dynamic mantle flow and crustal flexural mechanisms, whereas further to the west along the Snake River Plain, the topography is supported only by flexural mechanisms. Our analysis suggests that the eastward migration of the Snake River drainage divide lags behind the continued northeastward propagation of high standing topography associated with the Yellowstone hotspot by 1 to 2 million years.
Tectonic model for the Proterozoic growth of North America
Steven J. Whitmeyer, Department of Geology and Environmental Science, James Madison University, Harrisonburg, Virginia 22807, USA, and Karl E. Karlstrom.
Keywords: Proterozoic, Rodinia, Laurentia, continent assembly, North America
This paper presents a continent-scale model for the ancient (Precambrian) growth and evolution of North America. Major tectonic events are highlighted, beginning with the Trans-Hudson orogeny that assembled the Canadian Shield (cratonic core) of North America about 1.8 billion years ago. Subsequent collisional events, including the Yavapai and Mazatzal orogenies, added new crust to the southern margin of the enlarging North American continent. This accretionary sequence culminated with the collision of North America with components of present-day Africa and South America during the Grenville orogeny about 1 billion years ago. This collisional event is interpreted as the final stage in the assembly of the early supercontinent Rodinia. The approximately 800-million-year growth of ancient North America highlights processes through which originally juvenile (young) arc crust is stabilized into long-lasting continental crust.
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